Amazon Software Development Engineer

Entry-Level Questions (SDE I)

1. Data Structures & Algorithms: Prime Video Watch History Optimization

Level: SDE I (New Grad) - Amazon Prime Video

Question: “Given a string representing a watch history of episodes (e.g., ‘AABBAC’), return the minimum number of rewinds needed so that no two consecutive episodes are the same. You have 30 minutes, and the optimal solution uses a greedy + priority queue approach.”

Answer:

Problem Analysis:
We need to rearrange characters in a string such that no two consecutive characters are the same, using minimum “rewinds” (character moves).

Approach 1: Greedy + Priority Queue (Optimal)

import heapq
from collections import Counter
def min_rewinds_optimal(watch_history):
    """    Optimal solution using greedy approach with priority queue.    Time Complexity: O(n log k) where k is unique characters    Space Complexity: O(k)    """    if not watch_history:
        return 0    # Count frequency of each episode    freq_counter = Counter(watch_history)
    # Check if solution is possible    max_freq = max(freq_counter.values())
    if max_freq > (len(watch_history) + 1) // 2:
        return -1  # Impossible to arrange    # Priority queue: (-frequency, character)    # Use negative frequency for max heap behavior    max_heap = [(-freq, char) for char, freq in freq_counter.items()]
    heapq.heapify(max_heap)
    result = []
    rewinds = 0    while max_heap:
        # Get the most frequent character        first_freq, first_char = heapq.heappop(max_heap)
        first_freq = -first_freq
        # If result is empty or different from last character        if not result or result[-1] != first_char:
            result.append(first_char)
            first_freq -= 1            # Put back in heap if frequency > 0            if first_freq > 0:
                heapq.heappush(max_heap, (-first_freq, first_char))
        else:
            # Need to use second most frequent character            if not max_heap:
                # Only one type left and it matches last character                return -1            second_freq, second_char = heapq.heappop(max_heap)
            second_freq = -second_freq
            result.append(second_char)
            second_freq -= 1            rewinds += 1  # This counts as a rewind            # Put characters back in heap            if second_freq > 0:
                heapq.heappush(max_heap, (-second_freq, second_char))
            heapq.heappush(max_heap, (-first_freq, first_char))
    return rewinds
# Test casesdef test_min_rewinds():
    # Example: 'AABBAC' -> 'ABABAC' (1 rewind)    assert min_rewinds_optimal('AABBAC') == 1    assert min_rewinds_optimal('AAA') == -1  # Impossible    assert min_rewinds_optimal('ABAB') == 0   # Already optimal    assert min_rewinds_optimal('AAABBB') == 1 # 'ABABAB'    print("All test cases passed!")
test_min_rewinds()

Approach 2: Mathematical Solution (Alternative)

def min_rewinds_mathematical(watch_history):
    """    Mathematical approach based on frequency analysis.    Time Complexity: O(n)    Space Complexity: O(k)    """    if not watch_history:
        return 0    freq_counter = Counter(watch_history)
    frequencies = list(freq_counter.values())
    frequencies.sort(reverse=True)
    n = len(watch_history)
    max_freq = frequencies[0]
    # Check if arrangement is possible    if max_freq > (n + 1) // 2:
        return -1    # Calculate minimum rewinds needed    # The most frequent character determines the structure    even_positions = (n + 1) // 2    odd_positions = n // 2    rewinds = 0    remaining_freq = max_freq
    # Place most frequent character in even positions first    if remaining_freq <= even_positions:
        # Can fit all in even positions        rewinds = 0    else:
        # Need to use some odd positions        rewinds = remaining_freq - even_positions
    return rewinds

Key Insights:
1. Greedy Strategy: Always place the most frequent character that doesn’t conflict
2. Priority Queue: Efficiently manages character frequencies
3. Rewind Definition: Moving a character from its natural position
4. Impossibility Check: If max frequency > (n+1)/2, no solution exists

Edge Cases:
- Empty string: 0 rewinds
- Single character repeated: Check impossibility condition
- Already optimal arrangement: 0 rewinds
- All different characters: 0 rewinds

Amazon-Specific Considerations:
- Prime Video Context: Episodes must be watchable in sequence
- User Experience: Minimize disruption to viewing flow
- Performance: Sub-second response for real-time recommendations
- Scale: Handle thousands of episodes in user history

Follow-up Questions:
1. How would you handle streaming context where episodes arrive in real-time?
2. What if episodes have different priorities or user preferences?
3. How to optimize for memory usage with very long watch histories?

Mid-Level Questions (SDE II)

2. System Design: AWS Lambda Serverless Message Processing Pipeline

Level: SDE II - AWS Lambda Team

Question: “Design a serverless message-processing pipeline on AWS Lambda that ingests 1M events/sec from Kinesis, applies business logic, and writes results to DynamoDB. Include scaling, error handling, fault tolerance, and cost optimization.”

Answer:

High-Level Architecture:

┌─────────────────┐    ┌──────────────────┐    ┌─────────────────┐
│   Data Sources  │ -> │   Amazon Kinesis │ -> │   Lambda        │
│   (Apps/IoT)    │    │   Data Streams   │    │   Functions     │
└─────────────────┘    └──────────────────┘    └─────────────────┘
                                                         │
                       ┌─────────────────────────────────┴─────────────────────────────────┐
                       │                                                                   │
                ┌──────▼──────┐                                                    ┌──────▼──────┐
                │   DynamoDB  │                                                    │     SQS     │
                │   Tables    │                                                    │ Dead Letter │
                └─────────────┘                                                    │   Queue     │
                                                                                   └─────────────┘

Core Components Design:

1. Kinesis Data Streams Configuration:

import boto3
import json
from typing import Dict, List, Any
class KinesisStreamManager:
    """    Manages Kinesis streams for high-throughput event ingestion    """    def __init__(self):
        self.kinesis_client = boto3.client('kinesis')
    def create_optimized_stream(self, stream_name: str, target_throughput: int):
        """        Create Kinesis stream optimized for 1M events/sec        Args:            stream_name: Name of the stream            target_throughput: Target events per second        """        # Calculate shard count for 1M events/sec        # Each shard supports 1,000 records/sec or 1 MB/sec        avg_record_size = 1024  # 1KB average        throughput_per_shard = min(1000, 1024*1024 // avg_record_size)
        required_shards = (target_throughput // throughput_per_shard) + 1        stream_config = {
            'StreamName': stream_name,
            'ShardCount': min(required_shards, 500),  # AWS limit consideration            'StreamModeDetails': {
                'StreamMode': 'PROVISIONED'            },
            'Tags': [
                {'Key': 'Environment', 'Value': 'production'},
                {'Key': 'Team', 'Value': 'lambda-processing'},
                {'Key': 'AutoScale', 'Value': 'enabled'}
            ]
        }
        return self.kinesis_client.create_stream(**stream_config)
    def setup_auto_scaling(self, stream_name: str):
        """Configure auto-scaling for dynamic shard management"""        application_autoscaling = boto3.client('application-autoscaling')
        # Register scalable target        application_autoscaling.register_scalable_target(
            ServiceNamespace='kinesis',
            ResourceId=f'stream/{stream_name}',
            ScalableDimension='kinesis:shard:count',
            MinCapacity=10,
            MaxCapacity=500,
            RoleArn='arn:aws:iam::account:role/application-autoscaling-kinesis-role'        )
        # Create scaling policy        application_autoscaling.put_scaling_policy(
            PolicyName=f'{stream_name}-scaling-policy',
            ServiceNamespace='kinesis',
            ResourceId=f'stream/{stream_name}',
            ScalableDimension='kinesis:shard:count',
            PolicyType='TargetTrackingScaling',
            TargetTrackingScalingPolicyConfiguration={
                'TargetValue': 70.0,
                'PredefinedMetricSpecification': {
                    'PredefinedMetricType': 'KinesisStreamIncomingRecordsUtilization'                },
                'ScaleOutCooldown': 300,
                'ScaleInCooldown': 300            }
        )

2. Lambda Function Architecture:

import asyncio
import json
import time
from concurrent.futures import ThreadPoolExecutor
from dataclasses import dataclass
from typing import List, Dict, Any
@dataclassclass ProcessingResult:
    success: bool    record_id: str    processed_data: Dict[Any, Any] = None    error_message: str = Noneclass LambdaMessageProcessor:
    """    High-performance Lambda function for processing Kinesis events    """    def __init__(self):
        self.dynamodb = boto3.resource('dynamodb')
        self.sqs = boto3.client('sqs')
        self.table = self.dynamodb.Table('ProcessedEvents')
        self.dlq_url = 'https://sqs.region.amazonaws.com/account/dlq'        self.executor = ThreadPoolExecutor(max_workers=100)
    def lambda_handler(self, event: Dict, context) -> Dict:
        """        Main Lambda handler optimized for batch processing        """        start_time = time.time()
        batch_size = len(event['Records'])
        try:
            # Process records in parallel batches            results = self.process_batch_parallel(event['Records'])
            # Analyze results            successful_count = sum(1 for r in results if r.success)
            failed_count = batch_size - successful_count
            # Handle failed records            if failed_count > 0:
                self.handle_failed_records([r for r in results if not r.success])
            # Return batch item failures for partial retry            batch_item_failures = [
                {'itemIdentifier': r.record_id}
                for r in results if not r.success
            ]
            processing_time = time.time() - start_time
            # CloudWatch metrics            self.publish_metrics({
                'ProcessedRecords': successful_count,
                'FailedRecords': failed_count,
                'ProcessingTimeMs': processing_time * 1000,
                'BatchSize': batch_size
            })
            return {
                'batchItemFailures': batch_item_failures
            }
        except Exception as e:
            # Critical error - fail entire batch            self.publish_error_metrics(str(e), batch_size)
            raise e
    def process_batch_parallel(self, records: List[Dict]) -> List[ProcessingResult]:
        """        Process records in parallel for optimal throughput        """        # Group records for efficient DynamoDB batch operations        batch_groups = self.create_batch_groups(records, batch_size=25)
        all_results = []
        # Process each group in parallel        futures = []
        for group in batch_groups:
            future = self.executor.submit(self.process_record_group, group)
            futures.append(future)
        # Collect results        for future in futures:
            try:
                group_results = future.result(timeout=30)
                all_results.extend(group_results)
            except Exception as e:
                # Handle group processing failure                all_results.extend([
                    ProcessingResult(success=False, record_id='unknown', error_message=str(e))
                ])
        return all_results
    def process_record_group(self, records: List[Dict]) -> List[ProcessingResult]:
        """        Process a group of records with business logic        """        results = []
        processed_items = []
        for record in records:
            try:
                # Decode Kinesis record                data = json.loads(
                    base64.b64decode(record['kinesis']['data']).decode('utf-8')
                )
                # Apply business logic                processed_data = self.apply_business_logic(data)
                # Prepare for DynamoDB batch write                processed_items.append({
                    'PutRequest': {
                        'Item': {
                            'id': processed_data['id'],
                            'timestamp': processed_data['timestamp'],
                            'processed_at': int(time.time()),
                            'data': processed_data,
                            'source_shard': record['kinesis']['sequenceNumber']
                        }
                    }
                })
                results.append(ProcessingResult(
                    success=True,
                    record_id=record['kinesis']['sequenceNumber'],
                    processed_data=processed_data
                ))
            except Exception as e:
                results.append(ProcessingResult(
                    success=False,
                    record_id=record['kinesis']['sequenceNumber'],
                    error_message=str(e)
                ))
        # Batch write to DynamoDB        if processed_items:
            self.batch_write_to_dynamodb(processed_items)
        return results
    def apply_business_logic(self, data: Dict) -> Dict:
        """        Apply business transformation logic        """        # Example business logic transformations        processed = {
            'id': data.get('id'),
            'timestamp': data.get('timestamp'),
            'user_id': data.get('user_id'),
            'event_type': data.get('event_type'),
            'metadata': data.get('metadata', {}),
        }
        # Enrichment logic        if processed['event_type'] == 'user_action':
            processed['priority'] = 'high'        elif processed['event_type'] == 'system_event':
            processed['priority'] = 'medium'        else:
            processed['priority'] = 'low'        # Validation        if not processed['id'] or not processed['timestamp']:
            raise ValueError("Missing required fields: id or timestamp")
        return processed
    def batch_write_to_dynamodb(self, items: List[Dict]):
        """        Efficient batch write to DynamoDB with retry logic        """        table_name = 'ProcessedEvents'        try:
            response = self.dynamodb.batch_write_item(
                RequestItems={
                    table_name: items
                }
            )
            # Handle unprocessed items            unprocessed = response.get('UnprocessedItems', {})
            retry_count = 0            max_retries = 3            while unprocessed and retry_count < max_retries:
                time.sleep(2 ** retry_count)  # Exponential backoff                response = self.dynamodb.batch_write_item(
                    RequestItems=unprocessed
                )
                unprocessed = response.get('UnprocessedItems', {})
                retry_count += 1        except Exception as e:
            # Log error and send to DLQ            self.send_to_dlq(items, str(e))
            raise e

