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Tracing SDK

Table of Contents

Sampling

Sampling is a mechanism to control the noise and overhead introduced by OpenTelemetry by reducing the number of samples of traces collected and sent to the backend.

Sampling may be implemented on different stages of a trace collection. The earliest sampling could happen before the trace is actually created, and the latest sampling could happen on the Collector which is out of process.

The OpenTelemetry API has two properties responsible for the data collection:

  • IsRecording field of a Span. If false the current Span discards all tracing data (attributes, events, status, etc.). Users can use this property to determine if collecting expensive trace data can be avoided. Span Processor MUST receive only those spans which have this field set to true. However, Span Exporter SHOULD NOT receive them unless the Sampled flag was also set.
  • Sampled flag in TraceFlags on SpanContext. This flag is propagated via the SpanContext to child Spans. For more details see the W3C Trace Context specification. This flag indicates that the Span has been sampled and will be exported. Span Exporters MUST receive those spans which have Sampled flag set to true and they SHOULD NOT receive the ones that do not.

The flag combination SampledFlag == false and IsRecording == true means that the current Span does record information, but most likely the child Span will not.

The flag combination SampledFlag == true and IsRecording == false could cause gaps in the distributed trace, and because of this OpenTelemetry API MUST NOT allow this combination.

The following table summarizes the expected behavior for each combination of IsRecording and SampledFlag.

IsRecording Sampled Flag Span Processor receives Span? Span Exporter receives Span?
true true true true
true false true false
false true Not allowed Not allowed
false false false false

The SDK defines the interface Sampler as well as a set of built-in samplers and associates a Sampler with each [TracerProvider].

When asked to create a Span, the SDK MUST query the Sampler's ShouldSample method before actually creating the span, and act accordingly: see description of ShouldSample's return value below for how to set IsRecording and Sampled on the Span, and the table above on whether to pass the Span to SpanProcessors. A non-recording span MAY be implemented using the same mechanism as when a Span is created with no API-implementation installed (sometimes called a NoOpSpan or DefaultSpan).

Sampler

Sampler interface allows users to create custom samplers which will return a sampling SamplingResult based on information that is typically available just before the Span was created.

ShouldSample

Returns the sampling Decision for a Span to be created.

Required arguments:

  • Parent SpanContext. May be invalid to indicate a root span.
  • TraceId of the Span to be created. If the parent SpanContext contains a valid TraceId, they MUST always match.
  • Name of the Span to be created.
  • SpanKind of the Span to be created.
  • Initial set of Attributes of the Span to be created.
  • Collection of links that will be associated with the Span to be created. Typically useful for batch operations, see Links Between Spans.

Return value:

It produces an output called SamplingResult which contains:

  • A sampling Decision. One of the following enum values:
    • DROP - IsRecording() == false, span will not be recorded and all events and attributes will be dropped.
    • RECORD_ONLY - IsRecording() == true, but Sampled flag MUST NOT be set.
    • RECORD_AND_SAMPLE - IsRecording() == true AND Sampled flag` MUST be set.
  • A set of span Attributes that will also be added to the Span. The returned object must be immutable (multiple calls may return different immutable objects).
  • A Tracestate that will be associated with the Span through the new SpanContext. Note: If the sampler returns an empty Tracestate here, the Tracestate will be cleared, so samplers should normally return the passed-in Tracestate if they do not intend to change it.

GetDescription

Returns the sampler name or short description with the configuration. This may be displayed on debug pages or in the logs. Example: "TraceIdRatioBased{0.000100}".

Description MUST NOT change over time and caller can cache the returned value.

Built-in samplers

OpenTelemetry supports a number of built-in samplers to choose from. The default sampler is ParentBased(root=AlwaysOn).

AlwaysOn

  • Returns RECORD_AND_SAMPLE always.
  • Description MUST be AlwaysOnSampler.

AlwaysOff

  • Returns DROP always.
  • Description MUST be AlwaysOffSampler.

