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Distributed tracing is additional instrumentation layer on top of new or existing applications. In terms of distributed tracing, span represents a basic unit of work. For example, executing database query is a span. Spans are identified by a unique 128-bit ID. Spans also have other data, such as descriptions, timelines, key-value annotations, the ID of the span that caused them (parent), and process/tracer ID’s (normally IP address and process name). Spans are started and stopped, and they keep track of their timing information. Once span is created, it should be stopped at some point in the future. In turn, trace is a set of spans forming a tree-like structure. For example, if you are running a JAX-RS service, a trace might be formed by a PUT request and downstream work.

From implementation prospectiveperspective, and in context of Java applications, spans are attached to their threads (in general, thread which created the span should close it). However it is possible to transfer spans from thread to thread in order to model a complex execution flows. It is also possible to have many spans in the same thread, as long as they are properly created and closed. In the next sections we are going to see the examples of that.

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