3. Error Handling & Fault Tolerance:

class ErrorHandlingManager:
    """    Comprehensive error handling and fault tolerance    """    def __init__(self):
        self.cloudwatch = boto3.client('cloudwatch')
        self.sqs = boto3.client('sqs')
    def handle_failed_records(self, failed_results: List[ProcessingResult]):
        """        Handle failed records with appropriate retry strategy        """        for result in failed_results:
            error_type = self.classify_error(result.error_message)
            if error_type == 'retryable':
                # Return for Lambda's automatic retry                continue            elif error_type == 'throttling':
                # Implement exponential backoff                self.handle_throttling_error(result)
            else:
                # Send to DLQ for manual investigation                self.send_to_dlq(result)
    def classify_error(self, error_message: str) -> str:
        """        Classify errors for appropriate handling strategy        """        if 'throttling' in error_message.lower():
            return 'throttling'        elif 'timeout' in error_message.lower():
            return 'retryable'        elif 'validation' in error_message.lower():
            return 'permanent'        else:
            return 'retryable'    def implement_circuit_breaker(self, service_name: str):
        """        Circuit breaker pattern for downstream services        """        # Implement circuit breaker logic        pass

4. Cost Optimization Strategies:

class CostOptimizationManager:
    """    Cost optimization for Lambda and associated services    """    def optimize_lambda_configuration(self):
        """        Optimize Lambda function configuration for cost and performance        """        return {
            'memory_size': 1024,  # MB - optimal for CPU-bound tasks            'timeout': 300,       # seconds            'reserved_concurrency': 1000,  # Prevent runaway costs            'provisioned_concurrency': 100,  # For consistent performance            'architecture': 'arm64',  # 20% cost savings            'runtime': 'python3.9',
            'environment_variables': {
                'PYTHONPATH': '/opt/python',
                'BATCH_SIZE': '25',
                'MAX_RETRIES': '3'            }
        }
    def setup_auto_scaling_policies(self):
        """        Configure auto-scaling to optimize costs        """        scaling_policies = {
            'kinesis_shards': {
                'target_utilization': 70,
                'scale_out_cooldown': 300,
                'scale_in_cooldown': 900  # Longer cooldown to prevent thrashing            },
            'dynamodb_capacity': {
                'read_target_utilization': 70,
                'write_target_utilization': 70,
                'billing_mode': 'ON_DEMAND'  # For variable workloads            },
            'lambda_concurrency': {
                'reserved_concurrency': 1000,
                'target_tracking_config': {
                    'target_value': 0.7                }
            }
        }
        return scaling_policies

Performance Characteristics:

Scaling Metrics:
- Throughput: 1M+ events/second with auto-scaling
- Latency: P99 < 2 seconds end-to-end processing
- Availability: 99.99% with multi-AZ deployment
- Cost: ~$0.0001 per event processed

Monitoring & Alerting:
- CloudWatch metrics for all components
- X-Ray tracing for performance analysis
- Custom dashboards for operational visibility
- Automated alerting for threshold breaches

AWS-Specific Optimizations:
- ARM-based Lambda for 20% cost savings
- Provisioned concurrency for consistent performance
- Reserved capacity for predictable workloads
- Cross-region replication for disaster recovery

3. Object-Oriented Design: Alexa Audio Clip Caching System

Level: SDE II - Alexa Audio Team

Question: “Design an audio-clip caching system for Alexa that deduplicates clips across devices, enforces LRU eviction, and supports cache invalidation on skill updates. Show class diagrams and method signatures.”

Answer:

System Architecture Overview:

┌─────────────────┐    ┌──────────────────┐    ┌─────────────────┐
│   Alexa Device  │ -> │   Cache Manager  │ -> │   Audio Store   │
│   (Echo, etc.)  │    │   (LRU Policy)   │    │   (S3/Local)    │
└─────────────────┘    └──────────────────┘    └─────────────────┘
                                │
                       ┌────────┴────────┐
                       │                 │
                ┌──────▼──────┐   ┌──────▼──────┐
                │Deduplication│   │ Invalidation│
                │   Engine    │   │   Manager   │
                └─────────────┘   └─────────────┘

Core Class Design:

1. Audio Clip Model:

from abc import ABC, abstractmethod
from dataclasses import dataclass
from typing import Optional, Dict, Any, List
import hashlib
import time
from enum import Enum
class AudioFormat(Enum):
    MP3 = "mp3"    WAV = "wav"    OGG = "ogg"    FLAC = "flac"class CompressionLevel(Enum):
    LOW = 1    MEDIUM = 2    HIGH = 3@dataclass(frozen=True)
class AudioClipMetadata:
    """Immutable metadata for audio clips"""    clip_id: str    skill_id: str    content_hash: str    format: AudioFormat
    duration_ms: int    sample_rate: int    bit_rate: int    compression_level: CompressionLevel
    language: str    created_at: float    last_accessed: float    access_count: int = 0    def __post_init__(self):
        # Validate metadata        if self.duration_ms <= 0:
            raise ValueError("Duration must be positive")
        if self.sample_rate not in [8000, 16000, 22050, 44100, 48000]:
            raise ValueError("Invalid sample rate")
class AudioClip:
    """    Represents an audio clip with caching-specific properties    """    def __init__(self,
                 clip_id: str,
                 skill_id: str,
                 audio_data: bytes,
                 metadata: AudioClipMetadata):
        self._clip_id = clip_id
        self._skill_id = skill_id
        self._audio_data = audio_data
        self._metadata = metadata
        self._content_hash = self._calculate_hash()
        self._created_at = time.time()
        self._last_accessed = time.time()
        self._access_count = 0    @property    def clip_id(self) -> str:
        return self._clip_id
    @property    def skill_id(self) -> str:
        return self._skill_id
    @property    def content_hash(self) -> str:
        return self._content_hash
    @property    def audio_data(self) -> bytes:
        self._last_accessed = time.time()
        self._access_count += 1        return self._audio_data
    @property    def metadata(self) -> AudioClipMetadata:
        return self._metadata
    @property    def size_bytes(self) -> int:
        return len(self._audio_data)
    @property    def last_accessed(self) -> float:
        return self._last_accessed
    @property    def access_count(self) -> int:
        return self._access_count
    def _calculate_hash(self) -> str:
        """Calculate SHA-256 hash of audio content for deduplication"""        return hashlib.sha256(self._audio_data).hexdigest()
    def is_expired(self, ttl_seconds: int) -> bool:
        """Check if clip has expired based on TTL"""        return (time.time() - self._created_at) > ttl_seconds
    def __eq__(self, other) -> bool:
        if not isinstance(other, AudioClip):
            return False        return self._content_hash == other._content_hash
    def __hash__(self) -> int:
        return hash(self._content_hash)
    def __repr__(self) -> str:
        return (f"AudioClip(id={self._clip_id}, skill={self._skill_id}, "                f"size={self.size_bytes}, hash={self._content_hash[:8]}...)")

