n. / Tracing basics
Trace context propagation
Definition
Trace context propagation is passing the trace ID and current span ID from one service to the next, usually in an HTTP header, so the next service can continue the same trace instead of starting a new one.
In more detail
The common standard is the W3C traceparent header. The caller writes it on the outgoing request and the receiver reads it before starting its first span. If a hop drops the header, the trace splits in two, which is the most frequent reason a distributed trace looks incomplete. Queues and background jobs need the context stored in the message.
Example
A web server sends traceparent: 00-4bf92f3577b34da6a3ce929d0e0e4736-00f067aa0ba902b7-01 to the agent service. The agent's first span uses that trace ID and names the web span as its parent.
