How to Trigger an AWS Lambda from SNS

In this post we are going to use the AWS CDK to build an AWS Lambda Function that triggers from an AWS SNS message.

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

This should give you the following directory structure.

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

Create an SNS Topic

Install the SNS CDK package.

npm i @aws-cdk/aws-sns

Open lib/how-to-trigger-lambda-from-sns-stack.ts, add a new SNS topic, and deploy.

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

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

    const topic = new sns.Topic(this, 'OurSnsTopic', {
      displayName: 'Our SNS Topic',
    });
  }
}

Nice! Let's deploy the stack.

npm run cdk deploy

Create a Lambda

Install the Lambda CDK package.

npm i @aws-cdk/aws-lambda

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

Create src/index.js and paste the following code

exports.handler = async (event) => {
  event.Records.forEach((record) => {
    console.log('Record: %j', record);
  });
};

Open lib/how-to-trigger-lambda-from-sns-stack.ts, add a new Lambda function, and deploy.

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

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

    const topic = new sns.Topic(this, 'OurSnsTopic', {
      displayName: 'Our SNS Topic',
    });

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

Deploy again ...

npm run cdk deploy

You may need to confirm some IAM changes.

Create the Event Source

Install the Lambda Event Sources CDK package.

npm i @aws-cdk/aws-lambda-event-sources

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

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

    const topic = new sns.Topic(this, 'OurSnsTopic', {
      displayName: 'Our SNS Topic',
    });

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

    const eventSource = new lambdaEventSources.SnsEventSource(topic);

    lambdaFunction.addEventSource(eventSource);
  }
}

One more deployment ...

npm run cdk deploy

One last time, you may need to approve IAM changes.

Testing

We are going to make use of the AWS CLI to test our stack.

First, we need the ARN of our SNS topic, which you can get using the following command

aws sns list-topics --query "Topics[?contains(TopicArn, 'OurSnsTopic')]"

Next, using the TopicArn from the previous command, use the AWS CLI to publish a new message to Our SNS Topic. Your topic ARN may vary, but for me this command looks like this.

aws sns publish \
    --topic-arn "arn:aws:sns:us-east-2:472331918655:HowToTriggerLambdaFromSnsStack-OurSnsTopic2ED19057-1GAA1XT2U6XXH" \
    --message "Hello from SNS"

Verify that the Lambda executed by looking in CloudWatch. Find the LogGroup named /aws/lambda/SnsMessageHandler and open up the latest LogStream. You should see some log messages that look similar to this.

Clean Up

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

npm run cdk destroy

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