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W3C Trace Context

The W3C Trace Context specification defines a means of propagating context information about a specific HTTP transaction through a distributed system for the purposes of observability.

It is intended to replace vendor-specific implementations, which:

  • May not be correlated between vendors due to the lack of agreed identifiers.
  • May be dropped by intermediaries which don't support them.
  • Service providers can't guarantee support as there's no uniform standard.

Concepts

  • Two propagation fields, implemented as HTTP headers:
    • traceparent describes the position of a transaction in a trace graph. It has four hex-encoded, dash (-) delimited components:
      • version specifies the W3C Trace Context specification version.
      • trace-id uniquely identifies a distributed trace.
      • parent-id identifies the parent span.
      • trace-flags:
        • sampled indicates a caller may have recorded trace data.
    • tracestate optionally extends traceparent with key-value pairs. Vendor-specific/proprietary implementation detail is supported as a series of opaque values. These are expressed as a comma-delimited list of key-value pairs, e.g. field=value,otherField=otherValue. Keys may encode a tenant ID at the beginning of each key (some-tenant@field=value). Each key can appear only once, and should be rewritten.
  • Tools can have two levels of support:
    • At a minimum, propagation of the Traceparent and Tracestate headers.
    • Participation, where they modify the headers.
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