We discussed in Chapter 3, Distributed Tracing Fundamentals, that most tracing systems today use causal metadata propagation, also known as distributed context propagation, as the fundamental substrate for associating tracing events with individual executions. Unlike the trace data collection, the context propagation mechanism is always on for 100% of requests, regardless of the sampling decisions. In Chapter 4, Instrumentation Basics with OpenTracing, and Chapter 5, Instrumentation of Asynchronous Applications, we have seen that tracing instrumentation APIs, like the OpenTracing APIs, include primitives for implementing context propagation in the applications.
In other chapters, we used OpenTracing baggage, a form of general-purpose context propagation, to implement functionality completely unrelated to tracing: collecting metrics about a specific subset of requests (for example, from a given customer in the HotROD demo in Chapter 2, Take Tracing...