contextpropagation.md

Cross-Process Context Propagation

Following the OpenTracing standard, you must arrange for your application's Tracer to propagate a span context across process boundaries whenever a microservice sends a request to another microservice. Doing so enables you to represent the client's request as part of a continuing trace that consists of multiple connected spans.

The Tracer provides inject and extract methods for propagating span contexts across process boundaries. You can use these methods to propagate a childOf or followsFrom relationship between spans across processes or host boundaries.

• In code that makes an external call, such as an HTTP invocation, obtain the current span and its span context, create a carrier, and inject the span context into the carrier.
  Example:

  currentSpan = ...  // obtain the current span
  TextMap carrier = new TextMapInjectAdapter(new HashMap<>());
  tracer.inject(currentSpan.context(), Format.Builtin.HTTP_HEADERS, carrier); 
  
  // loop over the injected text map and set its contents on the HTTP request header...

• In code that responds to the call, such as receiving the HTTP request, extract the propagated span context.
  Example:

  TextMap carrier = new TextMapExtractAdapter(new HashMap<>());
  SpanContext ctx = tracer.extract(Format.Builtin.HTTP_HEADERS, carrier);
  Span receivingSpan = tracer.buildSpan("httpRequestOperationName").asChildOf(ctx).startActive(true);