2. LRU Cache Implementation:

from collections import OrderedDict
from threading import RLock
from typing import Optional, Callable
class LRUAudioCache:
    """    Thread-safe LRU cache optimized for audio clips    """    def __init__(self,
                 max_size_mb: int = 500,
                 max_items: int = 1000,
                 ttl_seconds: int = 3600):
        self._max_size_bytes = max_size_mb * 1024 * 1024        self._max_items = max_items
        self._ttl_seconds = ttl_seconds
        # Thread-safe data structures        self._cache: OrderedDict[str, AudioClip] = OrderedDict()
        self._content_hash_index: Dict[str, str] = {}  # hash -> clip_id        self._skill_index: Dict[str, set] = {}  # skill_id -> set of clip_ids        self._current_size_bytes = 0        self._lock = RLock()
        # Statistics        self._hits = 0        self._misses = 0        self._evictions = 0        # Event callbacks        self._on_eviction: Optional[Callable[[AudioClip], None]] = None        self._on_cache_full: Optional[Callable[[], None]] = None    def get(self, clip_id: str) -> Optional[AudioClip]:
        """        Retrieve audio clip and move to end (most recently used)        """        with self._lock:
            if clip_id not in self._cache:
                self._misses += 1                return None            clip = self._cache[clip_id]
            # Check TTL expiration            if clip.is_expired(self._ttl_seconds):
                self._remove_clip(clip_id)
                self._misses += 1                return None            # Move to end (most recently used)            self._cache.move_to_end(clip_id)
            self._hits += 1            return clip
    def put(self, clip: AudioClip) -> bool:
        """        Add audio clip to cache with deduplication        """        with self._lock:
            # Check for duplicate content            if clip.content_hash in self._content_hash_index:
                existing_clip_id = self._content_hash_index[clip.content_hash]
                # Update access pattern for existing clip                self._cache.move_to_end(existing_clip_id)
                return True            # Check capacity constraints            if not self._can_accommodate(clip):
                if not self._make_space_for(clip):
                    return False  # Cannot accommodate even after eviction            # Add to cache            self._add_clip(clip)
            return True    def remove(self, clip_id: str) -> bool:
        """        Explicitly remove clip from cache        """        with self._lock:
            if clip_id in self._cache:
                self._remove_clip(clip_id)
                return True            return False    def invalidate_by_skill(self, skill_id: str) -> int:
        """        Invalidate all clips associated with a skill        Returns number of clips removed        """        with self._lock:
            if skill_id not in self._skill_index:
                return 0            clip_ids = list(self._skill_index[skill_id])
            removed_count = 0            for clip_id in clip_ids:
                if self._remove_clip(clip_id):
                    removed_count += 1            return removed_count
    def invalidate_by_content_hash(self, content_hash: str) -> bool:
        """        Invalidate clip by content hash        """        with self._lock:
            if content_hash in self._content_hash_index:
                clip_id = self._content_hash_index[content_hash]
                return self._remove_clip(clip_id)
            return False    def clear(self) -> None:
        """Clear entire cache"""        with self._lock:
            self._cache.clear()
            self._content_hash_index.clear()
            self._skill_index.clear()
            self._current_size_bytes = 0    def get_stats(self) -> Dict[str, Any]:
        """Get cache statistics"""        with self._lock:
            total_requests = self._hits + self._misses
            hit_rate = self._hits / total_requests if total_requests > 0 else 0            return {
                'cache_size_mb': self._current_size_bytes / (1024 * 1024),
                'max_size_mb': self._max_size_bytes / (1024 * 1024),
                'item_count': len(self._cache),
                'max_items': self._max_items,
                'hit_rate': hit_rate,
                'hits': self._hits,
                'misses': self._misses,
                'evictions': self._evictions
            }

Performance Characteristics:
- Cache Hit Rate: Target >90% for frequently accessed clips
- Deduplication Rate: 15-30% storage savings
- Access Latency: <10ms for cached clips
- Memory Efficiency: Configurable size limits with LRU eviction

Amazon-Specific Considerations:
- Alexa Skills: Integration with skill lifecycle
- Device Constraints: Memory-aware caching for Echo devices
- Content Delivery: S3 integration for clip storage
- Privacy: Secure handling of user-generated audio

4. Amazon Leadership Principles: Customer Obsession

Level: SDE II - Retail Checkout Team

Question: “Tell me about a time you disagreed with a customer’s feature request. How did you ensure their needs were met while maintaining system integrity? Use the STAR method.”

Answer:

Situation:
While working as a software engineer on an e-commerce checkout system for a mid-sized online retailer, I received a feature request from one of our largest enterprise customers (a B2B client generating $2M+ annual revenue). They wanted the ability to modify order details (quantities, products, shipping addresses) after payment confirmation but before fulfillment. Their purchasing managers often caught errors post-checkout and wanted to avoid the friction of canceling and re-ordering.

Task:
My responsibility was to evaluate the technical feasibility and security implications of this feature while ensuring we met the customer’s underlying need for order flexibility. I needed to balance their specific request against system integrity, security requirements, and impacts on other customers.

Action:

1. Deep Dive into Customer’s True Need:

# Initial analysis framework I usedclass CustomerNeedAnalysis:
    def analyze_request(self, customer_request):
        return {
            'stated_requirement': customer_request,
            'underlying_business_need': self.identify_root_cause(),
            'current_pain_points': self.document_friction_points(),
            'impact_metrics': self.quantify_business_impact(),
            'technical_constraints': self.assess_system_limitations()
        }

I spent time with the customer to understand their workflow:
- Root Cause: Manual procurement process with multiple approval layers
- Pain Points: 15-20 minute re-order process for simple changes
- Impact: $50K+ in lost productivity annually due to order corrections
- Frequency: 200+ modification requests per month

2. Technical Assessment of Original Request:

# Security and data integrity concerns with post-payment modificationsclass OrderModificationRisks:
    def assess_risks(self):
        return {
            'payment_reconciliation': {
                'risk': 'HIGH',
                'issue': 'Payment amount vs final order value mismatch',
                'impact': 'Financial discrepancies, audit failures'            },
            'inventory_consistency': {
                'risk': 'HIGH',
                'issue': 'Reserved inventory released before modification',
                'impact': 'Overselling, stock inconsistencies'            },
            'compliance_violations': {
                'risk': 'MEDIUM',
                'issue': 'PCI-DSS requirements for payment data integrity',
                'impact': 'Regulatory compliance failures'            },
            'fraud_vector': {
                'risk': 'HIGH',
                'issue': 'Post-payment modifications could enable fraud',
                'impact': 'Financial losses, security breaches'            }
        }

3. Alternative Solution Development:
Instead of allowing post-payment modifications, I proposed a pre-checkout validation system that addressed their core need:

class EnhancedCheckoutValidation:
    """    Alternative solution maintaining system integrity while reducing order errors    """    def implement_validation_workflow(self):
        return {
            'intelligent_warnings': {
                'duplicate_detection': 'Warn about similar recent orders',
                'quantity_validation': 'Flag unusual quantity patterns',
                'shipping_verification': 'Confirm address accuracy',
                'approval_routing': 'Route high-value orders for review'            },
            'draft_order_system': {
                'save_progress': 'Allow saving incomplete orders',
                'collaboration': 'Multiple users can review before payment',
                'approval_workflow': 'Built-in approval chain',
                'bulk_ordering': 'Template-based repeat orders'            },
            'order_preview': {
                'detailed_summary': 'Comprehensive pre-payment review',
                'change_impact': 'Show cost/timeline implications',
                'confirmation_steps': 'Multi-step confirmation process',
                'mobile_friendly': 'Mobile-optimized review interface'            }
        }

4. Customer Collaboration Process:
I worked directly with their procurement team to design the solution:

# Collaborative design processdef customer_collaboration_framework():
    phases = {
        'requirements_gathering': {
            'stakeholder_interviews': 'Spoke with 5 different users',
            'workflow_mapping': 'Documented current vs desired state',
            'success_metrics': 'Defined measurable outcomes'        },
        'prototype_development': {
            'rapid_prototyping': '2-week MVP with core features',
            'user_testing': 'Weekly feedback sessions',
            'iterative_improvements': 'Incorporated real-time feedback'        },
        'pilot_implementation': {
            'limited_rollout': 'Started with one department',
            'success_tracking': 'Monitored error reduction metrics',
            'refinement': 'Adjusted based on usage patterns'        }
    }
    return phases

5. Implementation Details:

class OrderValidationSystem:
    def __init__(self):
        self.ml_model = DuplicateOrderDetector()
        self.approval_engine = WorkflowApprovalEngine()
    def validate_order_pre_checkout(self, order_data, user_context):
        """        Comprehensive pre-checkout validation        """        validations = {
            'duplicate_check': self.check_duplicate_orders(order_data),
            'quantity_analysis': self.analyze_quantity_patterns(order_data),
            'approval_required': self.determine_approval_needs(order_data),
            'shipping_validation': self.validate_shipping_details(order_data)
        }
        # Generate intelligent warnings        warnings = self.generate_contextual_warnings(validations)
        # Route for approval if needed        if validations['approval_required']:
            return self.route_for_approval(order_data, user_context)
        return {'validated': True, 'warnings': warnings}
    def check_duplicate_orders(self, order_data):
        """ML-based duplicate order detection"""        recent_orders = self.get_recent_orders(order_data['customer_id'])
        similarity_scores = self.ml_model.calculate_similarity(
            order_data, recent_orders
        )
        return {
            'potential_duplicates': [
                order for order, score in similarity_scores.items()
                if score > 0.8            ],
            'confidence': max(similarity_scores.values()) if similarity_scores else 0        }

Result:

Immediate Outcomes:
- Error Reduction: 75% decrease in order modification requests (200+ → 50 per month)
- Process Efficiency: 8-minute average reduction in order completion time
- Customer Satisfaction: 98% approval rating from pilot users
- System Integrity: Zero payment/order mismatches maintained

Long-term Impact:
- Customer Retention: Renewed $2.5M annual contract with 20% increase
- Feature Adoption: 85% of other enterprise customers adopted the new workflow
- Team Learning: Became template for future customer-driven feature development

Technical Metrics:

# Success metrics after 6 monthsresults = {
    'order_accuracy': {
        'before': '92%',
        'after': '98.5%',
        'improvement': '+6.5%'    },
    'customer_productivity': {
        'time_saved_per_order': '8 minutes',
        'annual_productivity_gain': '$75K+',
        'user_satisfaction': '98%'    },
    'system_reliability': {
        'zero_payment_discrepancies': True,
        'fraud_incidents': 0,
        'compliance_violations': 0    }
}

Customer Obsession Principles Demonstrated:

Listen and Understand: Spent time understanding the real business need behind the request

Think Long-term: Proposed solution that scaled to benefit other customers

Security & Trust: Maintained system integrity while solving customer pain

Innovation: Created new capabilities that exceeded original expectations

Ownership: Took responsibility for both customer success and technical excellence

Key Learning:
Sometimes the best way to serve customers is not to give them exactly what they ask for, but to understand their underlying need and provide a better solution. By maintaining technical integrity while innovating on the customer experience, we delivered more value than the original request would have provided.

Follow-up Questions I’d Be Prepared For:
- “How did you handle resistance from the customer when proposing an alternative?”
- “What would you have done if the customer insisted on their original approach?”
- “How did you measure the success of your alternative solution?”
- “What did you learn about balancing customer needs with technical constraints?”

Senior-Level Questions (SDE III)

5. Distributed Systems Design: Prime Logistics Real-Time Parcel Tracking

Level: SDE III - Prime Logistics

Question: “Design a real-time parcel-tracking service that updates user dashboards within 200 ms of scan events worldwide. Include architecture using SQS, DynamoDB global tables, and CloudFront.”

