wshtlib: a shared internal library without the framework

Eight Lambda functions and one FastAPI app need structured logging, request context, CloudWatch metrics, and Lambda warming support.

AWS Lambda Powertools covers all of this. wshtlib is a smaller alternative — stdlib only, no external dependencies, ~400 lines.

Structure

A single directory, copied into each Lambda image at build time:

COPY backend/v1/wshtlib/ ./wshtlib/

Six modules:

• context — request-scoped store via contextvars (trace_id, user_id, correlation_id)
• logger — structured JSON logger with runtime auto-detection (Lambda vs ECS vs local)
• middleware — FastAPI middleware that initialises context and logs every request
• decorators — @lambda_handler for warming, context init, and error handling
• metrics — CloudWatch Embedded Metrics Format output
• env — strict env var loading (raises on missing, no silent defaults)

Lambda Usage

@lambda_handler
def handler(event, context):
    logger.info("processing", shoot_id=event["shoot_id"])
    metrics.count("PhotosProcessed")
    metrics.flush()

@lambda_handler handles warming events, initialises context, and catches unhandled exceptions. The logger picks up trace_id from context automatically. Metrics emit EMF JSON to stdout — CloudWatch picks it up without a sidecar.

FastAPI Usage

app.add_middleware(WshtlibMiddleware)

Same context initialisation, same structured request logs, same X-Trace-Id response header.

What It Doesn’t Cover

No X-Ray tracing integration, no event parsing utilities, no idempotency helpers.