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Tracing API

Table of Contents

Tracing API consist of a few main classes:

  • Tracer is used for all operations. See Tracer section.
  • Span is a mutable object storing information about the current operation execution. See Span section.

Data types

While languages and platforms have different ways of representing data, this section defines some generic requirements for this API.

Time

OpenTelemetry can operate on time values up to nanosecond (ns) precision. The representation of those values is language specific.

Timestamp

A timestamp is the time elapsed since the Unix epoch.

  • The minimal precision is milliseconds.
  • The maximal precision is nanoseconds.

Duration

A duration is the elapsed time between two events.

  • The minimal precision is milliseconds.
  • The maximal precision is nanoseconds.

Tracer

The OpenTelemetry library achieves in-process context propagation of Spans by way of the Tracer.

The Tracer is responsible for tracking the currently active Span, and exposes functions for creating and activating new Spans. The Tracer is configured with Propagators which support transferring span context across process boundaries.

Obtaining a Tracer

New Tracer instances can be created via a TracerProvider and its getTracer function. This function expects two string arguments:

TracerProviders are generally expected to be used as singletons. Implementations SHOULD provide a single global default TracerProvider.

Some applications may use multiple TracerProvider instances, e.g. to provide different settings (e.g. SpanProcessors) to each of those instances and - in further consequence - to the Tracer instances created by them.

  • name (required): This name must identify the instrumentation library (e.g. io.opentelemetry.contrib.mongodb) and not the instrumented library. In case an invalid name (null or empty string) is specified, a working default Tracer implementation as a fallback is returned rather than returning null or throwing an exception. A library, implementing the OpenTelemetry API may also ignore this name and return a default instance for all calls, if it does not support "named" functionality (e.g. an implementation which is not even observability-related). A TracerProvider could also return a no-op Tracer here if application owners configure the SDK to suppress telemetry produced by this library.
  • version (optional): Specifies the version of the instrumentation library (e.g. semver:1.0.0).

Implementations might require the user to specify configuration properties at TracerProvider creation time, or rely on external configuration, e.g. when using the provider pattern.

Runtimes with multiple deployments/applications

Runtimes that support multiple deployments or applications might need to provide a different TracerProvider instance to each deployment. To support this, the global TracerProvider registry may delegate calls to create new instances of TracerProvider to a separate Provider component, and the runtime may include its own Provider implementation which returns a different TracerProvider for each deployment.

Provider instances are registered with the API via some language-specific mechanism, for instance the ServiceLoader class in Java.

Tracer operations

The Tracer MUST provide functions to:

  • Create a new Span

The Tracer SHOULD provide methods to:

  • Get the currently active Span
  • Make a given Span as active

The Tracer MUST internally leverage the Context in order to get and set the current Span state and how Spans are passed across process boundaries.

When getting the current span, the Tracer MUST return a placeholder Span with an invalid SpanContext if there is no currently active Span.

When creating a new Span, the Tracer MUST allow the caller to specify the new Span's parent in the form of a Span or SpanContext. The Tracer SHOULD create each new Span as a child of its active Span unless an explicit parent is provided or the option to create a span without a parent is selected, or the current active Span is invalid.

The Tracer SHOULD provide a way to update its active Span and MAY provide convenience functions to manage a Span's lifetime and the scope in which a Span is active. When an active Span is made inactive, the previously-active Span SHOULD be made active. A Span maybe finished (i.e. have a non-null end time) but still active. A Span may be active on one thread after it has been made inactive on another.

SpanContext

A SpanContext represents the portion of a Span which must be serialized and propagated along side of a distributed context. SpanContexts are immutable. SpanContext MUST be a final (sealed) class.

The OpenTelemetry SpanContext representation conforms to the w3c TraceContext specification. It contains two identifiers - a TraceId and a SpanId - along with a set of common TraceFlags and system-specific TraceState values.

TraceId A valid trace identifier is a 16-byte array with at least one non-zero byte.

SpanId A valid span identifier is an 8-byte array with at least one non-zero byte.

TraceFlags contain details about the trace. Unlike Tracestate values, TraceFlags are present in all traces. Currently, the only TraceFlags is a boolean sampled flag.

Tracestate carries system-specific configuration data, represented as a list of key-value pairs. TraceState allows multiple tracing systems to participate in the same trace.

IsValid is a boolean flag which returns true if the SpanContext has a non-zero TraceID and a non-zero SpanID.

IsRemote is a boolean flag which returns true if the SpanContext was propagated from a remote parent. When creating children from remote spans, their IsRemote flag MUST be set to false.

Please review the W3C specification for details on the Tracestate field.

