Posted on May 10 • Edited on May 11

AWS S3 Triggering Lambda

We’ll create an event-driven Lambda that listens for new images in an S3 bucket (photos-raw), resizes them using Node.js, and saves the output to another bucket (photos-resized).

You’ll learn how to:

Configure S3 trigger events in AWS SAM
Resize images in Lambda using sharp

📝 template.yml

AWSTemplateFormatVersion: '2010-09-09' Transform: AWS::Serverless-2016-10-31 Globals: Function: Timeout: 29 Runtime: nodejs22.x MemorySize: 512 Resources: ResizeFunction: Type: AWS::Serverless::Function Properties: Handler: src/resize.handler Policies: - S3ReadPolicy: BucketName: !Sub "photos-raw-${AWS::AccountId}" - S3WritePolicy: BucketName: !Sub "photos-resized-${AWS::AccountId}" Events: S3Upload: Type: S3 Properties: Bucket: !Sub "photos-raw-${AWS::AccountId}" Events: s3:ObjectCreated:Put Environment: Variables: RESIZED_BUCKET: !Sub "photos-resized-${AWS::AccountId}" RawBucket: Type: AWS::S3::Bucket Properties: BucketName: !Sub "photos-raw-${AWS::AccountId}" ResizedBucket: Type: AWS::S3::Bucket Properties: BucketName: !Sub "photos-resized-${AWS::AccountId}"

Whenever someone uploads a new file to the photos-raw S3 bucket, automatically trigger (run) the Lambda function.

🧠 Function

import { S3Client, GetObjectCommand, PutObjectCommand } from '@aws-sdk/client-s3'; import sharp from 'sharp'; import { S3Event } from 'aws-lambda'; import { Readable } from 'stream'; const s3 = new S3Client({}); const RESIZED_BUCKET = process.env.RESIZED_BUCKET const streamToBuffer = async (stream: Readable): Promise<Buffer> => { const chunks: Uint8Array[] = []; for await (const chunk of stream) chunks.push(chunk); return Buffer.concat(chunks); }; export const handler = async (event: S3Event) => { for (const record of event.Records) { const bucket = record.s3.bucket.name; const key = decodeURIComponent(record.s3.object.key.replace(/\+/g, ' ')); const { Body } = await s3.send(new GetObjectCommand({ Bucket: bucket, Key: key })); if (!Body || !(Body instanceof Readable)) { console.error('Invalid body stream'); return; } const imageBuffer = await streamToBuffer(Body); const resized = await sharp(imageBuffer) .resize(800) .toBuffer(); await s3.send(new PutObjectCommand({ Bucket: RESIZED_BUCKET, Key: key, Body: resized, ContentType: resized.ContentType, })); } };