Correlation IDs Go in Middleware, Not App Code

Problem

Distributed tracing across 15+ microservices was broken. Developers were supposed to pass correlation IDs in every request, but they kept forgetting. Some services generated their own IDs, others didn't pass them at all.

Key Insight

Infrastructure concerns belong in infrastructure layers. Injecting correlation IDs in HTTP middleware means app code never touches them:

@app.middleware("http")
async def correlation_middleware(request, call_next):
    correlation_id = request.headers.get(
        "X-Correlation-ID", str(uuid4())
    )
    # Attach to request state + response headers
    request.state.correlation_id = correlation_id
    response = await call_next(request)
    response.headers["X-Correlation-ID"] = correlation_id
    return response

After this change, 100% of requests had correlation IDs. Zero developer effort required.

Takeaway

Don't rely on developers remembering to do infrastructure tasks. Make it the path of least resistance — automatic, invisible, and impossible to skip.