pants to build aws lambdas

Working in a CI/CD pipeline requires the ability to create our build structures on the fly without a significant amount of hassle. While tooling is great, sometimes it becomes cumbersome if it does "too many things" or if you're trying to glue too many tools together.

With Pants I've found quite the opposite, a fantastic use case for building deployment dependencies and keeping them squared away the way we want.

Requirements

Use case

I'm going to demo a quick AWS Lambda build that will have a few non-traditional components to the build process.

Setup

Our lambda will be set up in its own folder, with a lambda handler declared inside a file.

# aws/test_lambda/lambda_handler.py

def lambda_handler(event, context):
    print("hello world")
    return

Additionally, we'll keep our requirements in a separate area that will have a specific file for our lambda. I like to keep dependencies separate between services as best we can.

# requirements/lambda.txt
# these are python libraries
requests
beautifulsoup4

The BUILD process setup

When working with Pants we'll need to specify BUILD files throughout our project. This is easily done with the builtin setup command that Pants has, then with some minor tweaking afterward we can configure our lambda environment.

Pants has some great docs on the AWS lambda support

To generate BUILD files:

# from root of directory
./pants tailor

Editing our lambda BUILD file:

# aws/test_lambda/BUILD

# The default `sources` field will include our handler file.
python_sources(name="test_lambda0")

python_awslambda(
    name="lambda",
    runtime="python3.8",
    handler="lambda_handler.py:lambda_handler",
)

python_requirements(
    source="../../requirements/lambda.txt"
)

This will allow our requirements to be specifically called out for 3rd party packages. The documentation on this is pretty spot on, if we want to get more specific in the future.

To inspect our dependencies, we can run:

./pants peek <your file path>

Building the lambda

To build the lambda, we'll utilize our Pants installation to create a .zip of the project space. This will then be deployed using the AWS CLI to be used in the creation of our lambda function.

I'm working off this set of docs for building lambdas

Building our .zip file:

# from project root
./pants package aws/test_lambda/lambda_handler.py

Deploying our lambda to AWS:

cd dist/aws.test_lambda/

aws create-function \
    --function-name test_lambda \
    --runetime python3.8 \
    --zip-file fileb://test_lambda.zip \
    --handler lambdex_handler.handler \
    --role arn:aws:iam:123456789012:role/service-role/MyTestFunction-role-asdf

To update our lambda after a separate build:

cd dist/aws.test_lambda/

aws update-function-code \
    --function-name test_lambda \
    --zip-file fileb://test_lambda.zip