Tracking Logs by Request Using Correlation ID

When dealing with complex distributed systems, it can be challenging to correlate log messages from different components to understand the flow of a request. One effective technique is to use a correlation ID. This is a unique identifier that can be propagated through the entire request lifecycle, allowing you to trace the request’s journey and correlate related log messages.

Generating a Correlation ID

You can generate a unique correlation ID using a UUID library or a custom function. Here’s an example using the uuid module:

Python

import uuid

def generate_correlation_id():
    return str(uuid.uuid4())

Adding the Correlation ID to the Request

Create a middleware function to add the correlation ID to the request context:

Python

from fastapi import Request, Response, HTTPException
from starlette.middleware.base import BaseHTTPMiddleware, RequestResponse   

class CorrelationIDMiddleware(BaseHTTPMiddleware):
    async def dispatch(self, request: Request, call_next: RequestResponse) -> Response:
        correlation_id = request.headers.get("X-Correlation-ID") or generate_correlation_id()
        request.state.correlation_id = correlation_id
        response = await call_next(request)
        response.headers["X-Correlation-ID"] = correlation_id
        return response

Using the Correlation ID in Endpoints

Access the correlation ID in your endpoints:

Python

from fastapi import APIRouter, Depends, Request

router = APIRouter()

@router.get("/items/{item_id}")
async def read_item(item_id: int, request: Request):
    correlation_id = request.state.correlation_id
    logger.info(f"Received GET request for item {item_id} (Correlation ID: {correlation_id})")
    # ...

Propagating the Correlation ID

If your application calls external services or other components, ensure that the correlation ID is propagated to them. You can include the correlation ID in headers or as a query parameter.

Logging with Correlation ID

Include the correlation ID in your log messages to correlate related events:

Python

logger.info(f"Processing request: {request.url} (Correlation ID: {correlation_id})")

Additional Terms

Unique ID Generation: Ensure that the correlation ID generation algorithm produces unique values.
Header Propagation: If your application interacts with external services, configure them to propagate the correlation ID header.
Log Aggregation: Consider using a log aggregation tool like Elasticsearch to centralize and analyze log data.
Security: Avoid including sensitive information in the correlation ID.

By using correlation IDs, you can effectively track the flow of requests through your FastAPI application, making it easier to diagnose and troubleshoot issues.