In the previous exercise, we implemented a distributed context propagation mechanism in our microservices. It is easy to see that neither in-process nor inter-process context propagation mechanisms are very specific to distributed tracing metadata: they are generic enough to pass other types of metadata throughout the call graph of a distributed execution. The OpenTracing authors recognized that fact and defined another general-purpose container for arbitrary metadata called "baggage." The term was originally coined by Prof. Rodrigo Fonseca, one of the authors of the X-Trace system. The OpenTracing baggage is an arbitrary collection of key/value pairs that are defined and used by the application itself. In Chapter 10, Distributed Context Propagation, we will discuss various uses of this mechanism. In this exercise, we want to try it out in the Hello application.
One of the biggest advantages of baggage is that it can be used to pass data through the call graph without...