Add Initial OpenTracing compatibility requirements

specification/opentracing-compatibility.md

@@ -0,0 +1,152 @@
+# OpenTracing Compatibility
+
+<details>
+<summary>Table of Contents</summary>
+
+* [Abstract](#abstract)
+* [OpenTracing Tracer](#opentracing-tracer)
+  * [Create a Tracer shim](#create-a-tracer-shim)
+  * [Inject](#inject)
+  * [Extract](#extract)
+* [ScopeManager](#scopemanager)
+  * [Activate a Span](#activate-a-span)
+  * [Get the active Span](#get-the-active-span)
+* [Span](#span)
+  * [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](#spancontext)
+    * [Get Baggage Items](#get-baggage-items)
+
+</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.
+
+## OpenTracing Tracer
+
+The OpenTracing Tracer operations MUST be implemented using the OpenTelemetry
+API:
+
+- An OpenTelemetry `Tracer` MUST be used to create `Span`s.
+- A `CorrelationContextManager` MUST be used to create the `CorrelationContext`
+  objects used to implement the OpenTracing `Span` baggage.
+- A set of `Propagator` MUST be leveraged in order to perform inject and extract operations.
+
+### Create a Tracer Shim
+
+The operation MUST accept the following parameters:
+
+- An OpenTelemetry `Tracer`.
+- A `CorrelationContextManager`.
+- A set of `Propagator`s of the supported types, usually `Composite Propagator`s.
+
+The API MUST return an OpenTracing `Tracer`.
+
+### Inject
+
+Inject the underlying `SpanContext` using the registered `Propagator`s:
+
+- `TEXT_MAP` and `HTTP_HEADERS` formats MUST use the registered `HTTPTextPropagator`, if any.
+
+Errors MUST NOT be raised if the specified OpenTracing `Format` is `BINARY`.
+
+### Extract
+
+Extract an OpenTelemetry `SpanContext` from a carrier using the registered `Propagator`s:
+
+- `TEXT_MAP` and `HTTP_HEADERS` formats MUST use the registered `HTTPTextPropagator`, if any.
+
+Returns an OpenTracing `SpanContext` with the underlying extracted `SpanContext`, or null if
+either the OpenTracing `Format` is `BINARY` or no value could be extracted.
+
+## ScopeManager.
+
+For OpenTracing languages implementing the `ScopeManager` interface,  its operations
+MUST be implemented using the OpenTelemetry context propagation API.
+
+### Activate a Span.
+
+Sets the specified `Span` as active for the current `Context`.
+
+### Get the active Span.
+
+Gets the active `Span` for the current `Context`. The API MUST return
+null if none exist.
+
+## Span
+
+The `Span` operations MUST be implemented using an underlying OpenTelemetry `Span`
+in conjunction with an associated `CorrelationContext` object used to implement its
+baggage part.
+
+OpenTracing `SpanContext` shim objects MUST act as objects associating an
+underlying `SpanContext` with a `CorrelationContext`. Every invocation to the
+`Set Baggage Item` operation will result in a newly associated `SpanContext` shim object
+with a new `CorrelationContext` containing the specified name/value pair.
+
+Each underlying `Span` can only be associated with one `SpanContext` shim at a time
+for all the execution units.
+
+The `Log` operations MUST be implemented using `Span`'s `Add Events`.
+
+The `Set Tag` operations MUST be implemented using `Span`'s `Set Attributes`.
+
+### Get Context
+
+Gets the currently associated `SpanContext` shim.
+
+### Get Baggage Item
+
+Gets a value for the specified name in the `CorrelationContext` of the associated `SpanContext`
+shim or null if none exists.
+
+### Set Baggage Item
+
+Creates a newly associated `SpanContext` shim object with a new `CorrelationContext`
+containing the specified name/value pair.
+
+### Set Tag
+
+Calls `Set Attribute` on the underlying `Span` with the specified name/value pair.
+
+If the type of the specified value is not supported, the value MUST NOT be set and
+a warning SHOULD be generated following the [error handling](error-handling.md)
+section.
+
+### Log
+
+Calls `Add Event` on the underlying `Span`.
+
+If an explicit timestamp is specified, it MUST be converted from microseconds
+to nanoseconds.
+
+### Finish
+
+Calls `End` on the underlying `Span`.
+
+If an explicit timestamp is specified, it MUST be converted from microseconds
+to nanoseconds.
+
+### SpanContext
+
+The `SpanContext` interface MUST be implemented using a OpenTelemetry `SpanContext`
+in conjunction with an associated `CorrelationContext`.
+
+Observe that the assigned `SpanContext` and `CorrelationContext` MUST be read-only
+and MUST NOT change after creation.
+
+#### Get Baggage Items
+
+Returns a shim exposing the associated `CorrelationContext` values.