관측 가능성과 상관관계 ID — 분산 요청의 흔적 잇기

# FastAPI — correlation ID middleware
import uuid, logging
from fastapi import FastAPI, Request
from starlette.middleware.base import BaseHTTPMiddleware

log = logging.getLogger(__name__)

class CorrelationIDMiddleware(BaseHTTPMiddleware):
    async def dispatch(self, request: Request, call_next):
        # 있으면 client-공급 ID 재사용 (일부 client 가 client-driven tracing 함)
        request_id = request.headers.get('x-request-id') or str(uuid.uuid4())
        request.state.request_id = request_id

        # 모든 log 줄에 첨부 (production 엔 structlog 나 contextvars)
        log.info('request.start', extra={'request_id': request_id, 'path': request.url.path})

        response = await call_next(request)

        # Response 에 echo, client 가 봄
        response.headers['X-Request-ID'] = request_id
        log.info('request.end', extra={'request_id': request_id, 'status': response.status_code})
        return response

app = FastAPI()
app.add_middleware(CorrelationIDMiddleware)

@app.get('/api/anything')
async def anything(request: Request):
    rid = request.state.request_id
    log.info('handler.work', extra={'request_id': rid})
    return {'rid': rid}

# httpx 통해 downstream service 에 전파
import httpx
from fastapi import Request

async def call_upstream(request: Request, payload: dict):
    rid = request.state.request_id
    async with httpx.AsyncClient() as client:
        resp = await client.post(
            'https://upstream.example.com/process',
            json=payload,
            headers={'X-Request-ID': rid},  # downstream 전파
        )
        return resp.json()

# Upstream service 가 같은 X-Request-ID header 봄,
# 같은 ID 와 log, 자기 response 에 돌려줌 — loop 닫음.

# W3C Trace Context — 표준 형식 (OpenTelemetry-호환)
import uuid
from fastapi import Request

def make_traceparent(trace_id: str | None = None, span_id: str | None = None) -> str:
    trace_id = trace_id or uuid.uuid4().hex                      # 32 hex 글자
    span_id  = span_id  or uuid.uuid4().hex[:16]                 # 16 hex 글자
    return f'00-{trace_id}-{span_id}-01'                          # 00=version, 01=sampled

# 들어오는 traceparent 읽기 (있으면); 아니면 하나 생성
def get_trace_context(request: Request) -> dict:
    incoming = request.headers.get('traceparent')
    if incoming:
        # parse: version-trace_id-span_id-flags
        _, trace_id, _, _ = incoming.split('-')
        return {'trace_id': trace_id, 'traceparent': make_traceparent(trace_id)}
    return {'trace_id': uuid.uuid4().hex, 'traceparent': make_traceparent()}