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The OpenTracing Semantic Specification

Version: 1.0

Document Overview

This is the "formal" OpenTracing semantic specification. Since OpenTracing must work across many languages, this document takes care to avoid language-specific concepts. That said, there is an understanding throughout that all languages have some concept of an "interface" which encapsulates a set of related capabilities.

Versioning policy

The OpenTracing specification uses a Major.Minor version number but has no .Patch component. The major version increments when backwards-incompatible changes are made to the specification. The minor version increments for non-breaking changes like the introduction of new standard tags, log fields, or SpanContext reference types. (You can read more about the motivation for this versioning scheme at Issue specification#2)

The OpenTracing Data Model

Traces in OpenTracing are defined implicitly by their Spans. In particular, a Trace can be thought of as a directed acyclic graph (DAG) of Spans, where the edges between Spans are called References.

For example, the following is an example Trace made up of 8 Spans:

Causal relationships between Spans in a single Trace

        [Span A]  ←←←(the root span)
     |             |
 [Span B]      [Span C] ←←←(Span C is a `ChildOf` Span A)
     |             |
 [Span D]      +---+-------+
               |           |
           [Span E]    [Span F] >>> [Span G] >>> [Span H]
                         (Span G `FollowsFrom` Span F)

Sometimes it's easier to visualize Traces with a time axis as in the diagram below:

Temporal relationships between Spans in a single Trace

––|–––––––|–––––––|–––––––|–––––––|–––––––|–––––––|–––––––|–> time

 [Span A···················································]
   [Span B··············································]
      [Span D··········································]
    [Span C········································]
         [Span E·······]        [Span F··] [Span G··] [Span H··]

Each Span encapsulates the following state:

  • An operation name
  • A start timestamp
  • A finish timestamp
  • A set of zero or more key:value Span Tags. The keys must be strings. The values may be strings, bools, or numeric types.
  • A set of zero or more Span Logs, each of which is itself a key:value map paired with a timestamp. The keys must be strings, though the values may be of any type. Not all OpenTracing implementations must support every value type.
  • A SpanContext (see below)
  • References to zero or more causally-related Spans (via the SpanContext of those related Spans)

Each SpanContext encapsulates the following state:

  • Any OpenTracing-implementation-dependent state (for example, trace and span ids) needed to refer to a distinct Span across a process boundary
  • Baggage Items, which are just key:value pairs that cross process boundaries

References between Spans

A Span may reference zero or more other SpanContexts that are causally related. OpenTracing presently defines two types of references: ChildOf and FollowsFrom. Both reference types specifically model direct causal relationships between a child Span and a parent Span. In the future, OpenTracing may also support reference types for Spans with non-causal relationships (e.g., Spans that are batched together, Spans that are stuck in the same queue, etc).

ChildOf references: A Span may be the ChildOf a parent Span. In a ChildOf reference, the parent Span depends on the child Span in some capacity. All of the following would constitute ChildOf relationships:

  • A Span representing the server side of an RPC may be the ChildOf a Span representing the client side of that RPC
  • A Span representing a SQL insert may be the ChildOf a Span representing an ORM save method
  • Many Spans doing concurrent (perhaps distributed) work may all individually be the ChildOf a single parent Span that merges the results for all children that return within a deadline

These could all be valid timing diagrams for children that are the ChildOf a parent.

    [-Parent Span---------]
         [-Child Span----]

    [-Parent Span--------------]
         [-Child Span A----]
          [-Child Span B----]
        [-Child Span C----]
         [-Child Span D---------------]
         [-Child Span E----]

FollowsFrom references: Some parent Spans do not depend in any way on the result of their child Spans. In these cases, we say merely that the child Span FollowsFrom the parent Span in a causal sense. There are many distinct FollowsFrom reference sub-categories, and in future versions of OpenTracing they may be distinguished more formally.

These can all be valid timing diagrams for children that "FollowFrom" a parent.

    [-Parent Span-]  [-Child Span-]

    [-Parent Span--]
     [-Child Span-]

    [-Parent Span-]
                [-Child Span-]

The OpenTracing API

There are three critical and inter-related types in the OpenTracing specification: Tracer, Span, and SpanContext. Below, we go through the behaviors of each type; roughly speaking, each behavior becomes a "method" in a typical programming language, though it may actually be a set of related sibling methods due to type overloading and so on.

When we discuss "optional" parameters, it is understood that different languages have different ways to construe such concepts. For example, in Go we might use the "functional Options" idiom, whereas in Java we might use a builder pattern.


The Tracer interface creates Spans and understands how to Inject (serialize) and Extract (deserialize) them across process boundaries. Formally, it has the following capabilities:

Start a new Span

Required parameters

  • An operation name, a human-readable string which concisely represents the work done by the Span (for example, an RPC method name, a function name, or the name of a subtask or stage within a larger computation). The operation name should be the most general string that identifies a (statistically) interesting class of Span instances. That is, "get_user" is better than "get_user/314159".

