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How to Trigger an AWS Lambda Function from a DynamoDB Stream Event

Alex Kates
Senior Software Engineer at https://creditgenie.com. Passionate about full-stack JavaScript, AWS Serverless, and Extreme Programming.
Updated on ・3 min read

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 setup 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
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This should give you the following directory structure.

Alt Text

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
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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',
    });
  }
}

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npm run cdk deploy
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Lambda

Install the Lambda CDK package.

npm i @aws-cdk/aws-lambda
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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
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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);
    });
};
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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,
    });
  }
}

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npm run cdk deploy
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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-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,
    }));
  }
}

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npm run cdk deploy
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Testing

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

aws dynamodb put-item \
    --table-name Table \
    --item '{
        "id": {"S": "test123"}
      }'
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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
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2021-05-14T19:08:55.527Z    ffba16f2-846b-4c6e-9ff5-189c3293116e    INFO    Event Id: INSERT

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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"
}

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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

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