Serverless Highlights Summary

Every highlight bullet from the 7 pages in this section, gathered on one page and grouped by the page it came from.

The Serverless Execution Model

• Serverless doesn't mean no servers - it means the provider manages allocation, scaling, and lifecycle for you
• A function only runs in response to an event, inside a provider-managed execution environment
• Cold starts happen when a new execution environment must be created; warm invocations reuse one that already exists
• State outside a single invocation does not persist - handlers must treat every backing resource as external

Serverless Basics

• Lambda fits spiky, short-lived HTTP and event-driven workloads
• Always-on containers win for WebSockets, long jobs, and steady baseline traffic
• Node 24 on nodejs24.x runtime with ESM and top-level await
• Keep handlers stateless; persist to DB, S3, or ElastiCache
• Cold starts scale with bundle size and VPC attachment
• Use @aws-sdk v3 modular clients to trim zip size

Lambda Handler Patterns

• Set callbackWaitsForEmptyEventLoop false to end handler on return, not on empty event loop
• Put expensive setup (DB clients, config) in module scope to reuse across invocations
• Use getRemainingTimeInMillis to guard long loops before Lambda timeout
• Module-scope DB pools or RDS Proxy prevent connection exhaustion across invocations
• Prefer async event handlers over legacy callback-style for new TypeScript code
• Type event sources with imports from aws-lambda for HTTP API, SQS, S3, and others

Cold Start Mitigation

• Import specific functions like lodash/pick.js to minimize bundle size
• Keep lambda zip under 5 MB to reduce download and init latency
• Defer heavy imports with dynamic import() to skip parse cost until needed
• Provisioned concurrency costs 24/7, use only for latency-critical functions
• Use RDS Proxy or Data API instead of VPC connections to avoid ENI delays
• Cache secrets after first fetch, use env for non-rotating configuration

@aws-sdk v3

• Import only AWS SDK v3 services you need to reduce bundle size vs monolithic v2
• Reuse SDK clients across Lambda warm invocations to avoid reinitializing on each request
• Command classes are immutable and safe to construct fresh for each request
• Mark @aws-sdk packages external in esbuild since Node.js 18+ Lambda includes v3
• Use middleware stack for cross-cutting concerns like logging, not business logic
• Lambda execution role automatically supplies credentials via the credential chain

serverless-express / aws-lambda-fastify

• Wrap existing Express or Fastify apps for Lambda without rewriting routes
• Fastify cold start lighter than Express but heavier than raw handlers
• Avoid for WebSockets, file uploads over 6 MB, or p99 latency under 50ms
• Split functions or lazy-load routers to avoid bloating cold start
• Await Fastify app.ready() before wrapping or face 404 errors
• Disable callbackWaitsForEmptyEventLoop or use RDS Proxy with DB pools

Serverless Best Practices

• Handlers stay stateless; persist to DynamoDB, Postgres, or S3
• SDK clients and env parsing in module scope, not per request
• esbuild bundles with @aws-sdk/* external on managed runtimes
• Idempotency keys for SQS and EventBridge at-least-once delivery
• Set callbackWaitsForEmptyEventLoop = false with connection pools
• Provisioned concurrency only for latency-critical aliases