AWS Lambda Function URLs with AWS CDK

NOTE
Since I wrote this post, the CDK has released an API to do this, so it's no longer necessary to use custom resources like I detail below.

For the lazy:

# Can be a Function or an Alias
# fn: lambda.Function
# my_role: iam.Role


fn_url = fn.add_function_url()
fn_url.grant_invoke_url(my_role)

CfnOutput(self, "TheUrl",
    # The .url attributes will return the unique Function URL
    value=fn_url.url
)

AWS recently released a great new feature for Lambda: AWS Lambda Function URLs

Function URLs promise to replace the API Gateway Lambda Proxy pattern as they allow a simpler way to do HTTP GET operations on your Lambda functions. They also don't suffer from the API gateway timeout problem of 30 sec.

The Github repository can be found here.

CDK init & deploy

I won’t cover setting up CDK and bootstrapping the environment. You can find that information here.

Once you have set up CDK, we need to set up the project:

1. mkdir CDK_Lambda_URL && cd CDK_Lambda_URL

2. cdk init --language python

3. source .venv/bin/activate

4. Optional: If you need additional libraries in your Lambda function, add aws-cdk.aws-lambda-python-alpha to requirements.txt to allow custom builds during stack deployment using Docker.

5. pip install -r requirements.txt && pip install -r requirements-dev.txt

   Now deploy empty stack to AWS:

6. cdk deploy

Stack design

This stack will deploy a lambda function using aws-lambda-python-alpha to build the function with all its additional libraries using a docker container. Make sure to have Docker installed and the daemon running before running cdk deploy.

from aws_cdk import CfnResource, Stack
from aws_cdk import aws_lambda as _lambda
from aws_cdk import aws_lambda_python_alpha as _lambda_python
from constructs import Construct


class LambdaStack(Stack):
    def __init__(self, scope: Construct, construct_id: str, **kwargs) -> None:
        super().__init__(scope, construct_id, **kwargs)

        # in __init__ I like to initialize the infrastructure I will be creating
        self.example_lambda = None

        self.build_infrastructure()

    def build_infrastructure(self):
        # For convenience, consolidate infrastructure construction
        self.build_lambda()

    def build_lambda(self):
        self.example_lambda = _lambda_python.PythonFunction(
            scope=self,
            id="ExampleLambda",
            # entry points to the directory
            entry="lambda_funcs/LambdaURL",
            # index is the file name
            index="URL_lambda.py",
            # handler is the function entry point name in the lambda.py file
            handler="handler",
            runtime=_lambda.Runtime.PYTHON_3_9,
            # name of function on AWS
            function_name="ExampleLambdaFunctionURL",
        )

        # Set up the Lambda Function URL
        cfnFuncUrl = CfnResource(
            scope=self,
            id="lambdaFuncUrl",
            type="AWS::Lambda::Url",
            properties={
                "TargetFunctionArn": self.example_lambda.function_arn,
                "AuthType": "NONE",
                "Cors": {"AllowOrigins": ["*"]},
            },
        )

        # Give everyone permission to invoke the Function URL
        CfnResource(
            scope=self,
            id="funcURLPermission",
            type="AWS::Lambda::Permission",
            properties={
                "FunctionName": self.example_lambda.function_name,
                "Principal": "*",
                "Action": "lambda:InvokeFunctionUrl",
                "FunctionUrlAuthType": "NONE",
            },
        )

        # Get the Function URL as output
        Output(
            scope=self,
            id="funcURLOutput",
            value=cfnFuncUrl.get_att(attribute_name="FunctionUrl").to_string(),
        )

Minimum Working Lambda Function

You can see here an example of the response format the API gateway is expecting.

from logging import getLogger

logger = getLogger()
logger.setLevel(level="DEBUG")


def handler(event, context):
    logger.debug(msg=f"Initial event: {event}")
    response = {
        "isBase64Encoded": False,
        "statusCode": 200,
        "headers": {
            "Access-Control-Allow-Origin": "*",
        },
        "body": f"Nice! You said {event['queryStringParameters']['q']}",
    }
    return response

Now you can do cdk deploy. The lambda function will be built using docker and uploaded to the bootstrapped ECR repository. Once the project is built, it will synth a CloudFormation template and begin deploying the infrastructure. You can watch your stack deploy on AWS CloudFormation - it should be quick since the infrastructure is relatively simple.

You can use the URL output by the stack or else navigate to your Lambda function on the AWS Console to find your Lambda Function URL

Query the Function URL

To query the Function URL and get a response back from your lambda function, just send a GET request using requests or Postman

That's all! Make sure to cdk destroy when you're done to avoid any charges.