For example, here are potential operation names for a Span that gets hypothetical account information:

Operation Name Guidance
get Too general
get_account/792 Too specific
get_account Good, and account_id=792 would make a nice Span tag

Optional parameters

  • Zero or more references to related SpanContexts, including a shorthand for ChildOf and FollowsFrom reference types if possible.
  • An optional explicit start timestamp; if omitted, the current walltime is used by default
  • Zero or more tags

Returns a Span instance that's already started (but not Finished)

Inject a SpanContext into a carrier

Required parameters

  • A SpanContext instance
  • A format descriptor (typically but not necessarily a string constant) which tells the Tracer implementation how to encode the SpanContext in the carrier parameter
  • A carrier, whose type is dictated by the format. The Tracer implementation will encode the SpanContext in this carrier object according to the format.

Extract a SpanContext from a carrier

Required parameters

  • A format descriptor (typically but not necessarily a string constant) which tells the Tracer implementation how to decode SpanContext from the carrier parameter
  • A carrier, whose type is dictated by the format. The Tracer implementation will decode the SpanContext from this carrier object according to format.

Returns a SpanContext instance suitable for use as a reference when starting a new Span via the Tracer.

Note: required formats for injection and extraction

Both injection and extraction rely on an extensible format parameter that dictates the type of the associated "carrier" as well as how a SpanContext is encoded in that carrier. All of the following formats must be supported by all Tracer implementations.

  • Text Map: an arbitrary string-to-string map with an unrestricted character set for both keys and values
  • HTTP Headers: a string-to-string map with keys and values that are suitable for use in HTTP headers (a la RFC 7230. In practice, since there is such "diversity" in the way that HTTP headers are treated in the wild, it is strongly recommended that Tracer implementations use a limited HTTP header key space and escape values conservatively.
  • Binary: a (single) arbitrary binary blob representing a SpanContext


With the exception of the method to retrieve the Span's SpanContext, none of the below may be called after the Span is finished.

Retrieve the Spans SpanContext

There should be no parameters.

Returns the SpanContext for the given Span. The returned value may be used even after the Span is finished.

Overwrite the operation name

Required parameters

  • The new operation name, which supersedes whatever was passed in when the Span was started

Finish the Span

Optional parameters

  • An explicit finish timestamp for the Span; if omitted, the current walltime is used implicitly.

With the exception of the method to retrieve a Span's SpanContext, no method may be called on a Span instance after it's finished.

Set a Span tag

Required parameters

  • The tag key, which must be a string
  • The tag value, which must be either a string, a boolean value, or a numeric type

Note that the OpenTracing project documents certain "standard tags" that have prescribed semantic meanings.

Log structured data

Required parameters

  • One or more key:value pairs, where the keys must be strings and the values may have any type at all. Some OpenTracing implementations may handle more (or more of) certain log values than others.

Optional parameters

  • An explicit timestamp. If specified, it must fall between the local start and finish time for the span.

Note that the OpenTracing project documents certain "standard log keys" which have prescribed semantic meanings.

Set a baggage item

Baggage items are key:value string pairs that apply to the given Span, its SpanContext, and all Spans which directly or transitively reference the local Span. That is, baggage items propagate in-band along with the trace itself.

Baggage items enable powerful functionality given a full-stack OpenTracing integration (for example, arbitrary application data from a mobile app can make it, transparently, all the way into the depths of a storage system), and with it some powerful costs: use this feature with care.

Use this feature thoughtfully and with care. Every key and value is copied into every local and remote child of the associated Span, and that can add up to a lot of network and cpu overhead.

Required parameters

  • The baggage key, a string
  • The baggage value, a string

Get a baggage item

Required parameters

  • The baggage key, a string

Returns the corresponding baggage value, xor some indication that such a value was missing.


The SpanContext is more of a "concept" than a useful piece of functionality at the generic OpenTracing layer. That said, it is of critical importance to OpenTracing implementations and does present a thin API of its own. Most OpenTracing users only interact with SpanContext via references when starting new Spans, or when injecting/extracting a trace to/from some transport protocol.

In OpenTracing we force SpanContext instances to be immutable in order to avoid complicated lifetime issues around Span finish and references.

Iterate through all baggage items

This is modeled in different ways depending on the language, but semantically the caller should be able to efficiently iterate through all baggage items in one pass given a SpanContext instance.


All OpenTracing language APIs must also provide some sort of NoopTracer implementation which can be used to flag-control OpenTracing or inject something harmless for tests (et cetera). In some cases (for example, Java) the NoopTracer may be in its own packaging artifact.

Optional API Elements

Some languages also provide utilities to pass an active Span and/or SpanContext around a single process. For instance, opentracing-go provides helpers to set and get the active Span in Go's context.Context mechanism.