This repository elaborates identifiers used to place an operation in a trace tree. These attributes are propagated in-process, and eventually downstream (often via http headers), to ensure all activity originating from the same root are collected together. A sampling decision is made at the root of the trace, and this indicates if trace details should be collected and reported to the tracing system (usually Zipkin) or not.
The most propagation use case is to copy a trace context from a client sending an RPC request to a server receiving it.
In this case, the same trace IDs are used, which means that both the client and server side of an operation end up in the same node in the trace tree.
Here's an example flow, assuming an HTTP request carries the propagated trace:
Client Tracer Server Tracer
┌──────────────────┐ ┌──────────────────┐
│ │ │ │
│ TraceContext │ Http Request Headers │ TraceContext │
│ ┌──────────────┐ │ ┌───────────────────┐ │ ┌──────────────┐ │
│ │ TraceId │ │ │ X─B3─TraceId │ │ │ TraceId │ │
│ │ │ │ │ │ │ │ │ │
│ │ ParentSpanId │ │ Extract │ X─B3─ParentSpanId │ Inject │ │ ParentSpanId │ │
│ │ ├─┼─────────>│ ├────────┼>│ │ │
│ │ SpanId │ │ │ X─B3─SpanId │ │ │ SpanId │ │
│ │ │ │ │ │ │ │ │ │
│ │ Sampled │ │ │ X─B3─Sampled │ │ │ Sampled │ │
│ └──────────────┘ │ └───────────────────┘ │ └──────────────┘ │
│ │ │ │
└──────────────────┘ └──────────────────┘
Trace identifiers are 64 or 128-bit, but all span identifiers within a trace are 64-bit. All identifiers are opaque.
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.
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.
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, the ParentSpanId is absent.
The following flags are reported either in a flag set or separate attributes.
When the Sampled flag is 1, report this span to the tracing system. When it is 0, do not. When B3 attributes are sent without the Sampled flag, the receiver should make the decision. Once Sampled is set to 0 or 1, the same value should be consistently sent downstream.
It may not be obvious why you'd send Sampled=0 to the next hop. Imagine a service decides not to trace an operation and makes 2 out-going calls, and these branched out further. If 0 ("don't trace") isn't propagated, the system might receive only parts of the operation, confusing users.
Leaving Sampled absent is special-case. The only known use-cases are the following:
- Debug trace: When setting Flags to 1, sampling is implicit
- Externally provisioned IDs: When you want to control IDs, but not sampling policy
Unless it is a debug trace, leaving sampled unset is typically for ID correlation. For example, someone re-uses a global identifier from another system, or correlating in logs. In these cases, the caller knows the ID they want, but allows the next hop to decide if it will be traced or not. The caller should not report a span to the tracing system using this ID unless they propagate Sampled=1.
When Debug is set, the trace should be reported to the tracing system and also override any collection-tier sampling policy. Debug implies Sampled.
B3 attributes are most commonly propagated as Http headers. All B3 headers follows the convention of X-B3-${name} with special-casing for flags. When reading headers, the first value wins.
The X-B3-TraceId header is required and is encoded as 32 or 16 hex characters. For example, a 128-bit TraceId header might look like: X-B3-TraceId: 463ac35c9f6413ad48485a3953bb6124
The X-B3-SpanId header is required and is encoded as 16 hex characters. For example, a SpanId header might look like: X-B3-SpanId: a2fb4a1d1a96d312
The X-B3-ParentSpanId header must be present on a child span and absent on the root span. It is encoded as 16 hex characters. For example, a ParentSpanId header might look like: X-B3-ParentSpanId: 0020000000000001
The X-B3-Sampled header is encoded as "1" or "0". Absent means defer the decision to the receiver of this header. For example, a Sampled header might look like: X-B3-Sampled: 1
Debug is encoded as X-B3-Flags: 1. Since Debug implies Sampled, so don't also send "X-B3-Sampled: 1".
B3 attributes can also be propagated as ASCII headers in the Custom Metadata of a request. The encoding is exactly the same as Http headers, except the names are explicitly or implicitly down-cased.
For example, the Http header X-B3-ParentSpanId: 0020000000000001 would become
an ASCII header x-b3-parentspanid with the same value.
In B3, the trace context is extracted from incoming headers. Timing and
metadata for the client and server side of an operation are recorded with
the same context. The ParentSpanId is the ID of the operation that caused
the current RPC. For example, it could be the ID of another server request
or a scheduled job. ParentSpanId is propagated so that when data is reported
to Zipkin, it can be placed in the correct spot in the trace tree.
Here's an example of a B3 library extracting a trace context from incoming http request headers:
┌───────────────────┐
Incoming Headers │ TraceContext │
┌───────────────────┐ │ ┌───────────────┐ │
│ X─B3-TraceId │──────┼─┼> TraceId │ │
│ │ │ │ │ │
│ X─B3-ParentSpanId │──────┼─┼> ParentSpanId │ │
│ │ │ │ │ │
│ X─B3-SpanId │──────┼─┼> SpanId │ │
│ │ │ └───────────────┘ │
└───────────────────┘ └───────────────────┘
Some propagation formats look similar to B3, but don't propagate a field named
parent. Instead, they propagate a span ID field which serves the same purpose
as ParentSpanId. Unlike B3, these systems use a different span ID for the
client and server side of an RPC. When a server reads headers like this, it is
expected to provision a new span ID for itself, and use the one it extracted as
its parent.
Here's an example of an alternate library composing a trace context with incoming http request headers and an ID generator:
┌───────────────────┐
Incoming Headers │ TraceContext │
┌───────────────────┐ │ ┌───────────────┐ │
│ XXXX─TraceId │──────┼─┼> TraceId │ │
│ │ │ │ │ │
│ XXXX─SpanId │──────┼─┼> ParentSpanId │ │
└───────────────────┘ │ │ │ │ ┌──────────────┐
│ │ SpanId <┼─┼──────│ ID Generator │
│ └───────────────┘ │ └──────────────┘
└───────────────────┘
In both B3 and the above example, incoming headers contain the parent's span ID, and three IDs (trace, parent, span) end up in the trace context. The difference is that B3 uses the same span ID for the client and server side of an RPC, where the latter does not.