Correlation IDs are identifiers assigned to a request or workflow and copied into logs, messages, and downstream calls so engineers can find related events across services.

In distributed systems, timestamps and service names are not enough to tell which log lines belong to the same request. Correlation IDs provide a shared handle that makes scattered work searchable as one request story.

In this chapter, you will learn how to generate, propagate, validate, and use correlation IDs across services, queues, async work, logs, and traces.

Consider a checkout request that crosses several services:

Each service logs something:

Those seven lines look related because we placed them together. In production, they are mixed with thousands of other log lines from the same second. Without a shared identifier, you match timestamps, hostnames, and guesses about the call flow.

That works on quiet systems. It fails under load.