b3-propagation

• B3 Propagation is a specification for the header “b3” and those that start with “x-b3-“
• used for trace context propagation across service boundaries
• propagated in-process, and eventually downstream (often via http headers)
• should be collected and reported to the tracing system (usually Zipkin) or not.

Identifiers

• Trace identifiers are 64 or 128-bit, but all span identifiers within a trace are 64-bit.
• TraceId: The TraceId is 64 or 128-bit in length and indicates the overall ID of the trace. Every span in a trace shares this ID.
• SpanId: The SpanId is 64-bit in length and indicates the position of the current operation in the trace tree. The value should not be interpreted: it may or may not be derived from the value of the TraceId.
• ParentSpanId: The ParentSpanId is 64-bit in length and indicates the position of the parent operation in the trace tree. When the span is the root of the trace tree, there is no ParentSpanId.

Http Encodings

• There are two encodings of B3
  • Multiple header encoding uses an X-B3- prefixed header per item in the trace context.
  • Single header delimits the context into into a single entry named b3.

Multiple Headers

• B3 attributes are most commonly propagated as multiple http headers. All B3 headers follows the convention of X-B3-${name} with special-casing for flags.
• The X-B3-TraceId header is encoded as 32 or 16 lower-hex characters.
• The X-B3-SpanId header is encoded as 16 lower-hex characters.
• The X-B3-ParentSpanId header may be present on a child span and must be absent on the root span.

Single Header

• A single header named b3 standardized in late 2018 for use in JMS and w3c tracestate.
• In simplest terms b3 maps propagation fields into a hyphen delimited string.
• b3={TraceId}-{SpanId}-{SamplingState}-{ParentSpanId}, where the last two fields are optional. For example, the following state encoded in multiple headers:

  1
  2
  3
  4
  X-B3-TraceId: 80f198ee56343ba864fe8b2a57d3eff7
  X-B3-ParentSpanId: 05e3ac9a4f6e3b90
  X-B3-SpanId: e457b5a2e4d86bd1
  X-B3-Sampled: 1
  

  Becomes one b3 header, for example:

  1
  b3: 80f198ee56343ba864fe8b2a57d3eff7-e457b5a2e4d86bd1-1-05e3ac9a4f6e3b90