Serverless Patterns

Production patterns for AWS Lambda and serverless architectures — cold starts, event routing, orchestration, and observability.

When to Activate

• Diagnosing Lambda cold start latency
• Designing event-driven workflows (S3 → Lambda, SQS batch processing)
• Building Step Functions state machines
• Adding structured logging/tracing with Lambda Powertools
• Calculating serverless cost vs. container cost
• Implementing idempotency for at-least-once delivery
• Choosing between Provisioned Concurrency, SnapStart, or keep-warm pings for a latency-sensitive function
• Debugging partial batch failures in SQS or DynamoDB Streams event source mappings

---

Cold Start Deep Dive

What Causes Cold Starts

Cold Start = Container Boot + Runtime Init + Function Init
             ~100-500ms    + ~50-200ms    + your code

Warm Start = (container already running)
             ~1-10ms (just your function code)

Cold Start by Runtime (approximate):

Runtime	Cold Start
Rust (custom runtime)	~1ms
Go	~5ms
Node.js 20	~80–150ms
Python 3.12	~100–300ms
Java 21 (without SnapStart)	~1000–3000ms
Java 21 (with SnapStart)	~100–200ms

Mitigation Strategies

1. Provisioned Concurrency (eliminates cold starts)

# serverless.yml / SAM template
ProvisionedConcurrencyConfig:
  ProvisionedConcurrentExecutions: 5  # Always warm instances

# Schedule scaling (cost optimization)
# Scale up before peak, down after
aws application-autoscaling register-scalable-target \
  --service-namespace lambda \
  --resource-id function:my-function:prod \
  --scalable-dimension lambda:function:ProvisionedConcurrency

aws application-autoscaling put-scheduled-action \
  --service-namespace lambda \
  --resource-id function:my-function:prod \
  --scalable-dimension lambda:function:ProvisionedConcurrency \
  --scheduled-action-name scale-up-morning \
  --schedule "cron(0 7 * * ? *)" \
  --scalable-target-action MinCapacity=10,MaxCapacity=10

2. SnapStart (Java only — Firecracker MicroVM snapshot)

# SAM template
MyFunction:
  Type: AWS::Serverless::Function
  Properties:
    SnapStart:
      ApplyOn: PublishedVersions
    AutoPublishAlias: prod

3. Keep-Warm Pings (free mitigation)

// handler — detect and discard warm pings
export const handler = async (event: any) => {
  if (event.source === 'keep-warm') {
    return { statusCode: 200 };
  }
  return handleRealRequest(event);
};

# EventBridge rule — ping every 5 minutes
KeepWarmRule:
  Type: AWS::Events::Rule
  Properties:
    ScheduleExpression: rate(5 minutes)
    Targets:
      - Id: MyLambdaTarget
        Arn: !GetAtt MyFunction.Arn
        Input: '{"source": "keep-warm"}'

4. Minimize Package Size

# Check your Lambda deployment size
aws lambda get-function --function-name my-function \
  --query 'Configuration.CodeSize'

# Reduce: only bundle what you need
# Node.js: esbuild with tree-shaking
esbuild src/handler.ts --bundle --minify --target=node20 \
  --platform=node --outfile=dist/handler.js

# Python: pip install --no-deps, use Lambda Layers for shared deps

5. Move Initialization Outside Handler

// WRONG: DB connection created on every invocation
export const handler = async (event) => {
  const db = new Database(process.env.DB_URL);  // ❌ Cold start every time
  return db.query('SELECT ...');
};

// CORRECT: Initialize once, reuse across warm invocations
const db = new Database(process.env.DB_URL);  // ✅ Runs once per container

export const handler = async (event) => {
  return db.query('SELECT ...');
};