Span

A Span represents a single operation within a trace. Spans can be nested to form a trace tree. Each trace contains a root span, which typically describes the end-to-end latency and, optionally, one or more sub-spans for its sub-operations.

Spans encapsulate:

The span name is a human-readable string which concisely identifies the work represented 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 span name should be the most general string that identifies a (statistically) interesting class of Spans, rather than individual Span instances. That is, "get_user" is a reasonable name, while "get_user/314159", where "314159" is a user ID, is not a good name due to its high cardinality.

For example, here are potential span names for an endpoint that gets a hypothetical account information:

Span Name Guidance
get Too general
get_account/42 Too specific
get_account Good, and account_id=42 would make a nice Span attribute
get_account/{accountId} Also good (using the "HTTP route")

The Span's start and end timestamps reflect the elapsed real time of the operation. A Span's start time SHOULD be set to the current time on span creation. After the Span is created, it SHOULD be possible to change the its name, set its Attributes, and add Links and Events. These MUST NOT be changed after the Span's end time has been set.

Spans are not meant to be used to propagate information within a process. To prevent misuse, implementations SHOULD NOT provide access to a Span's attributes besides its SpanContext.

Vendors may implement the Span interface to effect vendor-specific logic. However, alternative implementations MUST NOT allow callers to create Spans directly. All Spans MUST be created via a Tracer.

Span Creation

Implementations MUST provide a way to create Spans via a Tracer. By default, the currently active Span is set as the new Span's parent. The Tracer MAY provide other default options for newly created Spans.

Span creation MUST NOT set the newly created Span as the currently active Span by default, but this functionality MAY be offered additionally as a separate operation.

The API MUST accept the following parameters:

  • The span name. This is a required parameter.

  • The parent Span or a Context containing a parent Span or SpanContext, and whether the new Span should be a root Span. API MAY also have an option for implicit parenting from the current context as a default behavior. See Determining the Parent Span from a Context for guidance on Span parenting from explicit and implicit Contexts.

  • SpanKind, default to SpanKind.Internal if not specified.

  • Attributes - A collection of key-value pairs, with the same semantics as the ones settable with Span::SetAttributes. Additionally, these attributes may be used to make a sampling decision as noted in sampling description. An empty collection will be assumed if not specified.

    Whenever possible, users SHOULD set any already known attributes at span creation instead of calling SetAttribute later.

  • Links - see API definition here. Empty list will be assumed if not specified.

  • Start timestamp, default to current time. This argument SHOULD only be set when span creation time has already passed. If API is called at a moment of a Span logical start, API user MUST not explicitly set this argument.

Each span has zero or one parent span and zero or more child spans, which represent causally related operations. A tree of related spans comprises a trace. A span is said to be a root span if it does not have a parent. Each trace includes a single root span, which is the shared ancestor of all other spans in the trace. Implementations MUST provide an option to create a Span as a root span, and MUST generate a new TraceId for each root span created. For a Span with a parent, the TraceId MUST be the same as the parent. Also, the child span MUST inherit all TraceState values of its parent by default.

A Span is said to have a remote parent if it is the child of a Span created in another process. Each propagators' deserialization must set IsRemote to true on a parent SpanContext so Span creation knows if the parent is remote.

Determining the Parent Span from a Context

When a new Span is created from a Context, the Context may contain:

  • A current Span
  • An extracted SpanContext
  • A current Span and an extracted SpanContext
  • Neither a current Span nor an extracted Span context

The parent should be selected in the following order of precedence:

  • Use the current Span, if available.
  • Use the extracted SpanContext, if available.
  • There is no parent. Create a root Span.

Add Links

During the Span creation user MUST have the ability to record links to other Spans. Linked Spans can be from the same or a different trace. See Links description.

A Link is defined by the following properties:

  • (Required) SpanContext of the Span to link to.
  • (Optional) One or more Attributes with the same restrictions as defined for Span Attributes.

The Link SHOULD be an immutable type.

The Span creation API should provide:

  • An API to record a single Link where the Link properties are passed as arguments. This MAY be called AddLink.
  • An API to record a single Link whose attributes or attribute values are lazily constructed, with the intention of avoiding unnecessary work if a link is unused. If the language supports overloads then this SHOULD be called AddLink otherwise AddLazyLink MAY be considered. In some languages, it might be easier to defer Link or attribute creation entirely by providing a wrapping class or function that returns a Link or formatted attributes. When providing a wrapping class or function it SHOULD be named LinkFormatter.

Links SHOULD preserve the order in which they're set.

Span operations

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

Get Context

The Span interface MUST provide:

  • An API that returns the SpanContext for the given Span. The returned value may be used even after the Span is finished. The returned value MUST be the same for the entire Span lifetime. This MAY be called GetContext.