TraceIdRatioBased

  • The TraceIdRatioBased MUST ignore the parent SampledFlag. To respect the parent SampledFlag, the TraceIdRatioBased should be used as a delegate of the ParentBased sampler specified below.
  • Description MUST be TraceIdRatioBased{0.000100}.

TODO: Add details about how the TraceIdRatioBased is implemented as a function of the TraceID.

Requirements for TraceIdRatioBased sampler algorithm
  • The sampling algorithm MUST be deterministic. A trace identified by a given TraceId is sampled or not independent of language, time, etc. To achieve this, implementations MUST use a deterministic hash of the TraceId when computing the sampling decision. By ensuring this, running the sampler on any child Span will produce the same decision.
  • A TraceIdRatioBased sampler with a given sampling rate MUST also sample all traces that any TraceIdRatioBased sampler with a lower sampling rate would sample. This is important when a backend system may want to run with a higher sampling rate than the frontend system, this way all frontend traces will still be sampled and extra traces will be sampled on the backend only.

ParentBased

  • This is a composite sampler. ParentBased helps distinguished between the following cases:
    • No parent (root span).
    • Remote parent (SpanContext.IsRemote() == true) with SampledFlag equals true
    • Remote parent (SpanContext.IsRemote() == true) with SampledFlag equals false
    • Local parent (SpanContext.IsRemote() == false) with SampledFlag equals true
    • Local parent (SpanContext.IsRemote() == false) with SampledFlag equals false

Required parameters:

  • root(Sampler) - Sampler called for spans with no parent (root spans)

Optional parameters:

  • remoteParentSampled(Sampler) (default: AlwaysOn)
  • remoteParentNotSampled(Sampler) (default: AlwaysOff)
  • localParentSampled(Sampler) (default: AlwaysOn)
  • localParentNotSampled(Sampler) (default: AlwaysOff)
Parent parent.isRemote() parent.IsSampled() Invoke sampler
absent n/a n/a root()
present true true remoteParentSampled()
present true false remoteParentNotSampled()
present false true localParentSampled()
present false false localParentNotSampled()

Tracer Creation

New Tracer instances are always created through a TracerProvider (see API). The name and version arguments supplied to the TracerProvider must be used to create an InstrumentationLibrary instance which is stored on the created Tracer.

Configuration (i.e., Span processors and Sampler) MUST be managed solely by the TracerProvider and it MUST provide some way to configure them, at least when creating or initializing it.

The TracerProvider MAY provide methods to update the configuration. If configuration is updated (e.g., adding a SpanProcessor), the updated configuration MUST also apply to all already returned Tracers (i.e. it MUST NOT matter whether a Tracer was obtained from the TracerProvider before or after the configuration change). Note: Implementation-wise, this could mean that Tracer instances have a reference to their TracerProvider and access configuration only via this reference.

Additional Span Interfaces

The API-level definition for Span's interface only defines write-only access to the span. This is good because instrumentations and applications are not meant to use the data stored in a span for application logic. However, the SDK needs to eventually read back the data in some locations. Thus, the SDK specification defines sets of possible requirements for Span-like parameters:

  • Readable span: A function receiving this as argument MUST be able to access all information that was added to the span, as listed in the API spec. In particular, it MUST also be able to access the InstrumentationLibrary and Resource information (implicitly) associated with the span. It must also be able to reliably determine whether the Span has ended (some languages might implement this by having an end timestamp of null, others might have an explicit hasEnded boolean).

    A function receiving this as argument might not be able to modify the Span.

    Note: Typically this will be implemented with a new interface or (immutable) value type. In some languages SpanProcessors may have a different readable span type than exporters (e.g. a SpanData type might contain an immutable snapshot and a ReadableSpan interface might read information directly from the same underlying data structure that the Span interface manipulates).

  • Read/write span: A function receiving this as argument must have access to both the full span API as defined in the API-level definition for span's interface and additionally must be able to retrieve all information that was added to the span (as with readable span).

