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0004-layer-and-packaging.md

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0004. Layer and packaging

Status: Accepted

Context

We have customers consuming Powertools for AWS Lambda (Python) through the public Lambda layer, built in layer_v3/, and customers packaging it themselves in a zip or container image. Both are first-class. The layer sits on sys.path ahead of the Lambda runtime's own packages, so anything we ship in it shadows what the runtime provides.

The runtime already provides boto3, botocore and their dependencies (urllib3, s3transfer, python-dateutil, six, jmespath). When we shipped urllib3 by accident (see issues 8555 and 8556), customers got a version of it they did not choose and that could differ from the SDK the runtime expects.

Decision

  • The layer never ships boto3, botocore or any of their dependencies. The runtime provides them, or the customer packages the version they want. Extras that pull them in are not installed into the layer.
  • The layer is built from the released package with the same build for every supported Python version and for x86_64 and arm64. The two architectures must be functionally identical.
  • The layer ships one compiled .pyc per .py (compileall with checked hashes) so cold starts do not pay for bytecode compilation.
  • The layer contains Powertools for AWS Lambda (Python) and its optional extras only. A dependency we would not accept in pyproject.toml does not enter the layer either (ADR 0007).
  • The zip package built from the wheel is the reference for customers who do not use the layer; what works in the layer must work in the zip and vice versa.

Consequences

  • Before a release the layer is built per version and architecture and exercised in real functions, both as a layer and as a zip, on cold and warm starts.
  • A change in pyproject.toml extras or dependencies is also a layer change and is reviewed as one.
  • Customers who need a specific boto3 version keep full control of it.

When reviewing or writing a change

If a PR touches dependencies, extras or layer_v3/, check what ends up in python/lib/python3.*/site-packages of a built layer. Nothing from the runtime's own set may appear there.