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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. OpenTelemetry API defines a Sampler interface that can be used at instrumentation points by libraries to check the sampling Decision early and optimize the amount of telemetry that needs to be collected.

All other sampling algorithms may be implemented on SDK layer in exporters, or even out of process in Agent or Collector.

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

  • IsRecording field of a Span. If true the current Span records tracing events (attributes, events, status, etc.), otherwise all tracing events are dropped. Users can use this property to determine if expensive trace events can be avoided. Span Processors will receive all spans with this flag set. However, Span Exporter will not receive them unless the Sampled flag was set.
  • Sampled flag in TraceFlags on SpanContext. This flag is propagated via the SpanContext to child Spans. For more details see the [W3C specification][trace-flags]. This flag indicates that the Span has been sampled and will be exported. Span Processor and Span Exporter will receive spans with the Sampled flag set for processing.

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 SDK defines the two interfaces Sampler and Decision as well as a set of built-in samplers.

Sampler

Sampler interface allows 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:

  • SpanContext of a parent Span. Typically extracted from the wire. Can be null.
  • TraceId of the Span to be created. It can be different from the TraceId in the SpanContext. Typically in situations when the Span to be created starts a new Trace.
  • SpanId of the Span to be created.
  • Name of the Span to be created.
  • SpanKind
  • Initial set of Attributes for the Span being constructed
  • 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:
    • NOT_RECORD - IsRecording() == false, span will not be recorded and all events and attributes will be dropped.
    • RECORD - IsRecording() == true, but Sampled flag MUST NOT be set.
    • RECORD_AND_SAMPLED - IsRecording() == true AND Sampled flag` MUST be set.
  • A set of span Attributes that will also be added to the Span.
    • The list of attributes returned by SamplingResult MUST be immutable. Caller may call this method any number of times and can safely cache the returned value.

GetDescription

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

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

Built-in samplers

These are the default samplers implemented in the OpenTelemetry SDK:

  • ALWAYS_ON
    • This will be used as a default.
    • Description MUST be AlwaysOnSampler.
  • ALWAYS_OFF
    • Description MUST be AlwaysOffSampler.
  • ALWAYS_PARENT
    • Returns RECORD_AND_SAMPLED if SampledFlag is set to true on parent SpanContext and NOT_RECORD otherwise.
    • Description MUST be AlwaysParentSampler.
  • Probability
    • The default behavior should be to trust the parent SampledFlag. However there should be configuration to change this.
    • The default behavior is to apply the sampling probability only for Spans that are root spans (no parent) and Spans with remote parent. However there should be configuration to change this to "root spans only", or "all spans".
    • Description MUST be ProbabilitySampler{0.000100}.

Probability Sampler algorithm

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

Tracer Creation

New Tracer instances are always created through a TracerFactory (see API). The name and version arguments supplied to the TracerFactory must be used to create a Resource instance which is stored on the created Tracer. The readable representations of all Span instances created by a Tracer must provide a getLibraryResource method that returns this Resource information held by the Tracer.

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. This interface must be used to implement span exporter to batch and convert spans.

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

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

  +-----+---------------+   +---------------------+   +-------------------+
  |     |               |   |                     |   |                   |
  |     |               |   | BatchProcessor      |   |    SpanExporter   |
  |     |               +---> SimpleProcessor     +--->  (JaegerExporter) |
  | SDK | SpanProcessor |   |                     |   |                   |
  |     |               |   +---------------------+   +-------------------+
  |     |               |
  |     |               |   +---------------------+
  |     |               |   |                     |
  |     |               +---> ZPagesProcessor     |
  |     |               |   |                     |
  +-----+---------------+   +---------------------+

Interface definition

OnStart(Span)

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 readable span object.

Returns: Void

OnEnd(Span)

OnEnd is called when a span is ended. This method is called synchronously on the execution thread, therefore it should not block or throw an exception.

Parameters:

  • Span - a readable span object.

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 Processor instance. After the call to shutdown subsequent calls to onStart or onEnd are not allowed.

Shutdown should not block indefinitely. Language library authors can decide if they want to make the shutdown timeout to be configurable.

Built-in span processors

Simple processor

The implementation of SpanProcessor that passes ended span directly to the configured SpanExporter.

Configurable parameters:

  • exporter - the exporter where the spans are pushed.
Batching processor

The implementation of the SpanProcessor that batches ended spans and pushes them to the configured SpanExporter.

First the spans are added to a synchronized queue, then exported to the exporter pipeline in batches. The implementation is responsible for managing the span queue and sending batches of spans to the exporters. This processor can cause high contention in a very high traffic service.

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.
  • 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 goals of the interface are:

  • Minimize burden of implementation for protocol-dependent telemetry exporters. The protocol exporter is expected to be primarily a simple telemetry data encoder and transmitter.
  • Allow implementing helpers as composable components that use the same chainable Exporter interface. SDK authors are encouraged to implement common functionality such as queuing, batching, tagging, etc. as helpers. This functionality will be applicable regardless of what protocol exporter is used.

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 telemetry data. 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 (typically FailedRetryable).

Parameters:

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

Note that the data type for a span for illustration purposes here is written as an imaginary type ExportableSpan (similarly for metrics it would be e.g. ExportableMetrics). The actual data type must be specified by language library authors, it should be able to represent the span data that can be read by the exporter.

Returns: ExportResult:

ExportResult is one of:

  • Success - batch is successfully exported. For protocol exporters this typically means that the data is sent over the wire and delivered to the destination server.
  • FailedNotRetryable - exporting failed. The caller must not retry exporting the same batch. The batch must be dropped. This for example can happen when the batch contains bad data and cannot be serialized.
  • FailedRetryable - cannot export to the destination. The caller should record the error and may retry exporting the same batch after some time. This for example can happen when the destination is unavailable, there is a network error or endpoint does not exist.
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 FailedNotRetryable error.

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 to be 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
    FailedNotRetryable
    FailedRetryable
)
Java SpanExporter Interface
public interface SpanExporter {
 public enum ResultCode {
   Success, FailedNotRetryable, FailedRetryable
 }

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