Answer:

Global System Architecture:

┌─────────────────┐    ┌──────────────────┐    ┌─────────────────┐
│ Scan Devices    │ -> │   Regional       │ -> │   Global        │
│ (Warehouses,    │    │   Ingestion      │    │   Event         │
│  Trucks, etc.)  │    │   Layer          │    │   Processing    │
└─────────────────┘    └──────────────────┘    └─────────────────┘
                                │                         │
                       ┌────────┴────────┐       ┌────────┴────────┐
                       │                 │       │                 │
                ┌──────▼──────┐   ┌──────▼──────▼──────┐   ┌──────▼──────┐
                │     SQS     │   │    DynamoDB        │   │ CloudFront  │
                │ Event Queue │   │ Global Tables      │   │    CDN      │
                └─────────────┘   └────────────────────┘   └─────────────┘
                                           │                        │
                                  ┌────────┴────────┐      ┌────────┴────────┐
                                  │                 │      │                 │
                           ┌──────▼──────┐   ┌──────▼──────▼──────┐   ┌──────▼──────┐
                           │ Lambda      │   │   User Dashboard   │   │  Mobile     │
                           │ Functions   │   │    (Web/Mobile)    │   │    Apps     │
                           └─────────────┘   └────────────────────┘   └─────────────┘

Core Components Design:

1. Event Ingestion Layer:

import asyncio
import boto3
import json
import time
from typing import Dict, List, Optional
from dataclasses import dataclass, asdict
from enum import Enum
class ScanEventType(Enum):
    PICKUP = "pickup"    IN_TRANSIT = "in_transit"
    OUT_FOR_DELIVERY = "out_for_delivery"    DELIVERED = "delivered"    EXCEPTION = "exception"    RETURNED = "returned"class LocationType(Enum):
    WAREHOUSE = "warehouse"    SORTING_CENTER = "sorting_center"    DELIVERY_VEHICLE = "delivery_vehicle"    CUSTOMER_ADDRESS = "customer_address"@dataclassclass ScanEvent:
    """    Immutable scan event structure    """    tracking_id: str    event_type: ScanEventType
    timestamp: float    location_id: str    location_type: LocationType
    coordinates: Optional[Dict[str, float]]  # lat, lng    scan_device_id: str    operator_id: str    metadata: Dict = None    def __post_init__(self):
        if self.metadata is None:
            self.metadata = {}
        # Add event processing metadata        self.metadata.update({
            'ingestion_timestamp': time.time(),
            'event_version': '2.0',
            'source_region': self._determine_region()
        })
    def _determine_region(self) -> str:
        """Determine AWS region based on location"""        # Simplified region mapping        region_mapping = {
            'US': 'us-east-1',
            'EU': 'eu-west-1',
            'ASIA': 'ap-southeast-1',
            'DEFAULT': 'us-east-1'        }
        # Logic to determine region from location_id or coordinates        return region_mapping.get(self.location_id[:2], 'us-east-1')
    def to_dict(self) -> Dict:
        """Convert to dictionary for JSON serialization"""        return asdict(self)
class EventIngestionService:
    """    High-throughput event ingestion with regional distribution    """    def __init__(self):
        self.sqs_clients = {}  # region -> SQS client        self.event_validator = EventValidator()
        self.deduplication_cache = DeduplicationCache()
        # Initialize regional SQS clients        regions = ['us-east-1', 'eu-west-1', 'ap-southeast-1']
        for region in regions:
            self.sqs_clients[region] = boto3.client('sqs', region_name=region)
    async def ingest_scan_event(self, scan_event: ScanEvent) -> Dict[str, str]:
        """        Ingest scan event with validation and regional routing        """        try:
            # Validate event structure and business rules            validation_result = await self.event_validator.validate(scan_event)
            if not validation_result.is_valid:
                return {
                    'status': 'error',
                    'message': f'Validation failed: {validation_result.errors}'                }
            # Check for duplicates            if await self.deduplication_cache.is_duplicate(scan_event):
                return {
                    'status': 'duplicate',
                    'message': 'Event already processed'                }
            # Route to appropriate regional queue            region = scan_event.metadata['source_region']
            queue_url = f"https://sqs.{region}.amazonaws.com/account/tracking-events-{region}"            # Send to SQS with message attributes for routing            message_attributes = {
                'tracking_id': {'StringValue': scan_event.tracking_id, 'DataType': 'String'},
                'event_type': {'StringValue': scan_event.event_type.value, 'DataType': 'String'},
                'priority': {'StringValue': self._calculate_priority(scan_event), 'DataType': 'String'},
                'region': {'StringValue': region, 'DataType': 'String'}
            }
            response = await self._send_to_sqs(
                region, queue_url, scan_event.to_dict(), message_attributes
            )
            # Cache for deduplication            await self.deduplication_cache.store(scan_event)
            return {
                'status': 'success',
                'message_id': response['MessageId'],
                'region': region
            }
        except Exception as e:
            # Log error and send to DLQ            await self._handle_ingestion_error(scan_event, str(e))
            return {
                'status': 'error',
                'message': f'Ingestion failed: {str(e)}'            }
    def _calculate_priority(self, scan_event: ScanEvent) -> str:
        """        Calculate message priority for processing order        """        priority_mapping = {
            ScanEventType.DELIVERED: 'high',      # Customer notification critical            ScanEventType.EXCEPTION: 'high',     # Issue resolution needed            ScanEventType.OUT_FOR_DELIVERY: 'medium',  # Customer tracking active            ScanEventType.IN_TRANSIT: 'low',     # Routine update            ScanEventType.PICKUP: 'medium'       # Start of journey        }
        return priority_mapping.get(scan_event.event_type, 'low')
    async def _send_to_sqs(self, region: str, queue_url: str,
                          message_body: Dict, attributes: Dict) -> Dict:
        """        Send message to SQS with retry logic        """        sqs_client = self.sqs_clients[region]
        max_retries = 3        for attempt in range(max_retries):
            try:
                response = sqs_client.send_message(
                    QueueUrl=queue_url,
                    MessageBody=json.dumps(message_body),
                    MessageAttributes=attributes,
                    # Use tracking ID for deduplication in FIFO queues                    MessageDeduplicationId=f"{message_body['tracking_id']}_{message_body['timestamp']}",
                    MessageGroupId=message_body['tracking_id']
                )
                return response
            except Exception as e:
                if attempt == max_retries - 1:
                    raise e
                await asyncio.sleep(2 ** attempt)  # Exponential backoff

2. Real-Time Event Processing:

class RealTimeEventProcessor:
    """    Lambda-based event processor for sub-200ms updates    """    def __init__(self):
        self.dynamodb = boto3.resource('dynamodb')
        self.tracking_table = self.dynamodb.Table('TrackingEvents')
        self.summary_table = self.dynamodb.Table('PackageSummary')
        self.notification_service = NotificationService()
    async def lambda_handler(self, event: Dict, context) -> Dict:
        """        Process SQS events from tracking queue        """        start_time = time.time()
        processed_count = 0        failed_count = 0        # Process each SQS record        for record in event['Records']:
            try:
                # Parse scan event                scan_event = ScanEvent(**json.loads(record['body']))
                # Process event in parallel operations                await asyncio.gather(
                    self._update_tracking_history(scan_event),
                    self._update_package_summary(scan_event),
                    self._trigger_notifications(scan_event),
                    self._update_analytics(scan_event)
                )
                processed_count += 1            except Exception as e:
                failed_count += 1                await self._handle_processing_error(record, str(e))
        processing_time = (time.time() - start_time) * 1000  # Convert to ms        # CloudWatch metrics        await self._publish_metrics({
            'ProcessedEvents': processed_count,
            'FailedEvents': failed_count,
            'ProcessingTimeMs': processing_time,
            'BatchSize': len(event['Records'])
        })
        return {
            'statusCode': 200,
            'processedCount': processed_count,
            'failedCount': failed_count,
            'processingTimeMs': processing_time
        }
    async def _update_tracking_history(self, scan_event: ScanEvent):
        """        Update detailed tracking history with global replication        """        item = {
            'tracking_id': scan_event.tracking_id,
            'event_timestamp': scan_event.timestamp,
            'event_type': scan_event.event_type.value,
            'location_id': scan_event.location_id,
            'location_type': scan_event.location_type.value,
            'coordinates': scan_event.coordinates,
            'scan_device_id': scan_event.scan_device_id,
            'operator_id': scan_event.operator_id,
            'metadata': scan_event.metadata,
            'ttl': int(time.time()) + (365 * 24 * 3600)  # 1 year retention        }
        # Use composite sort key for chronological ordering        item['sort_key'] = f"{scan_event.timestamp}#{scan_event.event_type.value}"        response = self.tracking_table.put_item(
            Item=item,
            ConditionExpression='attribute_not_exists(sort_key)'  # Prevent duplicates        )
        return response
    async def _update_package_summary(self, scan_event: ScanEvent):
        """        Update package summary for fast dashboard queries        """        # Use update expression for atomic operations        update_expression = """            SET                current_status = :status,                current_location = :location,                last_updated = :timestamp,                coordinates = :coords            ADD                event_count = :increment        """        expression_values = {
            ':status': scan_event.event_type.value,
            ':location': scan_event.location_id,
            ':timestamp': scan_event.timestamp,
            ':coords': scan_event.coordinates or {},
            ':increment': 1        }
        response = self.summary_table.update_item(
            Key={'tracking_id': scan_event.tracking_id},
            UpdateExpression=update_expression,
            ExpressionAttributeValues=expression_values,
            ReturnValues='UPDATED_NEW'        )
        return response
    async def _trigger_notifications(self, scan_event: ScanEvent):
        """        Trigger real-time notifications for critical events        """        critical_events = [
            ScanEventType.DELIVERED,
            ScanEventType.OUT_FOR_DELIVERY,
            ScanEventType.EXCEPTION
        ]
        if scan_event.event_type in critical_events:
            await self.notification_service.send_real_time_notification(
                tracking_id=scan_event.tracking_id,
                event_type=scan_event.event_type.value,
                location=scan_event.location_id,
                timestamp=scan_event.timestamp
            )

3. Global DynamoDB Strategy:

class GlobalTableManager:
    """    Manages DynamoDB Global Tables for worldwide consistency    """    def __init__(self):
        self.dynamodb_clients = {}
        self.regions = ['us-east-1', 'eu-west-1', 'ap-southeast-1']
        for region in self.regions:
            self.dynamodb_clients[region] = boto3.client('dynamodb', region_name=region)
    def setup_global_tables(self):
        """        Configure Global Tables for cross-region replication        """        table_configs = {
            'TrackingEvents': {
                'partition_key': 'tracking_id',
                'sort_key': 'sort_key',
                'gsi': [
                    {
                        'index_name': 'LocationIndex',
                        'partition_key': 'location_id',
                        'sort_key': 'event_timestamp'                    }
                ],
                'billing_mode': 'ON_DEMAND',
                'point_in_time_recovery': True            },
            'PackageSummary': {
                'partition_key': 'tracking_id',
                'billing_mode': 'ON_DEMAND',
                'point_in_time_recovery': True,
                'stream_enabled': True            }
        }
        for table_name, config in table_configs.items():
            self._create_global_table(table_name, config)
    def _create_global_table(self, table_name: str, config: Dict):
        """        Create table with global replication        """        # Create table in primary region first        primary_region = 'us-east-1'        self._create_table(primary_region, table_name, config)
        # Add replicas in other regions        for region in self.regions:
            if region != primary_region:
                try:
                    self.dynamodb_clients[primary_region].create_global_table(
                        GlobalTableName=table_name,
                        ReplicationGroup=[
                            {'RegionName': primary_region},
                            {'RegionName': region}
                        ]
                    )
                except Exception as e:
                    print(f"Global table creation failed for {region}: {e}")

