Add OpenTracing compatibility section.

CHANGELOG.md

@@ -22,6 +22,11 @@ release.
 
 ### Semantic Conventions
 
+### Compatibility
+
+- Add initial OpenTracing compatibility section.
+  ([#1101](https://github.com/open-telemetry/opentelemetry-specification/pull/1101))
+
 ## v1.1.0 (2021-03-11)
 
 ### Traces

specification/compatibility/opentracing.md

@@ -0,0 +1,343 @@
+# OpenTracing Compatibility
+
+**Status**: [Experimental](../document-status.md).
+
+<details>
+<summary>Table of Contents</summary>
+
+* [Abstract](#abstract)
+* [Create an OpenTracing Tracer Shim](#create-an-opentracing-tracer-shim)
+* [Tracer Shim](#tracer-shim)
+  * [Inject](#inject)
+  * [Extract](#extract)
+* [OpenTelemetry Span and SpanContext Shim relationship](#opentelemetry-span-and-spancontext-shim-relationship)
+* [Span Shim](#span-shim)
+  * [Get Context](#get-context)
+  * [Get Baggage Item](#get-baggage-item)
+  * [Set Baggage Item](#set-baggage-item)
+  * [Set Tag](#set-tag)
+  * [Log](#log)
+  * [Finish](#finish)
+* [SpanContext Shim](#spancontext-shim)
+  * [Get Baggage Items](#get-baggage-items)
+* [ScopeManager Shim](#scopemanager-shim)
+  * [Activate a Span](#activate-a-span)
+  * [Get the active Span](#get-the-active-span)
+
+</details>
+
+## Abstract
+
+The OpenTelemetry project aims to provide backwards compatibility with the
+[OpenTracing](https://opentracing.io) project in order to ease migration of
+instrumented codebases.
+
+This functionality will be provided as a bridge layer implementing the
+[OpenTracing API](https://github.com/opentracing/specification) 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.
+
+The OpenTracing Shim and the OpenTelemetry API/SDK are expected to be consumed
+simultaneously in a running service, in order to ease migration from the former
+to the latter.
+
+## Create an OpenTracing Tracer Shim
+
+This operation is used to create a new OpenTracing `Tracer`:
+
+This operation MUST accept the following parameters:
+
+- An OpenTelemetry `Tracer`, used to create `Span`s.
+- OpenTelemetry `Propagator`s 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`.
+
+```java
+// Create a Tracer Shim relying on the global propagators.
+createTracerShim(tracer);
+
+// Create a Tracer Shim using:
+// 1) TraceContext propagator for TextMap
+// 2) Jaeger propagator for HttPHeaders.
+createTracerShim(tracer, OTPropagatorsBuilder()
+  .setTextMap(W3CTraceContextPropagator.getInstance())
+  .setHttpHeaders(JaegerPropagator.getInstance())
+  .build());
+```
+
+See OpenTracing Propagation
+[Formats](https://github.com/opentracing/specification/blob/master/specification.md#extract-a-spancontext-from-a-carrier).
+
+## Tracer Shim
+
+### Inject
+
+Parameters:
+
+- A `SpanContext`.
+- A `Format` descriptor.
+- A carrier.
+
+Inject the underlying OpenTelemetry `Span` and `Bagagge` using either the explicitly
+registered or the global OpenTelemetry `Propagator`s, as configured at construction time.
+
+- `TextMap` and `HttpHeaders` formats MUST use their explicitly specified `TextMapPropagator`,
+  if any, or else use the global `TextMapPropagator`.
+
+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 `Bagagge` using either the explicitly
+registered or the global OpenTelemetry `Propagator`s, as configured at construction time.
+
+- `TextMap` and `HttpHeaders` formats MUST use their explicitly specified `TextMapPropagator`,
+  if any, or else use the global `TextMapPropagator`.
+
+Returns a `SpanContext` Shim with the underlying extracted OpenTelemetry
+`Span` and `Baggage`. 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).
+
+## OpenTelemetry Span and SpanContext Shim relationship
+
+OpenTracing `SpanContext`, just as OpenTelemetry `SpanContext`, MUST be
+immutable, but it MUST also store `Baggage`. Hence, it MUST be replaced
+every time baggage is updated through the OpenTracing
+[Span Set Baggage Item](#set-baggage-item) operation. Special handling
+MUST be done by the Shim layer in order to retain these invariants.
+
+Because of the previous requirement, a given OpenTelemetry `Span`
+MUST be associated with ONE AND ONLY ONE `SpanContext` Shim object at all times
+for ALL execution units, in order to keep any linked `Baggage` consistent
+at all times. It MUST BE safe to get and set the associated `SpanContext` Shim
+object for a specified OpenTelemetry `Span` from different execution units.
+
+An example showing the need for these requirements is having an OpenTracing `Span`