IsRecording

Returns true if this Span is recording information like events with the AddEvent operation, attributes using SetAttributes, status with SetStatus, etc.

There should be no parameter.

This flag SHOULD be used to avoid expensive computations of a Span attributes or events in case when a Span is definitely not recorded. Note that any child span's recording is determined independently from the value of this flag (typically based on the sampled flag of a TraceFlag on SpanContext).

This flag may be true despite the entire trace being sampled out. This allows to record and process information about the individual Span without sending it to the backend. An example of this scenario may be recording and processing of all incoming requests for the processing and building of SLA/SLO latency charts while sending only a subset - sampled spans - to the backend. See also the sampling section of SDK design.

Users of the API should only access the IsRecording property when instrumenting code and never access SampledFlag unless used in context propagators.

Set Attributes

A Span MUST have the ability to set attributes associated with it.

An Attribute is defined by the following properties:

  • (Required) The attribute key, which MUST be a non-null and non-empty string.
  • (Required) The attribute value, which is either:
    • A primitive type: string, boolean or numeric.
    • An array of primitive type values. The array MUST be homogeneous, i.e. it MUST NOT contain values of different types.

The Span interface MUST provide:

  • An API to set a single Attribute where the attribute properties are passed as arguments. This MAY be called SetAttribute. To avoid extra allocations some implementations may offer a separate API for each of the possible value types.

Attributes SHOULD preserve the order in which they're set. Setting an attribute with the same key as an existing attribute SHOULD overwrite the existing attribute's value.

Attribute values expressing a numerical value of zero or an empty string are considered meaningful and MUST be stored and passed on to span processors / exporters. Attribute values of null are considered to be not set and get discarded as if that SetAttribute call had never been made. As an exception to this, if overwriting of values is supported, this results in clearing the previous value and dropping the attribute key from the set of attributes.

null values within arrays MUST be preserved as-is (i.e., passed on to span processors / exporters as null). If exporters do not support exporting null values, they MAY replace those values by 0, false, or empty strings. This is required for map/dictionary structures represented as two arrays with indices that are kept in sync (e.g., two attributes header_keys and header_values, both containing an array of strings to represent a mapping header_keys[i] -> header_values[i]).

Note that the OpenTelemetry project documents certain "standard attributes" that have prescribed semantic meanings.

Add Events

A Span MUST have the ability to add events. Events have a time associated with the moment when they are added to the Span.

An Event is defined by the following properties:

  • (Required) Name of the event.
  • (Optional) One or more Attributes with the same restrictions as defined for Span Attributes.
  • (Optional) Timestamp for the event.

The Event SHOULD be an immutable type.

The Span interface MUST provide:

  • An API to record a single Event where the Event properties are passed as arguments. This MAY be called AddEvent.
  • An API to record a single Event whose attributes or attribute values are lazily constructed, with the intention of avoiding unnecessary work if an event is unused. If the language supports overloads then this SHOULD be called AddEvent otherwise AddLazyEvent MAY be considered. In some languages, it might be easier to defer Event or attribute creation entirely by providing a wrapping class or function that returns an Event or formatted attributes. When providing a wrapping class or function it SHOULD be named EventFormatter.

Events SHOULD preserve the order in which they're set. This will typically match the ordering of the events' timestamps.

Note that the OpenTelemetry project documents certain "standard event names and keys" which have prescribed semantic meanings.

Set Status

Sets the Status of the Span. If used, this will override the default Span status, which is OK.

Only the value of the last call will be recorded, and implementations are free to ignore previous calls.

The Span interface MUST provide:

  • An API to set the Status where the new status is the only argument. This SHOULD be called SetStatus.

UpdateName

Updates the Span name. Upon this update, any sampling behavior based on Span name will depend on the implementation.

It is highly discouraged to update the name of a Span after its creation. Span name is often used to group, filter and identify the logical groups of spans. And often, filtering logic will be implemented before the Span creation for performance reasons. Thus the name update may interfere with this logic.

The function name is called UpdateName to differentiate this function from the regular property setter. It emphasizes that this operation signifies a major change for a Span and may lead to re-calculation of sampling or filtering decisions made previously depending on the implementation.

Alternatives for the name update may be late Span creation, when Span is started with the explicit timestamp from the past at the moment where the final Span name is known, or reporting a Span with the desired name as a child Span.

Required parameters:

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

End

Finish the Span. This call will take the current timestamp to set as Span's end time. Implementations MUST ignore all subsequent calls to End (there might be exceptions when Tracer is streaming event and has no mutable state associated with the Span).

Call to End of a Span MUST not have any effects on child spans. Those may still be running and can be ended later.

Parameters:

  • (Optional) Timestamp to explicitly set the end timestamp

This API MUST be non-blocking.

