OpenTracing Compatibility

Status: Stable.

Abstract

The OpenTelemetry project aims to provide backwards compatibility with the OpenTracing project in order to ease migration of instrumented codebases.

This functionality will be provided as a bridge layer implementing the OpenTracing API using the OpenTelemetry API. This layer MUST NOT rely on implementation specific details of any SDK.

More specifically, the intention is to allow OpenTracing instrumentation to be recorded using OpenTelemetry. This Shim Layer MUST NOT publicly expose any upstream OpenTelemetry API.

This functionality MUST be defined in its own OpenTracing Shim Layer, not in the OpenTracing nor the OpenTelemetry API or SDK.

Semantic convention mapping SHOULD NOT be performed, with the exception of error mapping, as described in the Set Tag and Log sections.

Consuming both the OpenTracing Shim and the OpenTelemetry API in the same codebase is not recommended for the following scenarios:

If the OpenTracing-instrumented code consumes baggage, as the Baggage itself may not be properly propagated. See Span Shim and SpanContext Shim relationship.
For languages with implicit in-process propagation support in OpenTelemetry and none in OpenTracing (e.g. Javascript), as it breaks the expected propagation semantics and may lead to incorrect Context usage and incorrect traces. See Implicit and Explicit support mismatch.

Language version support

Users are encouraged to check and update their language and runtime components before using the Shim layer, as the OpenTelemetry APIs and SDKs may have higher version requirements than their OpenTracing counterparts.

For details, see the Language version support section of Migrating from OpenTracing.

Create an OpenTracing Tracer Shim

This operation is used to create a new OpenTracing Tracer:

This operation MUST accept the following parameters:

An OpenTelemetry TracerProvider. This operation MUST use this TracerProvider to obtain a Tracer with the name opentracing-shim along with the current shim library version.
OpenTelemetry Propagators to be used to perform injection and extraction for the the OpenTracing TextMap and HTTPHeaders formats. If not specified, no Propagator values will be stored in the Shim, and the global OpenTelemetry TextMap propagator will be used for both OpenTracing TextMap and HTTPHeaders formats.

The API MUST return an OpenTracing Tracer.

// Create a Tracer Shim relying on the global propagators.
createTracerShim(tracerProvider);

// Create a Tracer Shim using:
// 1) TraceContext propagator for TextMap
// 2) Jaeger propagator for HttPHeaders.
createTracerShim(tracerProvider, OTPropagatorsBuilder()
  .setTextMap(W3CTraceContextPropagator.getInstance())
  .setHttpHeaders(JaegerPropagator.getInstance())
  .build());

See OpenTracing Propagation Formats.

Tracer Shim

Start a new Span

Parameters:

The operation name, a string.
An optional list of Span references.
An optional list of tags.
An optional explicit start timestamp, a numeric value.

For OpenTracing languages implementing the ScopeManager interface, the following parameters are defined as well:

An optional boolean specifying whether the current Span should be ignored as automatic parent.

If a list of Span references is specified, the first SpanContext with Child Of type in the entire list is used as parent, else the the first SpanContext is used as parent. All values in the list MUST be added as Links with the reference type value as a Link attribute, i.e. opentracing.ref_type set to follows_from or child_of.

If a list of Span references is specified, the union of their Baggage values MUST be used as the initial Baggage of the newly created Span. It is unspecified which Baggage value is used in the case of repeated keys. If no such lisf of references is specified, the current Baggage MUST be used as the initial value of the newly created Span.

If an initial set of tags is specified, the values MUST be set at the creation time of the OpenTelemetry Span, as opposed to setting them after the Span is already created. This is done in order to make those values available to any pre-Span-creation hook, e.g. the reference SDK performs a sampling step that consults Span information, including the initial tags/attributes, to decide whether to sample or not.

If an initial set of tags is specified and the OpenTracing error tag is included, after the OpenTelemetry Span was created the Shim layer MUST perform the same error handling as described in the Set Tag operation.

If an explicit start timestamp is specified, a conversion MUST be done to match the OpenTracing and OpenTelemetry units.

