Lambda Handler Patterns

import type { APIGatewayProxyHandlerV2, Context } from "aws-lambda";
 
// Init phase (cold start) - runs once per execution environment
const tableName = process.env.TABLE_NAME!;
 
export const handler: APIGatewayProxyHandlerV2 = async (event, context) => {
  context.callbackWaitsForEmptyEventLoop = false;
 
  const id = event.pathParameters?.id;
  if (!id) {
    return { statusCode: 400, body: JSON.stringify({ error: "id required" }) };
  }
 
  const item = await getItem(tableName, id);
  return {
    statusCode: item ? 200 : 404,
    body: JSON.stringify(item ?? { error: "not found" }),
  };
};
 
async function getItem(table: string, id: string) {
  return { id, table };
}

// src/handlers/orders.ts
import type {
  APIGatewayProxyEventV2,
  APIGatewayProxyResultV2,
  Context,
  SQSBatchResponse,
  SQSEvent,
} from "aws-lambda";
import { processOrder } from "../services/orders.js";
 
// ---- HTTP API handler ----
export async function httpHandler(
  event: APIGatewayProxyEventV2,
  context: Context
): Promise<APIGatewayProxyResultV2> {
  context.callbackWaitsForEmptyEventLoop = false;
 
  if (event.requestContext.http.method === "POST" && event.rawPath === "/orders") {
    const body = event.body ? JSON.parse(event.body) : {};
    const order = await processOrder(body);
    return { statusCode: 201, body: JSON.stringify(order) };
  }
 
  return { statusCode: 404, body: JSON.stringify({ error: "not found" }) };
}
 
// ---- SQS batch with partial failures ----
export async function sqsHandler(event: SQSEvent): Promise<SQSBatchResponse> {
  const batchItemFailures: { itemIdentifier: string }[] = [];
 
  await Promise.all(
    event.Records.map(async (record) => {
      try {
        const msg = JSON.parse(record.body) as { orderId: string };
        await processOrder(msg);
      } catch {
        batchItemFailures.push({ itemIdentifier: record.messageId });
      }
    })
  );
 
  return { batchItemFailures };
}

// INIT (outside handler) - cache clients, parse env
import { DynamoDBClient } from "@aws-sdk/client-dynamodb";
const ddb = new DynamoDBClient({});
 
export const handler = async () => {
  // INVOKE (inside handler) - per-request logic only
  await ddb.send(/* ... */);
};

export const handler = async (_event: unknown, context: Context) => {
  console.log(JSON.stringify({
    awsRequestId: context.awsRequestId,
    functionName: context.functionName,
    remainingMs: context.getRemainingTimeInMillis(),
  }));
};