---

Event Source Mapping

S3 → Lambda

// S3 event structure
interface S3Event {
  Records: Array<{
    s3: {
      bucket: { name: string };
      object: { key: string; size: number; eTag: string };
    };
    eventName: string;  // 'ObjectCreated:Put', 'ObjectRemoved:Delete'
  }>;
}

export const handler = async (event: S3Event) => {
  for (const record of event.Records) {
    const { bucket, object } = record.s3;
    const key = decodeURIComponent(object.key.replace(/\+/g, ' '));

    const file = await s3.getObject({
      Bucket: bucket.name,
      Key: key,
    }).promise();

    await processFile(file.Body as Buffer, key);
  }
};

SQS → Lambda (Batch Processing)

import { SQSEvent, SQSBatchResponse } from 'aws-lambda';

export const handler = async (event: SQSEvent): Promise<SQSBatchResponse> => {
  const failures: SQSBatchResponse['batchItemFailures'] = [];

  await Promise.all(
    event.Records.map(async (record) => {
      try {
        const message = JSON.parse(record.body);
        await processMessage(message);
      } catch (error) {
        // Partial batch failure — only failed items go back to queue
        failures.push({ itemIdentifier: record.messageId });
      }
    })
  );

  return { batchItemFailures: failures };
};

# SAM/CDK — SQS trigger configuration
MyFunction:
  Events:
    SQSQueue:
      Type: SQS
      Properties:
        Queue: !GetAtt MyQueue.Arn
        BatchSize: 10
        FunctionResponseTypes:
          - ReportBatchItemFailures  # Required for partial failure handling
        MaximumBatchingWindowInSeconds: 5

DynamoDB Streams → Lambda (CDC)

import { DynamoDBStreamEvent, DynamoDBRecord } from 'aws-lambda';
import { unmarshall } from '@aws-sdk/util-dynamodb';

export const handler = async (event: DynamoDBStreamEvent) => {
  for (const record of event.Records) {
    const eventType = record.eventName;  // 'INSERT' | 'MODIFY' | 'REMOVE'

    if (eventType === 'INSERT' || eventType === 'MODIFY') {
      const newItem = unmarshall(record.dynamodb?.NewImage ?? {});
      await syncToSearchIndex(newItem);
    }

    if (eventType === 'REMOVE') {
      const oldItem = unmarshall(record.dynamodb?.OldImage ?? {});
      await removeFromSearchIndex(oldItem.id);
    }
  }
};

EventBridge → Lambda

// EventBridge event
interface OrderPlacedEvent {
  source: string;         // 'myapp.orders'
  'detail-type': string;  // 'Order Placed'
  detail: {
    orderId: string;
    customerId: string;
    amount: number;
  };
}

export const handler = async (event: EventBridge.EventBridgeEvent<'Order Placed', OrderDetail>) => {
  const { orderId, customerId, amount } = event.detail;
  await processOrder({ orderId, customerId, amount });
};

# EventBridge rule — content-based filtering
OrderRule:
  Type: AWS::Events::Rule
  Properties:
    EventPattern:
      source: ['myapp.orders']
      detail-type: ['Order Placed']
      detail:
        amount: [{ numeric: ['>', 100] }]  # Only large orders
    Targets:
      - Id: ProcessLargeOrder
        Arn: !GetAtt ProcessOrderFunction.Arn

---

AWS Step Functions

State Machine Types

Type	Duration	Pricing	Use Case
Standard	Up to 1 year	Per state transition	Long-running workflows, auditing
Express (Async)	Up to 5 minutes	Per invocation + duration	High-volume events
Express (Sync)	Up to 5 minutes	Per invocation + duration	Request-response

State Types

{
  "Comment": "Order processing workflow",
  "StartAt": "ValidateOrder",
  "States": {
    "ValidateOrder": {
      "Type": "Task",
      "Resource": "arn:aws:lambda:...:function:ValidateOrder",
      "Next": "ChargePayment",
      "Catch": [{
        "ErrorEquals": ["ValidationError"],
        "Next": "HandleValidationError"
      }],
      "Retry": [{
        "ErrorEquals": ["States.TaskFailed"],
        "IntervalSeconds": 2,
        "MaxAttempts": 3,
        "BackoffRate": 2.0
      }]
    },