4. CloudFront CDN Integration:

class TrackingAPICDN:
    """    CloudFront integration for global API performance    """    def __init__(self):
        self.cloudfront = boto3.client('cloudfront')
        self.api_gateway = boto3.client('apigateway')
    def setup_tracking_api_distribution(self):
        """        Setup CloudFront distribution for tracking API        """        distribution_config = {
            'CallerReference': f'tracking-api-{int(time.time())}',
            'Comment': 'Global tracking API distribution',
            'Enabled': True,
            'Origins': {
                'Quantity': 3,
                'Items': [
                    {
                        'Id': 'us-east-1-api',
                        'DomainName': 'api-us-east-1.tracking.amazon.com',
                        'CustomOriginConfig': {
                            'HTTPPort': 443,
                            'HTTPSPort': 443,
                            'OriginProtocolPolicy': 'https-only'                        }
                    },
                    {
                        'Id': 'eu-west-1-api',
                        'DomainName': 'api-eu-west-1.tracking.amazon.com',
                        'CustomOriginConfig': {
                            'HTTPPort': 443,
                            'HTTPSPort': 443,
                            'OriginProtocolPolicy': 'https-only'                        }
                    },
                    {
                        'Id': 'ap-southeast-1-api',
                        'DomainName': 'api-ap-southeast-1.tracking.amazon.com',
                        'CustomOriginConfig': {
                            'HTTPPort': 443,
                            'HTTPSPort': 443,
                            'OriginProtocolPolicy': 'https-only'                        }
                    }
                ]
            },
            'DefaultCacheBehavior': {
                'TargetOriginId': 'us-east-1-api',
                'ViewerProtocolPolicy': 'redirect-to-https',
                'MinTTL': 0,
                'DefaultTTL': 300,  # 5 minutes                'MaxTTL': 86400,   # 24 hours                'Compress': True,
                'CachePolicyId': 'tracking-api-cache-policy'            },
            'CacheBehaviors': {
                'Quantity': 2,
                'Items': [
                    {
                        'PathPattern': '/api/v1/tracking/*',
                        'TargetOriginId': 'closest-api-origin',
                        'ViewerProtocolPolicy': 'redirect-to-https',
                        'MinTTL': 0,
                        'DefaultTTL': 60,   # 1 minute for tracking data                        'MaxTTL': 300,      # 5 minutes max                        'OriginRequestPolicyId': 'tracking-origin-policy'                    },
                    {
                        'PathPattern': '/api/v1/static/*',
                        'TargetOriginId': 's3-static-origin',
                        'ViewerProtocolPolicy': 'redirect-to-https',
                        'MinTTL': 86400,    # 24 hours for static content                        'DefaultTTL': 86400,
                        'MaxTTL': 31536000  # 1 year                    }
                ]
            }
        }
        return self.cloudfront.create_distribution(
            DistributionConfig=distribution_config
        )

5. Performance Optimization:

class PerformanceOptimizer:
    """    Optimizations for sub-200ms global response times    """    def __init__(self):
        self.cache_layer = ElastiCacheManager()
        self.connection_pool = ConnectionPoolManager()
    async def optimized_tracking_query(self, tracking_id: str,
                                     user_region: str) -> Dict:
        """        Optimized tracking query with multiple performance strategies        """        start_time = time.time()
        # Strategy 1: Check regional cache first        cached_result = await self.cache_layer.get_tracking_data(
            tracking_id, user_region
        )
        if cached_result and not self._is_stale(cached_result):
            return {
                'data': cached_result,
                'source': 'cache',
                'latency_ms': (time.time() - start_time) * 1000            }
        # Strategy 2: Query nearest DynamoDB region        nearest_region = self._get_nearest_region(user_region)
        # Strategy 3: Parallel queries for speed        summary_task = self._get_package_summary(tracking_id, nearest_region)
        events_task = self._get_recent_events(tracking_id, nearest_region, limit=5)
        package_summary, recent_events = await asyncio.gather(
            summary_task, events_task
        )
        # Strategy 4: Compile optimized response        response_data = {
            'tracking_id': tracking_id,
            'current_status': package_summary.get('current_status'),
            'current_location': package_summary.get('current_location'),
            'coordinates': package_summary.get('coordinates'),
            'last_updated': package_summary.get('last_updated'),
            'recent_events': recent_events,
            'estimated_delivery': self._calculate_estimated_delivery(
                package_summary, recent_events
            )
        }
        # Strategy 5: Cache result for future queries        await self.cache_layer.set_tracking_data(
            tracking_id, response_data, ttl=300        )
        latency_ms = (time.time() - start_time) * 1000        return {
            'data': response_data,
            'source': 'database',
            'latency_ms': latency_ms,
            'region': nearest_region
        }
    def _get_nearest_region(self, user_region: str) -> str:
        """        Determine nearest AWS region based on user location        """        region_mapping = {
            'north_america': 'us-east-1',
            'europe': 'eu-west-1',
            'asia_pacific': 'ap-southeast-1',
            'south_america': 'us-east-1',
            'africa': 'eu-west-1'        }
        return region_mapping.get(user_region, 'us-east-1')

Performance Characteristics:

Global Response Times:
- P50 Latency: <100ms globally
- P95 Latency: <200ms globally

- P99 Latency: <500ms globally
- Cache Hit Rate: >85% for frequently tracked packages

Scalability Metrics:
- Event Ingestion: 1M+ scan events per second globally
- Concurrent Users: 10M+ simultaneous dashboard users
- Global Replication: <5 seconds cross-region consistency
- Auto-scaling: 10x traffic spike handling

Reliability & Availability:
- System Uptime: 99.99% availability SLA
- Data Durability: 99.999999999% (11 9’s)
- Multi-region Failover: <30 seconds RTO
- Event Processing: At-least-once delivery guarantee

Amazon-Specific Optimizations:
- Prime Integration: Priority processing for Prime packages
- AWS Global Infrastructure: Leverages 25+ AWS regions
- Cost Optimization: On-demand pricing with reserved capacity for peaks
- Compliance: GDPR, SOC2, PCI DSS compliant tracking data handling

6. AWS-Specific Architecture: CloudFormation Drift Detection at Scale

Level: SDE III - AWS CloudFormation

Question: “Explain how you would implement drift detection in CloudFormation stacks at scale, ensuring compliance across millions of resources, with minimal performance overhead.”

Answer:

Architecture Overview:

┌─────────────────┐    ┌──────────────────┐    ┌─────────────────┐
│   CloudFormation│ -> │   Drift Detection│ -> │   Compliance    │
│     Stacks      │    │     Engine       │    │   Dashboard     │
│   (Millions)    │    │                  │    │                 │
└─────────────────┘    └──────────────────┘    └─────────────────┘
                                │
                       ┌────────┴────────┐
                       │                 │
                ┌──────▼──────┐   ┌──────▼──────┐
                │  Resource   │   │  Change     │
                │  Scanner    │   │  Tracker    │
                └─────────────┘   └─────────────┘

Core Implementation:

import asyncio
import boto3
from typing import Dict, List, Optional, Set
from dataclasses import dataclass
from enum import Enum
import json
import time
class DriftStatus(Enum):
    IN_SYNC = "IN_SYNC"    MODIFIED = "MODIFIED"    DELETED = "DELETED"    NOT_CHECKED = "NOT_CHECKED"class ResourceType(Enum):
    EC2_INSTANCE = "AWS::EC2::Instance"    S3_BUCKET = "AWS::S3::Bucket"    IAM_ROLE = "AWS::IAM::Role"    LAMBDA_FUNCTION = "AWS::Lambda::Function"    # ... hundreds more@dataclassclass DriftDetectionResult:
    """Result of drift detection for a resource"""    stack_name: str    logical_resource_id: str    physical_resource_id: str    resource_type: str    drift_status: DriftStatus
    property_differences: Dict = None    timestamp: float = None    def __post_init__(self):
        if self.timestamp is None:
            self.timestamp = time.time()
class ScalableDriftDetectionEngine:
    """    Scalable drift detection system for millions of CloudFormation resources    """    def __init__(self):
        self.cloudformation = boto3.client('cloudformation')
        self.config = boto3.client('config')
        self.dynamodb = boto3.resource('dynamodb')
        # Tables for tracking drift state        self.drift_results_table = self.dynamodb.Table('DriftDetectionResults')
        self.resource_inventory_table = self.dynamodb.Table('ResourceInventory')
        # Performance optimization        self.batch_size = 50        self.max_concurrent_detections = 100        self.drift_cache = DriftCache()
    async def detect_drift_at_scale(self,
                                  stack_names: List[str] = None) -> Dict[str, int]:
        """        Perform drift detection across all stacks or specified stacks        """        if stack_names is None:
            stack_names = await self._get_all_stacks()
        # Partition stacks for parallel processing        stack_batches = self._partition_stacks(stack_names, self.batch_size)
        # Process batches concurrently        semaphore = asyncio.Semaphore(self.max_concurrent_detections)
        tasks = []
        for batch in stack_batches:
            task = self._process_stack_batch_with_semaphore(semaphore, batch)
            tasks.append(task)
        batch_results = await asyncio.gather(*tasks, return_exceptions=True)
        # Aggregate results        total_stats = {
            'total_stacks': len(stack_names),
            'in_sync': 0,
            'drifted': 0,
            'errors': 0,
            'processing_time_ms': 0        }
        for result in batch_results:
            if isinstance(result, dict):
                total_stats['in_sync'] += result.get('in_sync', 0)
                total_stats['drifted'] += result.get('drifted', 0)
                total_stats['errors'] += result.get('errors', 0)
            else:
                total_stats['errors'] += 1        return total_stats
    async def _process_stack_batch_with_semaphore(self,
                                                semaphore: asyncio.Semaphore,
                                                stack_batch: List[str]) -> Dict:
        """        Process a batch of stacks with concurrency control        """        async with semaphore:
            return await self._process_stack_batch(stack_batch)
    async def _process_stack_batch(self, stack_names: List[str]) -> Dict:
        """        Process drift detection for a batch of stacks        """        batch_stats = {'in_sync': 0, 'drifted': 0, 'errors': 0}
        for stack_name in stack_names:
            try:
                # Check if we can use cached results                if await self._can_use_cached_result(stack_name):
                    cached_result = await self.drift_cache.get_drift_status(stack_name)
                    if cached_result.drift_status == DriftStatus.IN_SYNC:
                        batch_stats['in_sync'] += 1                    else:
                        batch_stats['drifted'] += 1                    continue                # Perform actual drift detection                drift_results = await self._detect_stack_drift(stack_name)
                # Store results                await self._store_drift_results(stack_name, drift_results)
                # Update stats                has_drift = any(r.drift_status != DriftStatus.IN_SYNC
                              for r in drift_results)
                if has_drift:
                    batch_stats['drifted'] += 1                else:
                    batch_stats['in_sync'] += 1            except Exception as e:
                batch_stats['errors'] += 1                await self._log_drift_detection_error(stack_name, str(e))
        return batch_stats
    async def _detect_stack_drift(self, stack_name: str) -> List[DriftDetectionResult]:
        """        Detect drift for all resources in a stack        """        try:
            # Initiate drift detection            response = self.cloudformation.detect_stack_drift(StackName=stack_name)
            drift_detection_id = response['StackDriftDetectionId']
            # Poll for completion with exponential backoff            drift_status = await self._wait_for_drift_detection(drift_detection_id)
            if drift_status != 'DETECTION_COMPLETE':
                raise Exception(f"Drift detection failed with status: {drift_status}")
            # Get detailed drift results            drift_results = await self._get_stack_drift_results(stack_name)
            return drift_results
        except Exception as e:
            raise Exception(f"Failed to detect drift for stack {stack_name}: {str(e)}")
    async def _wait_for_drift_detection(self, detection_id: str,
                                      max_wait_time: int = 300) -> str:
        """        Wait for drift detection to complete with intelligent polling        """        start_time = time.time()
        backoff_time = 1        max_backoff = 30        while (time.time() - start_time) < max_wait_time:
            try:
                response = self.cloudformation.describe_stack_drift_detection_status(
                    StackDriftDetectionId=detection_id
                )
                status = response['DetectionStatus']
                if status in ['DETECTION_COMPLETE', 'DETECTION_FAILED']:
                    return status
                # Exponential backoff with jitter                await asyncio.sleep(backoff_time + (backoff_time * 0.1))
                backoff_time = min(backoff_time * 2, max_backoff)
            except Exception as e:
                raise Exception(f"Error polling drift detection status: {str(e)}")
        raise Exception(f"Drift detection timed out after {max_wait_time} seconds")
    async def _get_stack_drift_results(self, stack_name: str) -> List[DriftDetectionResult]:
        """        Retrieve detailed drift results for all resources in a stack        """        results = []
        paginator = self.cloudformation.get_paginator('describe_stack_resource_drifts')
        async for page in paginator.paginate(StackName=stack_name):
            for resource_drift in page['StackResourceDrifts']:
                result = DriftDetectionResult(
                    stack_name=stack_name,
                    logical_resource_id=resource_drift['LogicalResourceId'],
                    physical_resource_id=resource_drift.get('PhysicalResourceId', ''),
                    resource_type=resource_drift['ResourceType'],
                    drift_status=DriftStatus(resource_drift['StackResourceDriftStatus']),
                    property_differences=resource_drift.get('PropertyDifferences', [])
                )
                results.append(result)
        return results
    async def _store_drift_results(self, stack_name: str,
                                 results: List[DriftDetectionResult]):
        """        Store drift detection results in DynamoDB for fast querying        """        # Batch write to DynamoDB        batch_items = []
        for result in results:
            item = {
                'stack_name': result.stack_name,
                'resource_id': f"{result.logical_resource_id}#{result.physical_resource_id}",
                'resource_type': result.resource_type,
                'drift_status': result.drift_status.value,
                'property_differences': json.dumps(result.property_differences),
                'detection_timestamp': result.timestamp,
                'ttl': int(time.time()) + (7 * 24 * 3600)  # 7 days retention            }
            batch_items.append({'PutRequest': {'Item': item}})
        # Write in batches of 25 (DynamoDB limit)        for i in range(0, len(batch_items), 25):
            batch = batch_items[i:i+25]
            try:
                response = self.dynamodb.batch_write_item(
                    RequestItems={
                        'DriftDetectionResults': batch
                    }
                )
                # Handle unprocessed items                unprocessed = response.get('UnprocessedItems', {})
                retry_count = 0                max_retries = 3                while unprocessed and retry_count < max_retries:
                    await asyncio.sleep(2 ** retry_count)
                    response = self.dynamodb.batch_write_item(
                        RequestItems=unprocessed
                    )
                    unprocessed = response.get('UnprocessedItems', {})
                    retry_count += 1            except Exception as e:
                await self._log_storage_error(stack_name, str(e))
    def _partition_stacks(self, stack_names: List[str], batch_size: int) -> List[List[str]]:
        """        Partition stacks into batches for parallel processing        """        return [stack_names[i:i + batch_size]
                for i in range(0, len(stack_names), batch_size)]
class DriftCache:
    """    Intelligent caching layer for drift detection results    """    def __init__(self):
        self.elasticache = boto3.client('elasticache')
        self.cache_ttl = 3600  # 1 hour default TTL    async def get_drift_status(self, stack_name: str) -> Optional[DriftDetectionResult]:
        """        Get cached drift status if available and not stale        """        # Implementation would use Redis/ElastiCache        pass    async def set_drift_status(self, stack_name: str,
                             result: DriftDetectionResult, ttl: int = None):
        """        Cache drift detection result        """        # Implementation would cache in Redis/ElastiCache        passclass ComplianceMonitor:
    """    Compliance monitoring and alerting system    """    def __init__(self):
        self.sns = boto3.client('sns')
        self.cloudwatch = boto3.client('cloudwatch')
    async def monitor_compliance_violations(self):
        """        Monitor for compliance violations and trigger alerts        """        # Query drift results        violations = await self._query_compliance_violations()
        if violations:
            await self._trigger_compliance_alerts(violations)
            await self._update_compliance_dashboard(violations)
    async def _query_compliance_violations(self) -> List[Dict]:
        """        Query for stacks with compliance violations        """        # Implementation would query DynamoDB for drifted resources        pass    async def _trigger_compliance_alerts(self, violations: List[Dict]):
        """        Send alerts for compliance violations        """        alert_message = {
            'violation_count': len(violations),
            'critical_violations': [v for v in violations if v['severity'] == 'CRITICAL'],
            'timestamp': time.time()
        }
        await self.sns.publish(
            TopicArn='arn:aws:sns:region:account:compliance-alerts',
            Message=json.dumps(alert_message),
            Subject='CloudFormation Drift Compliance Violations'        )
# Performance optimization strategiesclass PerformanceOptimizations:
    """    Performance optimization techniques for large-scale drift detection    """    @staticmethod    def intelligent_scheduling():
        """        Intelligent scheduling strategies to minimize performance impact        """        return {
            'off_peak_scheduling': 'Run during low-traffic hours (2-6 AM)',
            'geographic_distribution': 'Stagger detection across regions',
            'priority_based': 'Critical stacks checked more frequently',
            'resource_type_filtering': 'Skip read-only resource types',
            'incremental_detection': 'Only check recently modified stacks'        }
    @staticmethod    def resource_optimization():
        """        Resource optimization for minimal overhead        """        return {
            'lambda_concurrency': 'Reserved concurrency for drift detection',
            'dynamodb_on_demand': 'Auto-scaling for variable workloads',
            'cloudwatch_dashboards': 'Real-time performance monitoring',
            'cost_optimization': 'Spot instances for batch processing'        }

Performance Characteristics:
- Scale: Handle 1M+ CloudFormation resources
- Latency: <30 seconds for typical stack drift detection
- Throughput: 1000+ concurrent drift detections
- Accuracy: 99.9% drift detection accuracy
- Cost: <$0.01 per resource per detection cycle

AWS-Specific Optimizations:
- Config Rules: Leverage AWS Config for continuous compliance
- CloudWatch Events: Real-time stack change notifications
- Systems Manager: Automated remediation workflows
- Cost Explorer: Track drift detection costs and optimize

Principal-Level Questions (Principal SDE)

7. Scalability Challenges: Amazon Advertising Bid Optimization

Level: Principal SDE - Amazon Advertising

Question: “Design a bid-optimization service for Amazon DSP that processes real-time bidding requests (100K reqs/sec), applies ML models in <10 ms, and writes results to S3 for batch analysis. Include caching strategies, autoscaling, and A/B testing hooks.”

Answer:

Core Implementation:

import asyncio
import time
from typing import Dict, List
import boto3
class BidOptimizationService:
    def __init__(self):
        self.ml_cache = MLModelCache()
        self.feature_cache = FeatureCache()
        self.s3_writer = S3AsyncWriter()
    async def optimize_bid(self, request: Dict) -> Dict:
        """Main bid optimization with <10ms latency"""        start_time = time.time()
        # Parallel execution for speed        tasks = [
            self._extract_features(request),
            self._get_ml_prediction(request),
            self._check_budget_constraints(request)
        ]
        features, prediction, budget_ok = await asyncio.gather(*tasks)
        if not budget_ok:
            return {'bid': 0, 'reason': 'budget_exceeded'}
        # Calculate optimal bid        base_bid = prediction['expected_value'] * 0.8        contextual_multiplier = self._calculate_context_multiplier(features)
        final_bid = base_bid * contextual_multiplier
        # Async logging for batch analysis        asyncio.create_task(self._log_to_s3(request, prediction, final_bid))
        latency_ms = (time.time() - start_time) * 1000        return {
            'bid_amount': round(final_bid, 2),
            'confidence': prediction['confidence'],
            'latency_ms': latency_ms
        }
    async def _extract_features(self, request: Dict) -> Dict:
        """Fast feature extraction with caching"""        cache_key = f"features_{request['user_id']}_{request['context_hash']}"        if cached := await self.feature_cache.get(cache_key):
            return cached
        features = {
            'user_ltv': await self._get_user_lifetime_value(request['user_id']),
            'conversion_prob': await self._get_conversion_probability(request),
            'competition_level': await self._get_competition_metrics(request)
        }
        await self.feature_cache.set(cache_key, features, ttl=300)
        return features
    async def _get_ml_prediction(self, request: Dict) -> Dict:
        """ML model inference with caching"""        model_key = f"model_{request['campaign_type']}"        model = await self.ml_cache.get_model(model_key)
        # Vectorized prediction for speed        input_vector = self._vectorize_request(request)
        prediction = await model.predict_async(input_vector)
        return {
            'expected_value': prediction[0],
            'confidence': prediction[1],
            'model_version': model.version
        }
class MLModelCache:
    """High-performance ML model caching"""    def __init__(self):
        self.models = {}
        self.model_loader = ModelLoader()
    async def get_model(self, model_key: str):
        if model_key not in self.models:
            self.models[model_key] = await self.model_loader.load_optimized_model(model_key)
        return self.models[model_key]
class S3AsyncWriter:
    """Async S3 writer for batch analysis"""    def __init__(self):
        self.s3_client = boto3.client('s3')
        self.batch_buffer = []
        self.batch_size = 1000    async def write_bid_event(self, event_data: Dict):
        self.batch_buffer.append(event_data)
        if len(self.batch_buffer) >= self.batch_size:
            await self._flush_batch()
    async def _flush_batch(self):
        # Write batch to S3 for offline analysis        batch_data = '\n'.join(json.dumps(event) for event in self.batch_buffer)
        key = f"bid-events/{datetime.now().strftime('%Y/%m/%d/%H')}/{uuid.uuid4()}.jsonl"        await self.s3_client.put_object(
            Bucket='amazon-ads-analytics',
            Key=key,
            Body=batch_data.encode('utf-8')
        )
        self.batch_buffer.clear()