+have its [Set Baggage Item](#set-baggage-item) operation called from two different
+execution units (e.g. threads, coroutines), and afterwards have its
+[Context](#get-context) fetched in order to
+[iterate over its baggage values](#get-baggage-items).
+
+```java
+// Thread A: New SpanContextShim and Baggage values are created.
+openTracingSpan.setBaggageItem("1", "a")
+
+// Thread B: New SpanContextShim and Baggage values are created again.
+openTracingSpan.setBaggageItem("2", "b")
+
+// Thread C: Up-to-date SpanContextShim and Bagggage values are retrieved.
+for (Map.Entry<String, String> entry : openTracingSpan.context().baggageItems()) {
+  ...
+}
+```
+
+This is a simple graphical representation of the mentioned objects:
+
+```
+  Span Shim
+  +- OpenTelemetry Span
+  +- SpanContext Shim
+        +- OpenTelemetry SpanContext
+        +- OpenTelemetry Baggage
+```
+
+The OpenTelemetry `Span` in the `Span` Shim object is used to get and set
+its currently associated `SpanContext` Shim.
+
+Managing this one-to-one relationship between an OpenTelemetry `Span` and
+a `SpanContext` Shim object is an implementation detail. It can be implemented,
+for example, with the help of a global synchronized dictionary, or with an
+additional attribute in each OpenTelemetry `Span` object for dynamic languages.
+
+## Span Shim
+
+The OpenTracing `Span` operations MUST be implemented using 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 [associated](#opentelemetry-span-and-spancontext-shim-relationship)
+`SpanContext` Shim.
+
+### Get Baggage Item
+
+Parameters:
+
+- The baggage key, a string.
+
+Returns a value for the specified key in the OpenTelemetry `Baggage` of the
+associated `SpanContext` Shim or null if none exists.
+
+This is accomplished by getting the
+[associated](#opentelemetry-span-and-spancontext-shim-relationship)
+`SpanContext` Shim and do a lookup for the specified key in the OpenTelemetry
+`Baggage` instance.
+
+```java
+String getBaggageItem(String key) {
+  getSpanContextShim().getBaggage().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. The resulting `SpanContext` Shim is then
+[associated](#opentelemetry-span-and-spancontext-shim-relationship) to the underlying
+OpenTelemetry `Span`.
+
+```java
+void setBaggageItem(String key, String value) {
+  SpanContextShim spanContextShim = getSpanContextShim();
+
+  // Add value/key to the existing Baggage.
+  Baggage newBaggage = spanContextShim.getBaggage().toBuilder()
+    .put(key, value)
+    .build();
+
+  // Set a new SpanContext Shim object with the updated Baggage.
+  setSpanContextShim(spanContextShim.newWithBaggage(newBaggage));
+}
+```
+
+### 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.
+
+Certain values MUST be mapped from
+[OpenTracing Span Tags](https://github.com/opentracing/specification/blob/master/semantic_conventions.md#standard-span-tags-and-log-fields)
+to the respective OpenTelemetry `Attribute`:
+
+- `error` maps to [StatusCode](../trace/api.md##set-status):
+  - `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 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`.
+
+Gets the [associated](#opentelemetry-span-and-spancontext-shim-relationship)
+`SpanContext` Shim for the specified `Span` and puts its OpenTelemetry
+`Span`, `Baggage` and `Span` Shim objects in a new `Context`,
+which is then set as the currently active instance.
+
+```java
+Scope activate(Span span) {
+  SpanContextShim spanContextShim = getSpanContextShim(span);
+
+  // Put the associated Span and Baggage in the used Context.
+  Context context = Context.current()
+    .withValue(spanContextShim.getSpan())
+    .withValue(spanContextShim.getBaggage())
+    .withValue((SpanShim)spanShim);
+
+  // Set context as the current instance.
+  return context.makeCurrent();
+}
+```
+
+### Get the active Span
+
+Returns a `Span` Shim wrapping the currently active OpenTelemetry `Span`.
+
+If there are related OpenTelemetry `Span` and `Span` Shim objects in the
+current `Context`, the `Span` Shim MUST be returned. Else, a new `Span` Shim
+referencing the OpenTelemetry `Span` MUST be created and returned.
+
+The API MUST return null if no actual OpenTelemetry `Span` is set.
+
+```java
+Span active() {
+  io.opentelemetry.api.trace.Span span = Span.fromContext(Context.current());
+  SpanShim spanShim = SpanShim.fromContext(Context.current());
+
+  // 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.
+  new SpanShim(Span.current());
+}
+```