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Inject and Extract

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In order to trace across process boundaries and RPC calls in distributed systems, spanContext needs to propagated over the wire. The OpenTracing Go API provides two methods in the Tracer interface to do just that, inject(SpanContext, format, carrier) and extract(format, carrier).

Format Options and Carriers

The format parameter refers to one of the three standard encodings the OpenTracing API defines:

  1. TEXT_MAP where spanContext is encoded as a collection of string key-value pairs,
  2. BINARY where spanContext is encoded as an opaque byte array,
  3. HTTP_HEADERS, which is similar to TEXT_MAP except that the keys must be safe to be used as HTTP headers.

The carrier is an abstraction over the underlying RPC framework. For example, a carrier for TEXT_MAP format is an interface that allows the tracer to write key-value pairs via put(key, value) method, while a carrier for BINARY format is simply a ByteBuffer.

Injecting and Extracting HTTP

When your service calls a downstream service, it’s useful to pass down the SpanContext, so that Spans generated by this service could join the Spans from our service in a single trace. To do that, our service needs to Inject the SpanContext into the payload. On the other side of the connection, the downstream service needs then to Extract the SpanContext before it creates any Spans.

Injecting the spanContext to HTTP

Extracting the Span’s Context from Incoming Request

Injecting and Extracting TextMap

Injecting and Extracting BINARY