A/B Testing Integration:

class ABTestingFramework:
    async def get_bid_strategy(self, request: Dict) -> str:
        """Route requests to different bid strategies for A/B testing"""        user_hash = hash(request['user_id']) % 100        if user_hash < 10:  # 10% in experimental group            return 'ml_v2_aggressive'        elif user_hash < 20:  # 10% in control group            return 'ml_v1_conservative'        else:  # 80% in production            return 'ml_v1_production'

Auto-scaling Configuration:
- Lambda Concurrency: Reserved 10K, provisioned 2K for consistent <10ms latency
- DynamoDB: On-demand with auto-scaling for feature store
- ElastiCache: Redis cluster with read replicas for model caching
- Monitoring: CloudWatch alerts for latency >8ms or error rate >0.1%

Performance Characteristics:
- Throughput: 100K+ bid requests/second
- Latency: P99 <10ms, P50 <3ms
- Cost: ~$0.0001 per bid optimization
- A/B Testing: 20+ concurrent experiments supported

8. Embedded Systems/Security: Kindle Firmware Update Protocol

Level: SDE I - Kindle Firmware Team

Question: “Implement a firmware update protocol that ensures atomicity, rollback on failure, and encryption over low-bandwidth connections. Provide pseudocode and state-machine diagram.”

Answer:

State Machine Implementation:

from enum import Enum
import hashlib
import time
class UpdateState(Enum):
    IDLE = "idle"    DOWNLOADING = "downloading"    VERIFYING = "verifying"    APPLYING = "applying"    REBOOTING = "rebooting"    ROLLBACK = "rollback"    COMPLETE = "complete"    FAILED = "failed"class KindleFirmwareUpdater:
    def __init__(self):
        self.state = UpdateState.IDLE
        self.current_partition = "/dev/mmcblk0p1"  # Active partition        self.backup_partition = "/dev/mmcblk0p2"   # Inactive partition        self.crypto_key = self._load_device_key()
    async def update_firmware(self, manifest: dict) -> bool:
        """Main update flow with rollback protection"""        try:
            # Phase 1: Download and verify            self.state = UpdateState.DOWNLOADING
            if not await self._secure_download(manifest):
                raise Exception("Download failed")
            self.state = UpdateState.VERIFYING
            if not await self._verify_firmware(manifest):
                raise Exception("Verification failed")
            # Phase 2: Atomic application            self.state = UpdateState.APPLYING
            if not await self._atomic_apply():
                raise Exception("Application failed")
            # Phase 3: Test new firmware            self.state = UpdateState.REBOOTING
            await self._switch_boot_partition()
            # Post-reboot verification (this runs after reboot)            if await self._verify_boot_health():
                self.state = UpdateState.COMPLETE
                await self._cleanup_old_firmware()
                return True            else:
                await self._rollback()
                return False        except Exception as e:
            await self._rollback()
            self.state = UpdateState.FAILED
            return False    async def _secure_download(self, manifest: dict) -> bool:
        """Download with encryption and bandwidth optimization"""        total_size = manifest['firmware_size']
        chunk_size = 8192  # 8KB chunks for slow connections        downloaded = 0        with open('/tmp/firmware.enc', 'wb') as fw_file:
            for chunk_idx in range(0, total_size, chunk_size):
                # Download chunk with retry logic                for attempt in range(3):
                    try:
                        chunk_url = f"{manifest['base_url']}/chunk_{chunk_idx}"                        encrypted_chunk = await self._download_chunk(chunk_url)
                        fw_file.write(encrypted_chunk)
                        downloaded += len(encrypted_chunk)
                        # Progress reporting for slow connections                        self._report_progress(downloaded, total_size)
                        break                    except Exception as e:
                        if attempt == 2:  # Last attempt                            return False                        await asyncio.sleep(2 ** attempt)  # Exponential backoff        return True    async def _verify_firmware(self, manifest: dict) -> bool:
        """Cryptographic verification"""        # Decrypt firmware        decrypted_fw = self._decrypt_aes256('/tmp/firmware.enc', self.crypto_key)
        # Verify digital signature        if not self._verify_rsa_signature(decrypted_fw, manifest['signature']):
            return False        # Verify integrity        calculated_hash = hashlib.sha256(decrypted_fw).hexdigest()
        if calculated_hash != manifest['sha256_hash']:
            return False        # Save decrypted firmware        with open('/tmp/firmware.bin', 'wb') as f:
            f.write(decrypted_fw)
        return True    async def _atomic_apply(self) -> bool:
        """Atomic firmware application using A/B partitions"""        inactive_partition = self._get_inactive_partition()
        try:
            # Write to inactive partition first (no interruption to current system)            with open('/tmp/firmware.bin', 'rb') as fw:
                with open(inactive_partition, 'wb') as partition:
                    partition.write(fw.read())
            # Verify write was successful            if not self._verify_partition_integrity(inactive_partition):
                return False            # Update bootloader configuration atomically            bootloader_config = {
                'primary_partition': inactive_partition,
                'fallback_partition': self.current_partition,
                'boot_attempts': 0,
                'max_boot_attempts': 3            }
            self._update_bootloader(bootloader_config)
            return True        except Exception as e:
            return False    async def _verify_boot_health(self) -> bool:
        """Verify system health after firmware update"""        health_checks = [
            self._check_kernel_boot(),
            self._check_essential_services(),
            self._check_hardware_compatibility(),
            self._check_network_connectivity()
        ]
        # All health checks must pass        results = await asyncio.gather(*health_checks, return_exceptions=True)
        return all(result is True for result in results)
    async def _rollback(self) -> bool:
        """Automatic rollback to previous firmware"""        self.state = UpdateState.ROLLBACK
        # Switch back to previous partition        rollback_config = {
            'primary_partition': self.current_partition,
            'fallback_partition': None,
            'boot_attempts': 0        }
        self._update_bootloader(rollback_config)
        # Clean up failed update files        self._cleanup_temp_files()
        # Schedule reboot to old firmware        await self._schedule_reboot(delay_seconds=5)
        return True# Low-level hardware interfaceclass HardwareInterface:
    @staticmethod    def _update_bootloader(config: dict):
        """Update bootloader configuration atomically"""        # Write config to NVRAM/EEPROM        nvram_write(BOOTLOADER_CONFIG_ADDR, serialize(config))
    @staticmethod    def _get_inactive_partition() -> str:
        """Determine which partition is currently inactive"""        current = get_current_boot_partition()
        return "/dev/mmcblk0p2" if current == "/dev/mmcblk0p1" else "/dev/mmcblk0p1"    @staticmethod    def _verify_partition_integrity(partition: str) -> bool:
        """Verify partition can be read successfully"""        try:
            with open(partition, 'rb') as p:
                # Read first and last blocks to verify accessibility                p.read(4096)  # First block                p.seek(-4096, 2)  # Last block                p.read(4096)
            return True        except:
            return False# Encryption utilitiesclass CryptoUtils:
    @staticmethod    def _decrypt_aes256(encrypted_file: str, key: bytes) -> bytes:
        """AES-256 decryption optimized for embedded systems"""        from cryptography.hazmat.primitives.ciphers import Cipher, algorithms, modes
        with open(encrypted_file, 'rb') as f:
            iv = f.read(16)  # First 16 bytes are IV            encrypted_data = f.read()
        cipher = Cipher(algorithms.AES(key), modes.CBC(iv))
        decryptor = cipher.decryptor()
        decrypted = decryptor.update(encrypted_data) + decryptor.finalize()
        # Remove PKCS7 padding        padding_length = decrypted[-1]
        return decrypted[:-padding_length]
    @staticmethod    def _verify_rsa_signature(data: bytes, signature: str) -> bool:
        """RSA-2048 signature verification"""        from cryptography.hazmat.primitives import hashes, serialization
        from cryptography.hazmat.primitives.asymmetric import rsa, padding
        # Load public key (embedded in device)        public_key = load_embedded_public_key()
        try:
            public_key.verify(
                bytes.fromhex(signature),
                data,
                padding.PSS(
                    mgf=padding.MGF1(hashes.SHA256()),
                    salt_length=padding.PSS.MAX_LENGTH
                ),
                hashes.SHA256()
            )
            return True        except:
            return False

State Machine Diagram:

IDLE ──download──▶ DOWNLOADING ──verify──▶ VERIFYING ──apply──▶ APPLYING
 ▲                     │                      │                    │
 │                     ▼                      ▼                    ▼
 │                 FAILED ◀────────────────FAILED ◀──────────── FAILED
 │                     ▲                                           │
 │                     │                                           ▼
 └──rollback───── ROLLBACK ◀────────── REBOOTING ──success──▶ COMPLETE
                        ▲                    │
                        └─────boot_fail──────┘

Key Security Features:
- Encryption: AES-256-CBC for firmware delivery
- Authentication: RSA-2048 digital signatures
- Atomic Updates: A/B partition switching prevents brick scenarios
- Rollback: Automatic fallback on boot failure (3 attempts max)
- Bandwidth Optimization: 8KB chunks with resumable downloads
- Integrity: SHA-256 checksums and partition verification

9. Data Structures & Algorithms: Amazon Search Autocomplete

Level: SDE II - Amazon Search

Question: “Given a corpus of 1B search queries, design an autocomplete feature that returns top-k suggestions in <50 ms. Implement a trie with frequency weights and show how to update counts in real-time.”

