CDK Lambda Deployment

Question

Hello,
So I'm using CDK to build my infrastructure. I have created a Lambda Function in my CDK and another folder that holds the Lambda code. I used the following commands to deploy it to my AWS account in the command line:
> npm run build && cdk synth 
> cdk bootstrap
> cdk deploy
Everything worked fine the first time. However when I make any changes to my Lambda code and re-use these commands again my lambda code is not modified on the console. Seems like its not deploying the code because there is no changes in the CDK. How do I deploy my modified lambda code then? Can't find any documentation on that.

Accepted Answer

Since CDK uses CloudFormation to deploy its resources, any updates will need to trigger a CloudFormation update so that the code will be updated. As a test, I used the following CDK Lambda Code. Please see below:

```
const bucket = s3.Bucket.fromBucketName(this, 'bucket', '');

const lambdaFunc = new lambda.Function(this, 'lambdaFunc', {
          runtime: lambda.Runtime.PYTHON_3_9,
          code: new lambda.S3Code(bucket, "hello.py.zip", ''),
          handler: 'lambda.lambda_handler',
    });
```

Since I am leveraging S3 as the source of my code, any changes to the code will require the following steps:

1. Upload the Code to S3
 2. Obtain the Version Number in the S3 console in the `Versions` tab
 3. Add the Version Number to the third parameter in the `lambda.S3Code()` function
 4. run the `cdk deploy` command

After the Stack updated successfully, I was able to confirm that my Lambda code changed in the Lambda Console. Please see the test results below:

===
Original:
```
def handler(event, context):
    print('request: {}'.format(json.dumps(event)))
    return {
        'statusCode': 200,
        'headers': {
            'Content-Type': 'text/plain'
        },
        'body': 'Hello, CDK! You have a hit {}
'.format(event['path'])
    }
```

---

Updated: 
```
def handler(event, context):
    print('request: {}'.format(json.dumps(event)))
    return {
        'statusCode': 200,
        'headers': {
            'Content-Type': 'text/plain'
        },
        'body': 'Hola, CDK! Tienes un éxito {}
'.format(event['path'])
    }
```
===

Since there are various ways to source the Code in CDK for Lambda Constructs, a code snippet and CDK version will help me in provide the appropriate update steps for your use case.

Answer

I noticed that you mentioned that the Lambda code is in another file. My apologies for missing this detail earlier.

As a test, I deployed a CDK Stack that leveraged the Lambda construct. Please see the CDK code below:

```
 const lambdaFunc = new lambda.Function(this, 'lambdaFunc', {
          runtime: lambda.Runtime.PYTHON_3_9,
          code: lambda.Code.fromAsset('lambda'),
          handler: 'hello.handler',
    });
  }
```

The Lambda code will need to be in the in the CDK App directory but does not have to be in the `bin` nor the `lib` directory. In my example, the handler function is defined in the `hello.py` file that is in the `lambda` folder in my app.

Once my stack was deployed, I made the changes to the `hello.py` file and, then, ran the `cdk deploy` command. CDK was able to detect that the file changed and made the update to the Lambda function via CloudFormation. Please see the test results below:

===
Original: 
```
def handler(event, context):
    print('request: {}'.format(json.dumps(event)))
    return {
        'statusCode': 200,
        'headers': {
            'Content-Type': 'text/plain'
        },
        'body': 'Hola, CDK! Tienes un éxito {}
'.format(event['path'])
    }
```

---

Updated:
```
def handler(event, context):
    print('request: {}'.format(json.dumps(event)))
    return {
        'statusCode': 200,
        'headers': {
            'Content-Type': 'text/plain'
        },
        'body': 'Hello, CDK! You have a hit {}
'.format(event['path'])
    }
```
===

CDK Lambda Deployment

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