A correlation ID (also called a request ID) is a unique value generated when a request first enters your system, then passed along in a header and written to every log line the request produces. It lets you find everything that happened for one request across many services.
Why it matters
In a system with a load balancer, an app server, a queue, and a database, a single user error leaves fragments in four different log files. Without a shared identifier, you are matching timestamps by hand. With one, a single search returns the full story, which cuts MTTR considerably.
Example
GET /checkout HTTP/1.1
Host: shop.example.com
X-Request-ID: 7f3c9e1a-52b4-4c8d-9d0e-1a2b3c4d5e6fEach service logs the same value:
2026-10-08T09:14:02Z web id=7f3c9e1a status=502 upstream=payments
2026-10-08T09:14:02Z pay id=7f3c9e1a error="db timeout after 5000ms"Return the ID to the client in a response header or error page so support can ask for it. X-Request-ID is a common convention, and the W3C traceparent header standardizes the idea for distributed tracing.
Common mistakes
- Generating a new ID in each service instead of reusing the incoming one.
- Forgetting to forward it on outgoing calls and queue messages.
- Trusting client-supplied IDs without length or character validation, which allows log injection.
Related
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