    "ChargePayment": {
      "Type": "Task",
      "Resource": "arn:aws:states:::lambda:invoke.waitForTaskToken",
      "Parameters": {
        "FunctionName": "ChargePayment",
        "Payload": {
          "orderId.$": "$.orderId",
          "taskToken.$": "$$.Task.Token"
        }
      },
      "HeartbeatSeconds": 300,
      "Next": "FulfillOrder"
    },

    "FulfillOrder": {
      "Type": "Parallel",
      "Branches": [
        {
          "StartAt": "ShipItems",
          "States": {
            "ShipItems": { "Type": "Task", "Resource": "...", "End": true }
          }
        },
        {
          "StartAt": "SendConfirmation",
          "States": {
            "SendConfirmation": { "Type": "Task", "Resource": "...", "End": true }
          }
        }
      ],
      "Next": "UpdateInventory"
    },

    "WaitForRestock": {
      "Type": "Wait",
      "Seconds": 86400,
      "Next": "CheckInventory"
    },

    "OrderComplete": {
      "Type": "Succeed"
    },

    "HandleValidationError": {
      "Type": "Fail",
      "Error": "ValidationFailed",
      "Cause": "Order validation failed"
    }
  }
}

Map State (Fan-Out)

{
  "ProcessItems": {
    "Type": "Map",
    "ItemsPath": "$.items",
    "MaxConcurrency": 10,
    "Iterator": {
      "StartAt": "ProcessItem",
      "States": {
        "ProcessItem": {
          "Type": "Task",
          "Resource": "arn:aws:lambda:...:function:ProcessItem",
          "End": true
        }
      }
    },
    "Next": "Aggregate"
  }
}

---

Lambda Powertools

Unified library for structured logging, metrics, and tracing:

npm install @aws-lambda-powertools/logger @aws-lambda-powertools/metrics @aws-lambda-powertools/tracer

Logger

import { Logger } from '@aws-lambda-powertools/logger';

const logger = new Logger({
  serviceName: 'order-service',
  logLevel: 'INFO',
});

export const handler = async (event: any) => {
  // Automatically adds requestId, function name, cold start flag
  logger.info('Processing order', { orderId: event.orderId });

  try {
    const result = await processOrder(event.orderId);
    logger.info('Order processed', { orderId: event.orderId, result });
    return result;
  } catch (error) {
    logger.error('Order processing failed', { error, orderId: event.orderId });
    throw error;
  }
};

Metrics

import { Metrics, MetricUnit } from '@aws-lambda-powertools/metrics';

const metrics = new Metrics({ namespace: 'OrderService', serviceName: 'order-processor' });

export const handler = async (event: any) => {
  metrics.addMetric('OrdersReceived', MetricUnit.Count, 1);

  const start = Date.now();
  await processOrder(event);
  metrics.addMetric('ProcessingTime', MetricUnit.Milliseconds, Date.now() - start);

  metrics.addMetric('OrdersSucceeded', MetricUnit.Count, 1);
  metrics.publishStoredMetrics();  // Flush to CloudWatch
};

Tracer (X-Ray)

import { Tracer } from '@aws-lambda-powertools/tracer';

const tracer = new Tracer({ serviceName: 'order-service' });

export const handler = async (event: any) => {
  const segment = tracer.getSegment();
  const subsegment = segment?.addNewSubsegment('processOrder');

  try {
    const result = await processOrder(event);
    tracer.putAnnotation('orderId', event.orderId);
    tracer.putMetadata('result', result);
    return result;
  } catch (error) {
    subsegment?.addError(error as Error);
    throw error;
  } finally {
    subsegment?.close();
  }
};

---

For idempotency (Lambda Powertools idempotency, DynamoDB persistence), cost model (Lambda vs. Container break-even), and observability (CloudWatch Insights queries), see skill serverless-patterns-advanced.

Reference

• edge-patterns — Cloudflare Workers, Vercel Edge Middleware (CPU-constrained, no cold starts)
• observability — General observability patterns (OpenTelemetry, Grafana)
• chaos-engineering — Testing Lambda failure modes (throttling, DLQ exhaustion)