How to add structured logging to a FastAPI application

Structured Logging in FastAPI

Production logs must be machine-readable. JSON logging lets tools like Datadog, Loki, and CloudWatch parse and filter log fields without regex.

Middleware that logs every request

import logging
import time
import uuid
from fastapi import FastAPI, Request

logging.basicConfig(
    level=logging.INFO,
    format='{"time": "%(asctime)s", "level": "%(levelname)s", "message": "%(message)s"}'
)
logger = logging.getLogger(__name__)

app = FastAPI()

@app.middleware("http")
async def log_requests(request: Request, call_next):
    request_id = str(uuid.uuid4())[:8]
    start = time.time()
    response = await call_next(request)
    duration = round(time.time() - start, 4)
    logger.info(
        f"request_id={request_id} method={request.method} "
        f"path={request.url.path} status={response.status_code} "
        f"duration={duration}s"
    )
    return response

Add request_id to log entries so you can trace a full request chain across services. Include it in the response as a header too: response.headers["X-Request-Id"] = request_id.

For full JSON structured logging, use the structlog library which supports context binding, processor pipelines, and multiple output formats with minimal configuration.