How to Increase Your Python Aws Cdk Lambda Development Speed by Testing Locally with AWS SAM

Cross-post from: Medium
This article explains how to locally test, with AWS SAM (Serverless Application Model), a Lambda function + API Gateway created with AWS CDK (Cloud Development Kit). Python AWS CDK and SAM are a match made in heaven, or at least a match made in the cloud. This example uses the AWS CDK in Python.

Using Python AWS CDK and SAM

When using AWS CDK for your infrastructure as code, it can be a pain to figure out how to efficiently test your Lambda functions. Deploying your code to AWS with cdk deploy for every change is slow, having to run a deploy for every code change is not ideal. Luckily it is straightforward to use AWS SAM to test your function locally.

For scenarios where local dev isn’t feasible, get faster deployments to Lambda with cdk deploy --hotswap

Steps 0–2 help you set up a fresh CDK project. If you already have a CDK project which creates a Lambda function and API Gateway, skip ahead to step 3.

0) Prerequisites, Assumptions, and Environment

• NPM Installed
• Python Installed
• Docker Installed
• If you don’t have AWS CDK installed follow this guide
• If you don’t have AWS SAM installed, follow this guide

1) Initialize a Python CDK project

Note: If this is your first time using AWS CDK with Python, I recommend you try out the CDK Workshop from AWS: https://cdkworkshop.com/30-python/20-create-project/100-cdk-init.html

⚠️ Optionally: you can clone the completed repo found here and skip to step 3

Create a directory for your project and cd into it

$ mkdir aws-sam-cdk-demo
$ cd aws-sam-cdk-demo

Then, initialize your CDK project

$ cdk init app --language python

Create and source a python virtual environment

$ python3 -m venv .venv
$ source .venv/bin/activate

Install CDK requirements

$ pip install -r requirements.txt

Run CDK synth to ensure everything is setup correctly

$ cdk synth

You should see a small CloudFormation template get created with only the CDKMetadata resource and some conditions:

2) Create a Lambda function

In this example, I’ll be making an API that provides a greeting from a list of greetings I have hard coded.

Create a directory for the lambda code and create a file for the code

$ mkdir lambda$ touch lambda/greeting_generator.py

Go ahead and add some code to your function. You can copy the code from my function and tweak it as you see fit: https://github.com/chrishart0/aws-cdk-sam-demo/blob/master/lambda/greeting_generator.py

Install the lambda and API Gateway constructs

Add aws-cdk.aws-lambda and aws-cdk.aws-apigateway as lines to requirements.txt

$ pip install -r requirements.txt

Time to add lambda and API Gateway to your stack

The stack file is where you will define your infrastructure for CDK.

The stack file was created by cdk init, it follows the naming scheme same_as_your_parent_dir/same_as_your_parent_dir_stack.py

Example: Parent folder which init was run in is aws-sam-cdk-demo

• aws_sam_cdk_demo/aws_sam_cdk_demo_stack.py
• Example in GitHub: https://github.com/chrishart0/aws-cdk-sam-demo/blob/master/aws_sam_cdk_demo/aws_sam_cdk_demo_stack.py

Import lambda at the top of the stack file referenced above. How this file should look at the end is provided below or check the repo.

from aws_cdk import (
    core,
    aws_lambda as _lambda,
    aws_apigateway as apigateway,
)

Add code to your stack to generate a lambda function

greeting_function = _lambda.Function(
            self, 'greeting_handler',
            runtime=_lambda.Runtime.PYTHON_3_9,
            code=_lambda.Code.asset('lambda'),
            handler='greeting_generator.handler',
        )

Add code for the API Gateway. Ensure the handler is the same as the variable for the lambda function.

greeting_apg = apigateway.LambdaRestApi(
            self, 'greeting_endpoint',
            handler=greeting_function,
            description="Greeting API endpoint",
        )

It should now look like the image below:

Tip: Add links back to the CDK docs in your code; this will save you a lot of time hunting for the right page in the future.

Run cdk synth to ensure everything compiles. You should now see more resources in the CloudFormation output in your terminal.

3) Test that lambda function locally with SAM

Here is where things start to get a little… nutty 🐿️ (get it, like the SAM squirrel mascot???)

If you already have a CDK application with a lambda function start here!

Synthesize a template and write it to template.yaml

$ cdk synth --no-staging > template.yaml

Test the API with SAM

$ sam local start-api

In your browser or in another terminal hit the endpoint http://127.0.0.1:3000/

You can change the code of your lambda function on the fly, no need to restart SAM. Edit the source code in the lambda/ dir and call the endpoint again.

You can also test individual invocations with simulated events, this is ideal if you are not using API Gateway.

$ sam local invoke functionName -e event.json