    It MUST be possible for functions being called with this to somehow obtain the same Span instance and type that the span creation API returned (or will return) to the user (for example, the Span could be one of the parameters passed to such a function, or a getter could be provided).

Span processor

Span processor is an interface which allows hooks for span start and end method invocations. The span processors are invoked only when IsRecording is true.

Built-in span processors are responsible for batching and conversion of spans to exportable representation and passing batches to exporters.

Span processors can be registered directly on SDK TracerProvider and they are invoked in the same order as they were registered.

Each processor registered on TracerProvider is a start of pipeline that consist of span processor and optional exporter. SDK MUST allow to end each pipeline with individual exporter.

SDK MUST allow users to implement and configure custom processors and decorate built-in processors for advanced scenarios such as tagging or filtering.

The following diagram shows SpanProcessor's relationship to other components in the SDK:

  +-----+--------------+   +-------------------------+   +-------------------+
  |     |              |   |                         |   |                   |
  |     |              |   | Batching Span Processor |   |    SpanExporter   |
  |     |              +---> Simple Span Processor   +--->  (JaegerExporter) |
  |     |              |   |                         |   |                   |
  | SDK | Span.start() |   +-------------------------+   +-------------------+
  |     | Span.end()   |
  |     |              |
  |     |              |
  |     |              |
  |     |              |
  +-----+--------------+

Interface definition

OnStart

OnStart is called when a span is started. This method is called synchronously on the thread that started the span, therefore it should not block or throw exceptions.

Parameters:

  • span - a read/write span object for the started span. It SHOULD be possible to keep a reference to this span object and updates to the span SHOULD be reflected in it. For example, this is useful for creating a SpanProcessor that periodically evaluates/prints information about all active span from a background thread.
  • parentContext - the parent Context of the span that the SDK determined (the explicitly passed Context, the current Context or an empty Context if that was explicitly requested).

Returns: Void

OnEnd(Span)

OnEnd is called after a span is ended (i.e., the end timestamp is already set). This method MUST be called synchronously within the Span.End() API, therefore it should not block or throw an exception.

Parameters:

  • Span - a readable span object for the ended span. Note: Even if the passed Span may be technically writable, since it's already ended at this point, modifying it is not allowed.

Returns: Void

Shutdown()

Shuts down the processor. Called when SDK is shut down. This is an opportunity for processor to do any cleanup required.

Shutdown SHOULD be called only once for each SpanProcessor instance. After the call to Shutdown, subsequent calls to OnStart, OnEnd, or ForceFlush are not allowed. SDKs SHOULD ignore these calls gracefully, if possible.

Shutdown SHOULD provide a way to let the caller know whether it succeeded, failed or timed out.

Shutdown MUST include the effects of ForceFlush.

Shutdown SHOULD complete or abort within some timeout. Shutdown can be implemented as a blocking API or an asynchronous API which notifies the caller via a callback or an event. Language library authors can decide if they want to make the shutdown timeout configurable.

ForceFlush()

Exports all spans that have not yet been exported to the configured Exporter.

ForceFlush SHOULD provide a way to let the caller know whether it succeeded, failed or timed out.

ForceFlush SHOULD only be called in cases where it is absolutely necessary, such as when using some FaaS providers that may suspend the process after an invocation, but before the Processor exports the completed spans.

ForceFlush SHOULD complete or abort within some timeout. ForceFlush can be implemented as a blocking API or an asynchronous API which notifies the caller via a callback or an event. Language library authors can decide if they want to make the flush timeout configurable.

Built-in span processors

The standard OpenTelemetry SDK MUST implement both simple and batch processors, as described below. Other common processing scenarios should be first considered for implementation out-of-process in OpenTelemetry Collector

Simple processor

This is an implementation of SpanProcessor which passes finished spans and passes the export-friendly span data representation to the configured SpanExporter, as soon as they are finished.

Configurable parameters:

  • exporter - the exporter where the spans are pushed.

