Context Propagation: Context, Propagators and W3C Headers: OTCA practice questions

Context propagation is what lets OpenTelemetry connect telemetry produced in different services into one picture. Two ideas are involved. Context is the in-process object that carries execution-scoped values, such as the active span and baggage, across API boundaries within one process. Propagation is the mechanism that serializes that state into a message going to another process and deserializes it on the other side.

A Context is immutable. Writing a value never changes an existing Context; it returns a new Context that contains the old values plus the new one. Languages with implicit context also offer operations to get the current Context and to attach or detach one, and most users touch context through tracing or baggage APIs rather than directly.

Crossing a process boundary is the job of propagators. The Propagators API currently defines the TextMapPropagator, which injects values into and extracts values from a carrier as string key-value pairs, typically HTTP headers. Inject reads the value from a Context and writes it into a mutable carrier on the outgoing side. Extract reads the carrier on the incoming side and returns a new Context. If a value cannot be parsed, Extract must not throw and must not overwrite an existing valid value. A SpanContext created through extraction reports IsRemote as true, which is how samplers know the parent came from another process. Several propagators can be grouped into a composite propagator that runs them in order.

The default propagator format is W3C Trace Context. Its traceparent header has the form version-traceid-parentid-traceflags, for example 00-a0892f3577b34da6a3ce929d0e0e4736-f03067aa0ba902b7-01. The trace ID is 16 bytes, the span ID 8 bytes, and the trailing flags byte carries the sampled bit, so 01 means sampled. The optional tracestate header carries vendor-specific data. Baggage travels in its own W3C Baggage header and holds application-defined name and value pairs. The Baggage API works even when no SDK is installed, so baggage can still flow through a service that only has a propagator configured.

When an SDK is configured with environment variables, OTEL_PROPAGATORS selects propagators as a comma-separated list and defaults to tracecontext,baggage. Other known values include b3 for the single B3 header, b3multi for the separate X-B3 headers, xray, and none. The jaeger and ottrace values are deprecated in favour of W3C Trace Context. Without any configuration, the bare API uses no-op propagators, so nothing is injected.

Propagation also interacts with sampling. Under the default ParentBased sampler, a span with a sampled remote parent is sampled, and a span with an unsampled remote parent is dropped. Finally, treat incoming context from untrusted sources with care and never put secrets or personal data into baggage, because it is sent to downstream services.