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How to Trigger an AWS Lambda from S3

Alex Kates
Senior Software Engineer at https://creditgenie.com. Passionate about full-stack JavaScript, AWS Serverless, and Extreme Programming.
Updated on ・4 min read

In this post we are going to use the AWS CDK to build an AWS Lambda Function that triggers from AWS S3 Put requests.

All of the code can be found in this repository.

Setup

We need to run a few commands to setup our CDK app.

mkdir how-to-trigger-lambda-from-s3
cd how-to-trigger-lambda-from-s3
npx cdk init app --language typescript

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This should give you the following directory structure.
Directory structure after running the CDK init command

Also make sure you have your AWS CLI configured. For more information follow the AWS CLI quickstart guide.

Create an S3 Bucket

Install the S3 CDK package.

npm i @aws-cdk/aws-s3
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Open lib/how-to-trigger-lambda-from-s3-stack.ts, add a new S3 Bucket, and deploy.

import * as cdk from '@aws-cdk/core';
import * as s3 from '@aws-cdk/aws-s3';

export class HowToTriggerLambdaFromS3Stack extends cdk.Stack {
  constructor(scope: cdk.Construct, id: string, props?: cdk.StackProps) {
    super(scope, id, props);

    const bucket = new s3.Bucket(this, 'OurBucket', {
      /**
       * The following properties ensure the bucket is properly 
       * deleted when we run cdk destroy */
      autoDeleteObjects: true,
      removalPolicy: cdk.RemovalPolicy.DESTROY
    })
  }
}

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npm run cdk deploy

You may need to confirm some IAM changes.
Confirm IAM changes when running cdk deploy

Create a Lambda

Install the Lambda CDK package.

npm i @aws-cdk/aws-lambda
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Deploying a Lambda function requires bootstrapping your CDK app which gives us an S3 bucket where our Lambda's source code will live. This is a one time operation.

npm run cdk bootstrap
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Create src/index.js and paste the following code

exports.handler = async (event) => {
  event.Records.forEach((record) => {
    console.log('Event Name: %s', record.eventName);
    console.log('S3 Request: %j', record.s3);
  });
};
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Open lib/how-to-trigger-lambda-from-s3-stack.ts, add a new Lambda function, and deploy.

import * as cdk from '@aws-cdk/core';
import * as s3 from '@aws-cdk/aws-s3';
import * as lambda from '@aws-cdk/aws-lambda';

export class HowToTriggerLambdaFromS3Stack extends cdk.Stack {
  constructor(scope: cdk.Construct, id: string, props?: cdk.StackProps) {
    super(scope, id, props);

    const bucket = new s3.Bucket(this, 'OurBucket', {
      /**
       * The following properties ensure the bucket is properly 
       * deleted when we run cdk destroy */
      autoDeleteObjects: true,
      removalPolicy: cdk.RemovalPolicy.DESTROY
    });

    const lambdaFunction = new lambda.Function(this, 'Function', {
      code: lambda.Code.fromAsset('src'),
      handler: 'index.handler',
      functionName: 'BucketPutHandler',
      runtime: lambda.Runtime.NODEJS_12_X,
    });
  }
}

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Deploy again ...

npm run cdk deploy
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You may need to confirm more IAM changes.
More IAM changes after deploying stack with Lambda function

Create the Event Source

Install the Lambda Event Sources CDK package.

npm i @aws-cdk/aws-lambda-event-sources
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Open lib/how-to-trigger-lambda-from-s3-stack.ts, add a new S3EventSource to the Lambda Function.

import * as cdk from '@aws-cdk/core';
import * as s3 from '@aws-cdk/aws-s3';
import * as lambda from '@aws-cdk/aws-lambda';
import * as lambdaEventSources from '@aws-cdk/aws-lambda-event-sources';

export class HowToTriggerLambdaFromS3Stack extends cdk.Stack {
  constructor(scope: cdk.Construct, id: string, props?: cdk.StackProps) {
    super(scope, id, props);

    const bucket = new s3.Bucket(this, 'OurBucket', {
      /**
       * The following properties ensure the bucket is properly 
       * deleted when we run cdk destroy */
      autoDeleteObjects: true,
      removalPolicy: cdk.RemovalPolicy.DESTROY
    });

    const lambdaFunction = new lambda.Function(this, 'Function', {
      code: lambda.Code.fromAsset('src'),
      handler: 'index.handler',
      functionName: 'BucketPutHandler',
      runtime: lambda.Runtime.NODEJS_12_X,
    });

    const s3PutEventSource = new lambdaEventSources.S3EventSource(bucket, {
      events: [
        s3.EventType.OBJECT_CREATED_PUT
      ]
    });

    lambdaFunction.addEventSource(s3PutEventSource);
  }
}
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One more deployment ...

npm run cdk deploy
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One last time, you may need to approve IAM changes.

IAM changes approval after deploying the S3 event source

Testing

Create a text file that we will use to upload to the S3 bucket.

echo "This is a test" > testFile.txt
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Use the AWS CLI to upload this file into our S3 bucket from earlier. Your bucket name will vary, but for me this command looks like this.

aws s3 cp ./testFile.txt s3://howtotriggerlambdafroms3stack-ourbucket48caf57c-1p9g74rol1cza/testFile.txt
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Verify that the Lambda executed by looking in CloudWatch. Find the LogGroup named /aws/lambda/BucketPutHandler and open up the latest LogStream. You should see some log messages that look similar to this.

CloudWatch results after uploading an object to S3

Clean Up

Don't forget to delete your stack when you are finished!

npm run cdk destroy
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Thanks for reading! If you found this useful, please follow me here
https://dev.to/thealexkates
https://twitter.com/thealexkates

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