python 36 lines · 7 steps

Request ID tracing in FastAPI middleware

Assign every request a unique ID, stash it in a ContextVar, and thread it through logs and responses.

Explained by highlit
FastAPI Intermediate middleware context-variables request-tracing logging async
1import logging
2import uuid
3from contextvars import ContextVar
4 
5from starlette.middleware.base import BaseHTTPMiddleware
6from starlette.requests import Request
7from starlette.types import ASGIApp
8 
9request_id_ctx: ContextVar[str] = ContextVar("request_id", default="-")
10 
11 
12class RequestIDMiddleware(BaseHTTPMiddleware):
13 def __init__(self, app: ASGIApp, header_name: str = "X-Request-ID") -> None:
14 super().__init__(app)
15 self.header_name = header_name
16 
17 async def dispatch(self, request: Request, call_next):
18 request_id = request.headers.get(self.header_name) or uuid.uuid4().hex
19 token = request_id_ctx.set(request_id)
20 request.state.request_id = request_id
21 try:
22 response = await call_next(request)
23 finally:
24 request_id_ctx.reset(token)
25 response.headers[self.header_name] = request_id
26 return response
27 
28 
29class RequestIDLogFilter(logging.Filter):
30 def filter(self, record: logging.LogRecord) -> bool:
31 record.request_id = request_id_ctx.get()
32 return True
33 
34 
35def get_request_id() -> str:
36 return request_id_ctx.get()
01 / 01
STEP 01

Walkthrough

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Three takeaways
  1. 1A ContextVar carries per-request state safely across async boundaries without passing it through every function.
  2. 2Always reset a ContextVar with its token in a finally block so leaked values never contaminate later requests.
  3. 3A logging filter can enrich every record with contextual data pulled from the current execution context.

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