The API MUST return an OpenTracing Span.

Inject

Parameters:

A SpanContext.
A Format descriptor.
A carrier.

Inject the underlying OpenTelemetry Span and Baggage using either the explicitly registered or the global OpenTelemetry Propagators, as configured at construction time.

TextMap and HttpHeaders formats MUST use their explicitly specified TextMapPropagator, if any, or else use the global TextMapPropagator.

It MUST inject any non-empty Baggage even amidst no valid SpanContext.

Errors MAY be raised if the specified Format is not recognized, depending on the specific OpenTracing Language API (e.g. Go and Python do, but Java may not).

Extract

Parameters:

A Format descriptor.
A carrier.

Extract the underlying OpenTelemetry Span and Baggage using either the explicitly registered or the global OpenTelemetry Propagators, as configured at construction time.

TextMap and HttpHeaders formats MUST use their explicitly specified TextMapPropagator, if any, or else use the global TextMapPropagator.

The operation MUST return a SpanContext Shim instance with the extracted values if any of these conditions are met:

SpanContext is valid.
SpanContext is sampled.
SpanContext contains non-empty extracted Baggage.

Otherwise, the operation MUST return null or empty value.

if (!extractedSpanContext.isValid()
    && !extractedSpanContext.isSampled()
    && extractedBaggage.isEmpty()) {
  return null;
}

return SpanContextShim(extractedSpanContext, extractedBaggage);

Errors MAY be raised if either the Format is not recognized or no value could be extracted, depending on the specific OpenTracing Language API (e.g. Go and Python do, but Java may not).

Note: Invalid but sampled SpanContext instances are returned as a way to support jaeger-debug-id headers, which are used to force propagation of debug information.

Close

OPTIONAL operation. If this operation is implemented for a specific OpenTracing language, it MUST close the underlying TracerProvider if it implements a “closeable” interface or method; otherwise it MUST be defined as a no-op operation.

The Shim layer MUST protect against errors or exceptions raised while closing the underlying TracerProvider.

Note: Users are advised against calling this operation more than once per TracerProvider as it may have unexpected side effects, limitations or race conditions, e.g. a single Shim Tracer being closed multiple times or multiple Shim Tracer having their close operation being called.

Span Shim and SpanContext Shim relationship

As per the OpenTracing Specification, the OpenTracing SpanContext Shim MUST contain Baggage data and it MUST be immutable.

In turn, the OpenTracing Span Shim MUST contain a SpanContext Shim. When updating its associated baggage, the OpenTracing Span MUST set its OpenTracing SpanContext Shim to a new instance containing the updated Baggage.

This is a simple graphical representation of the mentioned objects:

  Span Shim
  +- OpenTelemetry Span (read-only)
  +- SpanContext Shim
        +- OpenTelemetry SpanContext (read-only)
        +- OpenTelemetry Baggage (read-only)

The OpenTracing Shim properly performs in-process and inter-process propagation of the OpenTelemetry Span along its associated Baggage leveraging the hierarchy of objects shown above.

As OpenTelemetry is not aware of this association, the related Baggage may not be properly propagated if the OpenTelemetry API is consumed along the OpenTracing Shim in the same codebase, as shown in the example below:

// methodOne consumes the OpenTelemetry API.
void methodOne(Span span) {
  try (Scope scope = span.makeCurrent()) {
    methodTwo();
  }
}

// methodTwo consumes the OpenTracing Shim.
void methodTwo() {
  io.opentracing.Span span = io.opentracing.util.GlobalTracer.get()
    .activeSpan();

  // Correctly set in the underlying io.opentelemetry.api.trace.Span
  span.setTag("tag", "value");

  // Value is set in the Shim layer -- it may not be later propagated
  // as OpenTelemetry is not aware of the Baggage associated
  // to this Span.
  span.setBaggageItem("baggage", "item");
}

Operations accessing the associated Baggage MUST be safe to be called concurrently.

Span Shim

The OpenTracing Span operations MUST be implemented using the underlying OpenTelemetry Span and Baggage values with the help of a SpanContext Shim object.

