aws-serverless

name	aws-serverless
type	reference
description	Provides AWS serverless architecture patterns for Lambda, API Gateway, DynamoDB, SQS, and SAM/CDK. Use when working with AWS serverless files (serverless.yml, CDK stacks) or when the user mentions Lambda, API Gateway, serverless, or AWS SAM.
paths	["**/serverless.yml","**/template.yaml","**/cdk/**","**/sam/**"]
effort	3
allowed-tools	Read, Glob, Grep
user-invocable	true
when_to_use	When building or deploying serverless applications on AWS with Lambda, API Gateway, DynamoDB, or SAM/CDK

AWS Serverless

Critical rules (non-obvious)

• Initialize clients OUTSIDE handler — Lambda reuses execution environments across invocations; creating clients inside costs 100-500ms per cold start
• context.callbackWaitsForEmptyEventLoop = false — prevents Node.js from hanging on open async handles (DB connections, etc.)
• SQS VisibilityTimeout = 6× Lambda timeout — if Lambda takes 30s, set 180s; otherwise messages return to queue mid-processing
• FunctionResponseTypes: [ReportBatchItemFailures] — partial batch failure; without this, any single failure retries the entire batch
• Never use * in Access-Control-Allow-Origin with credentials: true — browsers block it; use explicit origin

Lambda handler pattern

// Initialize once (reused across invocations = faster after cold start)
const { DynamoDBClient } = require("@aws-sdk/client-dynamodb");
const { DynamoDBDocumentClient, GetCommand } = require("@aws-sdk/lib-dynamodb");

const docClient = DynamoDBDocumentClient.from(new DynamoDBClient({}));

exports.handler = async (event, context) => {
  context.callbackWaitsForEmptyEventLoop = false;  // don't hang on open handles
  try {
    const body = typeof event.body === "string" ? JSON.parse(event.body) : event.;
     result =  docClient.( ({
      : process..,
      : { : body. },
    }));
     { : , : { :  }, : .(result.) };
  }  (err) {
    .(.({ : err., : context. }));
     { : err. ?? , : .({ : err. }) };
  }
};