Answer:

Core Trie Implementation:

import heapq
import threading
import time
from typing import List, Dict, Optional
class TrieNode:
    def __init__(self):
        self.children: Dict[str, 'TrieNode'] = {}
        self.is_end_word = False        self.frequency = 0        self.word = ""        # Performance optimization: cache top suggestions at each node        self.top_suggestions: List[tuple] = []  # [(frequency, word), ...]        self.cache_valid = Falseclass OptimizedAutocomplete:
    def __init__(self, max_suggestions=10):
        self.root = TrieNode()
        self.max_suggestions = max_suggestions
        self.update_lock = threading.RLock()
        self.suggestion_cache = {}  # LRU cache for frequent prefixes        self.cache_size = 50000    def insert(self, query: str, frequency: int = 1):
        """Insert query with frequency weight"""        with self.update_lock:
            current = self.root
            query_lower = query.lower().strip()
            # Traverse and create path            for char in query_lower:
                if char not in current.children:
                    current.children[char] = TrieNode()
                current = current.children[char]
                current.frequency += frequency  # Aggregate frequency at each node                current.cache_valid = False  # Invalidate cache            # Mark end of word            current.is_end_word = True            current.word = query
            current.frequency += frequency
            # Invalidate affected cache entries            self._invalidate_cache_prefix(query_lower)
    def get_suggestions(self, prefix: str, max_results: int = None) -> List[Dict]:
        """Get top-k suggestions with <50ms target latency"""        start_time = time.time()
        if max_results is None:
            max_results = self.max_suggestions
        prefix_lower = prefix.lower().strip()
        # Fast path: check cache        cache_key = f"{prefix_lower}:{max_results}"        if cache_key in self.suggestion_cache:
            cached = self.suggestion_cache[cache_key]
            if time.time() - cached['timestamp'] < 300:  # 5min cache                return cached['suggestions']
        # Find prefix node        current = self.root
        for char in prefix_lower:
            if char not in current.children:
                return []
            current = current.children[char]
        # Get suggestions efficiently        suggestions = self._get_top_suggestions_fast(current, prefix, max_results)
        # Cache result        result = {
            'suggestions': suggestions,
            'timestamp': time.time()
        }
        if len(self.suggestion_cache) < self.cache_size:
            self.suggestion_cache[cache_key] = result
        latency_ms = (time.time() - start_time) * 1000        return suggestions
    def _get_top_suggestions_fast(self, node: TrieNode, prefix: str, max_results: int) -> List[Dict]:
        """Fast suggestion retrieval using cached results when possible"""        # Check if we have valid cached suggestions at this node        if node.cache_valid and len(node.top_suggestions) >= max_results:
            top_cached = node.top_suggestions[:max_results]
            return [
                {
                    'query': word,
                    'frequency': freq,
                    'relevance_score': self._calculate_relevance(word, prefix, freq)
                }
                for freq, word in top_cached
            ]
        # Rebuild suggestions using optimized DFS        suggestions_heap = []
        def dfs(current: TrieNode, current_word: str, depth: int):
            # Limit search depth for performance            if depth > 50 or len(suggestions_heap) > max_results * 10:
                return            if current.is_end_word:
                # Use negative frequency for max-heap behavior                heapq.heappush(suggestions_heap, (-current.frequency, current_word))
            # DFS through children, prioritize high-frequency branches            children_by_freq = sorted(
                current.children.items(),
                key=lambda x: x[1].frequency,
                reverse=True            )
            for char, child in children_by_freq[:10]:  # Limit branching factor                dfs(child, current_word + char, depth + 1)
        dfs(node, prefix, 0)
        # Extract top results        top_suggestions = heapq.nlargest(max_results, suggestions_heap)
        # Cache suggestions at this node for future queries        node.top_suggestions = top_suggestions
        node.cache_valid = True        # Format response        return [
            {
                'query': word,
                'frequency': -freq,
                'relevance_score': self._calculate_relevance(word, prefix, -freq)
            }
            for freq, word in top_suggestions
        ]
    def update_frequency(self, query: str, delta: int):
        """Real-time frequency updates"""        with self.update_lock:
            current = self.root
            query_lower = query.lower().strip()
            # Update frequency along the path            path_nodes = [current]
            for char in query_lower:
                if char in current.children:
                    current = current.children[char]
                    path_nodes.append(current)
                else:
                    return  # Query doesn't exist            # Update frequencies and invalidate caches            for node in path_nodes:
                node.frequency += delta
                node.cache_valid = False            # Invalidate related cache entries            self._invalidate_cache_prefix(query_lower)
    def batch_update_frequencies(self, updates: List[tuple]):
        """Batch frequency updates for efficiency"""        with self.update_lock:
            for query, delta in updates:
                self.update_frequency(query, delta)
    def _calculate_relevance(self, suggestion: str, prefix: str, frequency: int) -> float:
        """Calculate relevance score for ranking"""        # Normalize frequency (log scale for better distribution)        import math
        freq_score = math.log(frequency + 1) / 20.0  # Normalize to [0,1] range        # Exact prefix match bonus        prefix_match_bonus = 0.3 if suggestion.lower().startswith(prefix.lower()) else 0.0        # Length penalty for very long suggestions        length_penalty = max(0, (len(suggestion) - 50) * 0.01)
        # Recent popularity boost (would integrate with real-time analytics)        recency_boost = 0.1  # Placeholder for trending queries        score = freq_score + prefix_match_bonus + recency_boost - length_penalty
        return round(max(0, min(1, score)), 3)
    def _invalidate_cache_prefix(self, prefix: str):
        """Invalidate cache entries that start with given prefix"""        keys_to_remove = [
            key for key in self.suggestion_cache.keys()
            if key.startswith(prefix.split(':')[0])
        ]
        for key in keys_to_remove:
            del self.suggestion_cache[key]
# Real-time update processorclass RealTimeUpdateProcessor:
    def __init__(self, autocomplete_engine: OptimizedAutocomplete):
        self.engine = autocomplete_engine
        self.update_queue = []
        self.batch_size = 1000    async def process_search_event(self, query: str, user_context: dict):
        """Process real-time search events"""        # Add to update queue        self.update_queue.append((query, 1))
        # Process batch when threshold reached        if len(self.update_queue) >= self.batch_size:
            await self._flush_updates()
    async def _flush_updates(self):
        """Flush batched updates efficiently"""        if not self.update_queue:
            return        # Group by query for efficiency        query_deltas = {}
        for query, delta in self.update_queue:
            query_deltas[query] = query_deltas.get(query, 0) + delta
        # Apply batch updates        batch_updates = list(query_deltas.items())
        self.engine.batch_update_frequencies(batch_updates)
        # Clear queue        self.update_queue.clear()
# Performance testingdef performance_test():
    """Test autocomplete performance with realistic data"""    engine = OptimizedAutocomplete()
    # Simulate popular Amazon search queries    popular_queries = [
        ("iphone 14", 50000000),
        ("iphone 14 case", 25000000),
        ("iphone 14 pro", 30000000),
        ("iphone charger", 40000000),
        ("amazon echo", 20000000),
        ("amazon prime video", 15000000),
        ("laptop", 35000000),
        ("laptop stand", 10000000),
    ]
    # Build trie    for query, freq in popular_queries:
        engine.insert(query, freq)
    # Test autocomplete performance    test_prefixes = ["iph", "lap", "ama", "i"]
    for prefix in test_prefixes:
        start_time = time.time()
        suggestions = engine.get_suggestions(prefix, 10)
        latency_ms = (time.time() - start_time) * 1000        print(f"Prefix '{prefix}': {latency_ms:.2f}ms, {len(suggestions)} suggestions")
        for suggestion in suggestions[:3]:
            print(f"  - {suggestion['query']} (freq: {suggestion['frequency']}, score: {suggestion['relevance_score']})")
if __name__ == "__main__":
    performance_test()

Optimization Strategies:
- Node-level Caching: Top suggestions cached at frequently accessed nodes
- Limited DFS: Depth and branching factor limits prevent slow queries
- Batch Updates: Real-time frequency updates processed in batches
- LRU Cache: Recent autocomplete results cached for fast retrieval
- Relevance Scoring: Multi-factor scoring for better result quality

Performance Characteristics:
- Latency: <50ms P95 for autocomplete queries
- Throughput: 10K+ autocomplete requests/second
- Memory: ~10GB for 1B queries with optimized node structure
- Updates: Real-time frequency updates with <100ms propagation
- Cache Hit Rate: >80% for frequently requested prefixes

10. Amazon Leadership Principles: Ownership & Diving Deep (IAM Security)

Level: SDE III - AWS IAM Security

Question: “Describe a time you took ownership of a security incident in production, dove deep into logs and metrics, and implemented a fix that prevented future recurrence. What metrics did you track?”

Answer:

Situation:
In Q3 2023, I was working as a Senior SDE on the AWS IAM team when our security monitoring detected an unusual spike in failed authentication attempts across multiple AWS regions. Within 30 minutes, I took ownership of the incident as the on-call security lead.

Task:
My responsibility was to investigate potential security threats, determine if customer accounts were compromised, implement immediate mitigations, and prevent future incidents of this scale.

Action - Taking Ownership & Diving Deep:

Phase 1: Immediate Analysis

# CloudWatch Logs Insights query to investigate attack patternsfields @timestamp, sourceIPAddress, userIdentity.type, errorCode
| filter errorCode="SigninFailure"
| stats count() by sourceIPAddress, userIdentity.userName
| sort count desc
| limit 100

Key Findings:
- 2.3M failed login attempts from 1,847 IP addresses in 6 hours
- 127 unique customer accounts targeted with credential stuffing attacks
- 596 accounts successfully compromised (0.23% success rate)

Phase 2: Emergency Mitigation

# Rate limiting implementation I deployedrate_limit_policy = {
    'auth_attempts_per_ip': 10,  # per 5-minute window    'auth_attempts_per_account': 20,  # per 5-minute window    'geo_velocity_check': True,  # Block impossible geo transitions    'automated_blocking': True  # Auto-block known bad IPs}

Phase 3: Long-term Solution

class EnhancedAuthSystem:
    def __init__(self):
        self.risk_engine = RiskScoringEngine()
        self.threat_intel = ThreatIntelligenceAPI()
    def evaluate_auth_request(self, request):
        risk_score = self.calculate_risk_score(request)
        if risk_score > 0.8:
            return "BLOCK"        elif risk_score > 0.6:
            return "REQUIRE_MFA"        else:
            return "ALLOW"

Result - Measurable Impact:

Immediate Results:
- Attack volume reduced by 99.5% (2.3M to 12K attempts/day)
- Zero successful credential stuffing attacks post-mitigation
- <0.1% false positive rate for legitimate users

Long-term Results:
- MTTR reduced from 8 hours to 45 minutes
- 99.7% attack detection rate within 2 minutes
- Security NPS improved from 7.2 to 8.9

Key Metrics Tracked:

ongoing_metrics = {
    'authentication_success_rate': '>99.5%',
    'mean_time_to_detect_attack': '<5 minutes',
    'false_positive_rate': '<0.1%',
    'attack_attribution_accuracy': '>95%',
    'customer_security_posture': 'MFA adoption rates',
    'incident_prevention_rate': 'Attacks stopped before impact'}

Leadership Principles Demonstrated:
- Ownership: Took complete responsibility for resolution and customer impact
- Diving Deep: Analyzed 47GB of logs and built custom threat analysis tools
- Bias for Action: Implemented fixes within 8 hours while designing long-term solutions
- Insist on Highest Standards: Created security benchmarks exceeding industry standards
- Innovation: 3 security enhancements shipped within 6 months

This comprehensive Amazon Software Development Engineer question bank demonstrates the technical depth, system design capabilities, and leadership principles required for Amazon engineering roles across all levels from SDE I through Principal SDE.