The Log operations MUST be implemented using the OpenTelemetry Span’s Add Events operations.

The Set Tag operations MUST be implemented using the OpenTelemetry Span’s Set Attributes operations.

Get Context

Returns the current SpanContext Shim.

This operation MUST be safe to be called concurrently.

Get Baggage Item

Parameters:

The baggage key, a string.

Returns the value for the specified key in the OpenTelemetry Baggage of the current SpanContext Shim, or null if none exists.

This operation MUST be safe to be called concurrently.

String getBaggageItem(String key) {
  synchronized(this) {
    // Get the current SpanContext's Baggage.
    io.opentelemetry.baggage.Baggage baggage = this.spanContextShim.getBaggage();

    // Return the value for key.
    return baggage.getEntryValue(key);
  }
}

Set Baggage Item

Parameters:

The baggage key, a string.
The baggage value, a string.

Creates a new SpanContext Shim with a new OpenTelemetry Baggage containing the specified Baggage key/value pair, and sets it as the current instance for this Span Shim.

This operation MUST be safe to be called concurrently.

void setBaggageItem(String key, String value) {
  synchronized(this) {
    // Add value/key to the existing Baggage.
    Baggage newBaggage = this.spanContextShim.getBaggage().toBuilder()
      .put(key, value)
      .build();

    // Create a new SpanContext with the updated Baggage.
    SpanContextShim newSpanContextShim = this.spanContextShim
      .newWithBaggage(newBaggage);

    // Update our SpanContext instance.
    this.spanContextShim = newSpanContextShim;
  }
}

Set Tag

Parameters:

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

Calls Set Attribute on the underlying OpenTelemetry Span with the specified key/value pair.

The error tag MUST be mapped to StatusCode:

true maps to Error.
false maps to Ok.
no value being set maps to Unset.

If the type of the specified value is not supported by the OTel API, the value MUST be converted to a string.

Log

Parameters:

A set of key/value pairs, where keys must be strings, and the values may have any type.

Calls Add Events on the underlying OpenTelemetry Span with the specified key/value pair set.

The Add Event’s name parameter MUST be the value with the event key in the pair set, or else fallback to use the log literal string.

If pair set contains an event=error entry, the values MUST be mapped to an Event with the conventions outlined in the Exception semantic conventions document:

If an entry with error.object key exists and the value is a language-specific error object, a call to RecordException(e) is performed along the rest of the specified key/value pair set as additional event attributes.
Else, a call to AddEvent is performed with name being set to exception, along the specified key/value pair set as additional event attributes, including mapping of the following key/value pairs:
- error.kind maps to exception.type.
- message maps to exception.message.
- stack maps to exception.stacktrace.

If an explicit timestamp is specified, a conversion MUST be done to match the OpenTracing and OpenTelemetry units.

Finish

Calls End on the underlying OpenTelemetry Span.

If an explicit timestamp is specified, a conversion MUST be done to match the OpenTracing and OpenTelemetry units.

SpanContext Shim

SpanContext Shim MUST be immutable and MUST contain the associated SpanContext and Baggage values.

Get Baggage Items

Returns a dictionary, collection, or iterator (depending on the requirements of the OpenTracing API for a specific language) backed by the associated OpenTelemetry Baggage values.

ScopeManager Shim

For OpenTracing languages implementing the ScopeManager interface, its operations MUST be implemented using the OpenTelemetry Context Propagation API in order to get and set active Context instances.

Activate a Span

Parameters:

A Span.

Stores the Span Shim and its underlying Span and Baggage in a new Context, which is then set as the currently active instance.

If the specified Span is null, it MUST be set to a NonRecordableSpan wrapping an invalid SpanContext, to signal there is no active Span nor Baggage.

