aws-serverless-patternslisted

# AWS Serverless Patterns > **Reuse-first** (per `~/.claude/rules-library/common/reuse-first.md`): > One source of truth per Lambda layer concern — one auth > middleware, one error responder, one body parser, one > idempotency helper, one DDB client factory, one outbox > publisher. Lambda layers (or shared `/lib/` for monorepo > deploys) are the canonical home. Each new handler imports the > shared primitives; never re-implement them per handler. Lambda + API Gateway + the surrounding event-driven AWS surface. The patterns here matter because the failure modes (cold starts, lost messages, double-processing, runaway concurrency) show up at customer scale, not in dev. ## When to Activate - Authoring or reviewing a Lambda handler - Designing a webhook ingest path (Stripe, Slack, ClickUp, Twilio, GitHub) - Adding SQS / SNS / EventBridge wiring - Diagnosing a cold-start tail or throttled-concurrency incident - Building a Step Functions workflow - Configuring `serverless.yml`, SAM `template.yaml`, or CDK stacks ## Cold Start: Minimize, Don't Eliminate Cold-start latency comes from two places: 1. **Container provisioning** — AWS-controlled; ~100-300 ms for Node 20 / 256 MB 2. **Module import** — your code; can balloon to 1-3 s with heavy SDK imports Fixes that work: - **Tree-shake heavy SDKs** — `import { DynamoDBClient } from "@aws-sdk/client-dynamodb"` only pulls the DDB client, not the whole `aws-sdk` v2 monolith. - **Top-of-file imports for the hot path** — keep `await