experimental lambda battery pack

[nikomatsakis]

Hi all, I think it'd be really helpful to have a Lambda battery pack that pulls together the various crates you need to stand up a simple lambda and gives an example or two of getting it done.

[jlizen]

I just cut awslabs/metrique#247 for a metrique-battery-pack and figured I'd jot a few thoughts down here as well.

Default set

Just lambda_runtime (with default features, which includes tracing).

Features

Feature	What it enables	Notes
http	lambda_http	Function URL / API Gateway handlers
events	lambda_events	Typed event structs for S3, SQS, SNS, etc.
extension	lambda_extension	For building Lambda extensions

Templates

All templates should demonstrate testability (e.g., extracting handler logic into a function that can be unit tested independently of the Lambda harness).

Template	What it scaffolds
default	Custom serde request/response structs, a simple handler, unit test
http	Function URL / API Gateway handler with lambda_http. Branches on framework variable (default: axum)
stream	Stream processing handler. Branches on source variable (sqs / kinesis)
events	Generic event handler. Branches on event variable (similar to cargo lambda). Down the road, can add special support for specific services
extension	Lambda extension skeleton

Each template should include a shared LAMBDA.md with a quick-start guide, links to the runtime docs, and common patterns (e.g., shared state, error handling with anyhow).

It would be nice to work in an example of testing against aws-sdk clients via shared state, but maybe that is better suited to an aws-sdk battery-pack :)

The battery pack should live in this repo so it stays in sync with runtime releases.

Open questions

• Metrics: ideally templates would include metrics setup out of the box. This probably belongs as a lambda_runtime API feature first, though, so leaving it as a future addition.