# How to capture Table changes with DynamoDB Streams & Lambda with AWS CDK

### Dynamo DB stream - Definition

What is a stream ?

According to the AWS Documentation : 

> A DynamoDB stream is an ordered flow of information about changes to items in a DynamoDB table.
Whenever an application creates, updates, or deletes items in the table, DynamoDB Streams writes a stream record with the primary key attributes of the items that were modified

### Use case

According to the definition, every time a change happens in a DynamoDB table, it is stored in a stream.

Okay, but what can we do with that stream ?

Well, you can use a stream to invoke a Lambda function.

![untitled@2x (6).png](https://cdn.hashnode.com/res/hashnode/image/upload/v1639414258125/Ry-tRS5W-.png)

For example, you have an application that creates new Users on a DynamoDB table.

It could be useful to send to automatically an email to your new users.

You can do that with DynamoDB streams.

### Let's do this

Let's create the main folder of the project 

```
mkdir streamLambdaExample
cd streamLambdaExample
npm init -y
``` 
Then install CDK and aws-lambda package

```
npm install -g aws-cdk
npm i @aws-cdk/aws-lambda
npm i @aws-cdk/aws-dynamodb
npm i @aws-cdk/aws-lambda-event-sources
mkdir cdk-folder
cdk init app --language typescript

#### CDK part

``` 

Open the file lib/cdk-folder-stack.ts

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

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

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

    const myLambdaFunction = new lambda.Function(this, 'my-lambda', {
      runtime: lambda.Runtime.NODEJS_14_X,
      memorySize: 256,
      timeout: cdk.Duration.seconds(60),
      handler: 'index.main',
      code: lambda.Code.fromAsset(path.join(__dirname, '/../app/my-lambda/')),
    });

    myLambdaFunction.addEventSource(new DynamoEventSource(usersTable, {
      startingPosition: lambda.StartingPosition.LATEST,
    }));
  }
}

```
#### Lambda part

Now the lambda code 

```
mkdir app
cd app
mkdir my-lambda
cd my-lambda
npm init -y 
touch index.js
```

We are just going to log the DynamoDB record.

```
exports.main = async (event) => {
    for (const record of event.Records){
        console.log('DynamoDB Record: %j', record.dynamodb);
    }
};
```

### Let's try it

Let's bootstrap first 

```
cdk bootstrap
⏳  Bootstrapping environment aws://XXXXXXXXXXXXX/us-east-1...
 ✅  Environment aws://XXXXXXXXXXXXXX/us-east-1 bootstrapped (no changes).
``` 

Second command, to create the Cloudformation template for our app :

```
cdk synth
``` 

And now, let's deploy !

```
cdk deploy
``` 

### On the console

First, let's see our DynamoDB table

Go to DynamoDB on the console (on the right region), to see your Users table : 


![Capture d’écran 2021-12-13 à 19.26.58.png](https://cdn.hashnode.com/res/hashnode/image/upload/v1639421427637/v8vooy9KT.png)

We are going to create a User on the table (click on Create Item)

![Capture d’écran 2021-12-13 à 19.27.21.png](https://cdn.hashnode.com/res/hashnode/image/upload/v1639421453316/RkUSSTrgb.png)

Fill with the info you want : 

![Capture d’écran 2021-12-13 à 19.27.46.png](https://cdn.hashnode.com/res/hashnode/image/upload/v1639421479952/O2aG2YS3X.png)

Hit save.

If we configured everything correctly, our lambda should be triggered and display the log.

Go to the Lambda console (you can see that the trigger is DynamoDB) : 


![Capture d’écran 2021-12-13 à 19.28.15.png](https://cdn.hashnode.com/res/hashnode/image/upload/v1639421645822/MwLT-lYUsL.png)

Go to the Cloudwatch Logs of your Lambda : 


![Capture d’écran 2021-12-13 à 19.48.06.png](https://cdn.hashnode.com/res/hashnode/image/upload/v1639421636451/HvwhtDQdF.png)

Your lambda has been triggered by the DynamoDB stream ! Well done ! 


### Cleaning

Don't forget to clean after this tutorial, i don't want you to have a huge bill on AWS! 


```
cdk destroy
``` 

### Conclusion

Fell free to ask me questions in the comments ! 
See you in the next one ! 🤖