In this post, we are going to use the AWS CDK to build an AWS Lambda Function that triggers from DynamoDB Stream Events.

All of the code can be found in this repository.

Setup

We need to run a few commands to set up our CDK app.

mkdir how-to-trigger-lambda-from-ddb-stream
cd how-to-trigger-lambda-from-ddb-stream
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.

DynamoDB

Install the DynamoDB CDK package.

npm i @aws-cdk/aws-dynamodb

Open lib/how-to-trigger-lambda-from-ddb-stream-stack.ts, add a new DynamoDB table, and deploy.

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

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

    const table = new dynamodb.Table(this, 'Table', {
      billingMode: dynamodb.BillingMode.PAY_PER_REQUEST,
      partitionKey: { name: 'id', type: dynamodb.AttributeType.STRING },
      stream: dynamodb.StreamViewType.NEW_AND_OLD_IMAGES,
      tableName: 'Table',
    });
  }
}

npm run cdk deploy

Lambda

Install the Lambda CDK package.

npm i @aws-cdk/aws-lambda

Deploying Lambda functions requires bootstrapping your CDK app which gives us an S3 bucket where our Lambda's source code will live.

npm run cdk bootstrap

Create src/index.js and paste the following code

exports.handler = async (event) => {
    event.Records.forEach((record) => {
        console.log('Event Id: %s', record.eventID);
        console.log('Event Id: %s', record.eventName);
        console.log('DynamoDB Record: %j', record.dynamodb);
    });
};

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

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

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

    const table = new dynamodb.Table(this, 'Table', {
      billingMode: dynamodb.BillingMode.PAY_PER_REQUEST,
      partitionKey: { name: 'id', type: dynamodb.AttributeType.STRING },
      tableName: 'Table',
    });

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

npm run cdk deploy

Event Source

Install the Lambda Event Sources CDK package.

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

Open lib/how-to-trigger-lambda-from-ddb-stream-stack.ts, add a new DynamoEventSource to the Lambda Function.

import * as cdk from '@aws-cdk/core';
import * as dynamodb from '@aws-cdk/aws-dynamodb';
import * as lambda from '@aws-cdk/aws-lambda';
import { DynamoEventSource } from '@aws-cdk/aws-lambda-event-sources';

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

    const table = new dynamodb.Table(this, 'Table', {
      billingMode: dynamodb.BillingMode.PAY_PER_REQUEST,
      partitionKey: { name: 'id', type: dynamodb.AttributeType.STRING },
      stream: dynamodb.StreamViewType.NEW_AND_OLD_IMAGES,
      tableName: 'Table',
    });

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

    lambdaFunction.addEventSource(new DynamoEventSource(table, {
      startingPosition: lambda.StartingPosition.LATEST,
    }));
  }
}

npm run cdk deploy

Testing

Using the AWS CLI, insert a new item into the DynamoDB table.

aws dynamodb put-item \
    --table-name Table \
    --item '{
        "id": {"S": "test123"}
      }'

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

2021-05-14T19:08:55.487Z    ffba16f2-846b-4c6e-9ff5-189c3293116e    INFO    Event Id: e6db4f872fd1997f0f459bcebb8163e8

2021-05-14T19:08:55.527Z    ffba16f2-846b-4c6e-9ff5-189c3293116e    INFO    Event Id: INSERT

2021-05-14T19:08:55.527Z    ffba16f2-846b-4c6e-9ff5-189c3293116e    INFO    DynamoDB Record: {
    "ApproximateCreationDateTime": 1621019335,
    "Keys": {
        "id": {
            "S": "test123"
        }
    },
    "NewImage": {
        "id": {
            "S": "test123"
        }
    },
    "SequenceNumber": "733400000000008471956488",
    "SizeBytes": 18,
    "StreamViewType": "NEW_AND_OLD_IMAGES"
}

Summary

Congratulations. If you've made it this far then you successfully ...

1. Scaffolded a new CDK app

2. Created a DynamoDB table

3. Created a Lambda Function

4. Created a DynamoEventSource for the Lambda Function