Scope activate(Span span) {
  if (span == null) {
    span = new SpanShim(io.opentelemetry.api.trace.Span.getInvalid());
  }

  SpanShim spanShim = (SpanShim)span;

  // Put the associated Span and Baggage in a new Context.
  Context context = Context.current()
    .withValue(spanShim)
    .withValue(spanShim.getSpan())
    .withValue(spanShim.getBaggage());

  // Set context as the current instance.
  return context.makeCurrent();
}

Unsampled OpenTelemetry Spans can be perfectly activated, as they have valid SpanContexts (albeit with the sampled flag set to false):

// The underlying OpenTelemetry TracerProvider's Sampler
// decided to NOT sample this Span, hence
// io.opentelemetry.api.trace.Span.getSpanContext().isSampled() == false.
Span span = tracer.buildSpan("operationName").start();

try (Scope scope = tracer.scopeManager().activate(span)) {
  // tracer.scopeManager().activeSpan() == span
}

Get the active Span

Returns a Span Shim wrapping the currently active OpenTelemetry Span.

This operation MUST immediately return null if the current OpenTelemetry Span’s SpanContext is invalid and the current Baggage is empty, to signal there is no active Span nor Baggage.

If the current OpenTelemetry Span’s SpanContext is invalid but the current Baggage is not empty, this operation MUST return a new Span Shim containing a no-op OpenTelemetry Span and the non-empty Baggage.

If there are matching OpenTelemetry Span and Span Shim objects in the current Context, the Span Shim MUST be returned. Else, a new Span Shim containing the current OpenTelemetry Span and Baggage MUST be returned.

Span active() {
  io.opentelemetry.api.trace.Span span = Span.fromContext(Context.current());
  io.opentelemetry.api.baggage.Baggage baggage = Baggage.fromContext(Context.current());
  SpanShim spanShim = SpanShim.fromContext(Context.current());

  // There is no actual currently active Span.
  if (!span.getSpanContext().isValid()) {
    // Immediately return null if there is no Baggage.
    if (baggage.isEmpty()) {
      return null;
    }

    // Else return a no-op Span with the Baggage.
    return SpanShim(baggage);
  }

  // Span was activated through the Shim layer, re-use it.
  if (spanShim != null && spanShim.getSpan() == span) {
    return spanShim;
  }

  // Span was NOT activated through the Shim layer,
  // do a best effort with the current values.
  new SpanShim(span, baggage);
}

Span References

As defined in the OpenTracing Specification, a Span may reference zero or more other SpanContexts that are causally related. The reference information itself consists of a SpanContext and the reference type.

OpenTracing defines two types of references:

Child Of: The parent Span depends on the child Span in some capacity.
Follows From: The parent Span does not depend in any way on the result of their child Spans.

OpenTelemetry does not define strict equivalent semantics for these references. These reference types must not be confused with the Link functionality. This information is however preserved as the opentracing.ref_type attribute.

In process Propagation exceptions

Implicit and Explicit support mismatch

For languages with implicit in-process propagation support in OpenTelemetry and none in OpenTracing (i.e. no ScopeManager support), the Shim MUST only use explicit context propagation in its operations (e.g. when starting a new Span). This is done to easily comply with the explicit-only propagation semantics of the OpenTracing API:

// Tracer Shim
startSpan(name: string, options: SpanOptions = {}): Span {
  const otelSpanOptions = ...;

  if (!options.childOf && !options.references) {
    // Do NOT get nor set the current Context/Span,
    // as it is part of the implicit propagation support.
    otelSpanOptions.root = true;
  }
  ...
}

Using both the OpenTracing Shim and the OpenTelemetry API in the same codebase may result in traces using the incorrect parent Span, given the different implicit/explicit propagation expectations. For this case, the Shim MAY offer in-Development integration with the OpenTelemetry implicit in-process propagation via an explicit setting, warning the user incorrect parent values may be consumed:

// Tracer Shim
startSpan(name: string, options: SpanOptions = {}): Span {
  const otelSpanOptions = ...;

  if (!options.childOf && !options.references) {
    if (otShimOptions.supportImplicitPropagation) {
      // Allow OpenTelemetry to consume the current Context
      // to fetch the parent Span.
      otelSpanOptions.root = false;
    }
    ...
  }
  ...
}

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