Span lifetime

Span lifetime represents the process of recording the start and the end timestamps to the Span object:

  • The start time is recorded when the Span is created.
  • The end time needs to be recorded when the operation is ended.

Start and end time as well as Event's timestamps MUST be recorded at a time of a calling of corresponding API.

Status

Status interface represents the status of a finished Span. It's composed of a canonical code in conjunction with an optional descriptive message.

StatusCanonicalCode

StatusCanonicalCode represents the canonical set of status codes of a finished Span, following the Standard GRPC codes:

  • Ok
    • The operation completed successfully.
  • Cancelled
    • The operation was cancelled (typically by the caller).
  • Unknown
    • An unknown error.
  • InvalidArgument
    • Client specified an invalid argument. Note that this differs from FailedPrecondition. InvalidArgument indicates arguments that are problematic regardless of the state of the system.
  • DeadlineExceeded
    • Deadline expired before operation could complete. For operations that change the state of the system, this error may be returned even if the operation has completed successfully.
  • NotFound
    • Some requested entity (e.g., file or directory) was not found.
  • AlreadyExists
    • Some entity that we attempted to create (e.g., file or directory) already exists.
  • PermissionDenied
    • The caller does not have permission to execute the specified operation. PermissionDenied must not be used if the caller cannot be identified (use Unauthenticated1 instead for those errors).
  • ResourceExhausted
    • Some resource has been exhausted, perhaps a per-user quota, or perhaps the entire file system is out of space.
  • FailedPrecondition
    • Operation was rejected because the system is not in a state required for the operation's execution.
  • Aborted
    • The operation was aborted, typically due to a concurrency issue like sequencer check failures, transaction aborts, etc.
  • OutOfRange
    • Operation was attempted past the valid range. E.g., seeking or reading past end of file. Unlike InvalidArgument, this error indicates a problem that may be fixed if the system state changes.
  • Unimplemented
    • Operation is not implemented or not supported/enabled in this service.
  • Internal
    • Internal errors. Means some invariants expected by underlying system has been broken.
  • Unavailable
    • The service is currently unavailable. This is a most likely a transient condition and may be corrected by retrying with a backoff.
  • DataLoss
    • Unrecoverable data loss or corruption.
  • Unauthenticated
    • The request does not have valid authentication credentials for the operation.

Status creation

API MUST provide a way to create a new Status.

Required parameters

  • StatusCanonicalCode of this Status.

Optional parameters

  • Description of this Status.

GetCanonicalCode

Returns the StatusCanonicalCode of this Status.

GetDescription

Returns the description of this Status. Languages should follow their usual conventions on whether to return null or an empty string here if no description was given.

GetIsOk

Returns true if the canonical code of this Status is Ok, otherwise false.

SpanKind

SpanKind describes the relationship between the Span, its parents, and its children in a Trace. SpanKind describes two independent properties that benefit tracing systems during analysis.

The first property described by SpanKind reflects whether the Span is a remote child or parent. Spans with a remote parent are interesting because they are sources of external load. Spans with a remote child are interesting because they reflect a non-local system dependency.

The second property described by SpanKind reflects whether a child Span represents a synchronous call. When a child span is synchronous, the parent is expected to wait for it to complete under ordinary circumstances. It can be useful for tracing systems to know this property, since synchronous Spans may contribute to the overall trace latency. Asynchronous scenarios can be remote or local.

In order for SpanKind to be meaningful, callers should arrange that a single Span does not serve more than one purpose. For example, a server-side span should not be used directly as the parent of another remote span. As a simple guideline, instrumentation should create a new Span prior to extracting and serializing the span context for a remote call.

These are the possible SpanKinds:

  • SERVER Indicates that the span covers server-side handling of a synchronous RPC or other remote request. This span is the child of a remote CLIENT span that was expected to wait for a response.
  • CLIENT Indicates that the span describes a synchronous request to some remote service. This span is the parent of a remote SERVER span and waits for its response.
  • PRODUCER Indicates that the span describes the parent of an asynchronous request. This parent span is expected to end before the corresponding child CONSUMER span, possibly even before the child span starts. In messaging scenarios with batching, tracing individual messages requires a new PRODUCER span per message to be created.
  • CONSUMER Indicates that the span describes the child of an asynchronous PRODUCER request.
  • INTERNAL Default value. Indicates that the span represents an internal operation within an application, as opposed to an operations with remote parents or children.

To summarize the interpretation of these kinds:

SpanKind Synchronous Asynchronous Remote Incoming Remote Outgoing
CLIENT yes yes
SERVER yes yes
PRODUCER yes maybe
CONSUMER yes maybe
INTERNAL