Deploy an API fronted Lambda function using AWS CDK

This project is a minimum working example of deploying a Lambda function accessible through an API Gateway using Python flavored AWS CDK. The API allows you to pass data directly into the Lambda function and get a response back with a result.

For maybe a better way to do this using the new Lambda Function URL feature recently released by AWS see my post here.

What is possible using this pattern?

1) Pass data to the function, get a machine learning prediction back.
2) Request a file stored on S3, get a secure link back.
3) Send parameters to the function, get complex calculations back.
4) Database retrieval.
etc...

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_API && cd CDK_Lambda_API

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.

# lambda_api_stack.py

from aws_cdk import Stack
from aws_cdk import aws_apigateway as apigw
from aws_cdk import aws_lambda as _lambda
from aws_cdk import aws_lambda_python_alpha as _lambda_python
from constructs import Construct


class LambdaModelPredictionsStack(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.prediction_lambda = None
        self.gateway = None
        # Additional useful infrastructure might include an S3 bucket,
        # an EFS store, SQS queue, etc.

        self.build_infrastructure()

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

    def build_lambda(self):
        self.prediction_lambda = _lambda_python.PythonFunction(
            scope=self,
            id="PredictionLambda",
            # entry points to the directory
            entry="lambda_funcs/APILambda",
            # index is the file name
            index="API_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="ExampleAPILambda",
        )

    def build_gateway(self):
        # This will attach an API gateway as a trigger to our
        # lambda function above. The return of the handler function also
        # gets routed back to the API gateway.
        self.gateway = apigw.LambdaRestApi(
            self, "Endpoint", handler=self.prediction_lambda
        )

Minimum Working Lambda Function

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

# lambda_funcs/APILambda/API_lambda.py

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.

Once the process is completed, CDK will output the endpoint URL of the API Gateway:

Query the API Gateway

To query the API gateway 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.