Batching processor

This is an implementation of the SpanProcessor which create batches of finished spans and passes the export-friendly span data representations to the configured SpanExporter.

Configurable parameters:

  • exporter - the exporter where the spans are pushed.
  • maxQueueSize - the maximum queue size. After the size is reached spans are dropped. The default value is 2048.
  • scheduledDelayMillis - the delay interval in milliseconds between two consecutive exports. The default value is 5000.
  • exportTimeoutMillis - how long the export can run before it is cancelled. The default value is 30000.
  • maxExportBatchSize - the maximum batch size of every export. It must be smaller or equal to maxQueueSize. The default value is 512.

Span Exporter

Span Exporter defines the interface that protocol-specific exporters must implement so that they can be plugged into OpenTelemetry SDK and support sending of telemetry data.

The goal of the interface is to minimize burden of implementation for protocol-dependent telemetry exporters. The protocol exporter is expected to be primarily a simple telemetry data encoder and transmitter.

Interface Definition

The exporter must support two functions: Export and Shutdown. In strongly typed languages typically there will be 2 separate Exporter interfaces, one that accepts spans (SpanExporter) and one that accepts metrics (MetricsExporter).

Export(batch)

Exports a batch of readable spans. Protocol exporters that will implement this function are typically expected to serialize and transmit the data to the destination.

Export() will never be called concurrently for the same exporter instance. Export() can be called again only after the current call returns.

Export() must not block indefinitely, there must be a reasonable upper limit after which the call must time out with an error result (Failure).

Any retry logic that is required by the exporter is the responsibility of the exporter. The default SDK SHOULD NOT implement retry logic, as the required logic is likely to depend heavily on the specific protocol and backend the spans are being sent to.

Parameters:

batch - a batch of readable spans. The exact data type of the batch is language specific, typically it is some kind of list, e.g. for spans in Java it will be typically Collection<SpanData>.

Returns: ExportResult:

ExportResult is one of:

  • Success - The batch has been successfully exported. For protocol exporters this typically means that the data is sent over the wire and delivered to the destination server.
  • Failure - exporting failed. The batch must be dropped. For example, this can happen when the batch contains bad data and cannot be serialized.

Note: this result may be returned via an async mechanism or a callback, if that is idiomatic for the language implementation.

Shutdown()

Shuts down the exporter. Called when SDK is shut down. This is an opportunity for exporter to do any cleanup required.

Shutdown should be called only once for each Exporter instance. After the call to Shutdown subsequent calls to Export are not allowed and should return a Failure result.

Shutdown should not block indefinitely (e.g. if it attempts to flush the data and the destination is unavailable). Language library authors can decide if they want to make the shutdown timeout configurable.

Further Language Specialization

Based on the generic interface definition laid out above library authors must define the exact interface for the particular language.

Authors are encouraged to use efficient data structures on the interface boundary that are well suited for fast serialization to wire formats by protocol exporters and minimize the pressure on memory managers. The latter typically requires understanding of how to optimize the rapidly-generated, short-lived telemetry data structures to make life easier for the memory manager of the specific language. General recommendation is to minimize the number of allocations and use allocation arenas where possible, thus avoiding explosion of allocation/deallocation/collection operations in the presence of high rate of telemetry data generation.

Examples

These are examples on what the Exporter interface can look like in specific languages. Examples are for illustration purposes only. Language library authors are free to deviate from these provided that their design remain true to the spirit of Exporter concept.

Go SpanExporter Interface
type SpanExporter interface {
    Export(batch []ExportableSpan) ExportResult
    Shutdown()
}

type ExportResult struct {
    Code         ExportResultCode
    WrappedError error
}

type ExportResultCode int

const (
    Success ExportResultCode = iota
    Failure
)
Java SpanExporter Interface
public interface SpanExporter {
 public enum ResultCode {
   Success, Failure
 }

 ResultCode export(Collection<ExportableSpan> batch);
 void shutdown();
}