Scopes and Threading


Introduction

In any given thread there is an “active” span primarily responsible for the work accomplished by the surrounding application code, called the ActiveSpan. The OpenTracing API allows for only one span in a thread to be active at any point in time. This is managed using a Scope, which formalizes the activation and deactivation of a Span.

Other spans that are involved with the same thread will satisfy either of the following conditions:

  • Started
  • Not finished
  • Not “active”

For example, there can be multiple spans on the same thread, if the spans are:

  • Waiting for I/O
  • Blocked on a child Span
  • Off of the critical path

Note that if a Scope exists when the developer creates a new Span then it will act as its parent, unless the programmer invokes ignoreActiveSpan() at buildSpan() time or specifies parent context explicitly.

Accessing the Current Active Span

As it is inconvenient to pass an active Span from function to function manually, so OpenTracing requires that every Tracer contain a ScopeManager. The ScopeManager API grants access to the active Span through a Scope. This means that a developer can access any active Span through a Scope.

Moving a span between threads

Using the ScopeManager API, a developer can transfer the spans among different threads. A Span’s lifetime might start in one thread and end in another. The ScopeManager API allows for a Span to be transferred to another thread or callback. Passing of scopes to another thread or callback is not supported. For more details, refer to the language specific documentation.