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Add OpenCensus migration plan (#3015)
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@dashpole
dashpole authored Jan 17, 2023
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3 changes: 3 additions & 0 deletions CHANGELOG.md
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### Compatibility

- Add Tracer.Close() to the OpenTracing Shim layer.
- Add OpenCensus migration guide and add BinaryPropagation as an option to gRPC
instrumentation for OpenCensus compatibility
([#3015](https://github.com/open-telemetry/opentelemetry-specification/pull/3015)).

### OpenTelemetry Protocol

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189 changes: 103 additions & 86 deletions specification/compatibility/opencensus.md
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**Status**: [Experimental](../document-status.md), Unless otherwise specified.

## Abstract

The OpenTelemetry project aims to provide backwards compatibility with the
[OpenCensus](https://opencensus.io) project in order to ease migration of
instrumented codebases.

This functionality will be provided as a bridge layer implementing the
[OpenCensus API](https://github.com/census-instrumentation/opencensus-specs)
using the OpenTelemetry API. This layer MUST NOT rely on implementation specific
details of the OpenTelemetry SDK.
## Migration Path

### Breaking Changes when migrating to OpenTelemetry

Migrating from OpenCensus to OpenTelemetry may require breaking changes to the telemetry produced
because of:

* Different or new semantic conventions for names and attributes (e.g. [`grpc.io/server/server_latency`](https://github.com/census-instrumentation/opencensus-specs/blob/master/stats/gRPC.md#server) vs [`rpc.server.duration`](https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/metrics/semantic_conventions/rpc-metrics.md#rpc-server))
* Data model differences (e.g. OpenCensus supports [SumOfSquaredDeviations](https://github.com/census-instrumentation/opencensus-proto/blob/v0.3.0/src/opencensus/proto/metrics/v1/metrics.proto#L195), OTLP does not)
* Instrumentation API feature differences (e.g. OpenCensus supports [context-based attributes](https://github.com/census-instrumentation/opencensus-specs/blob/master/stats/Record.md#recording-stats)), OTel does not)
* Differences between equivalent OC and OTel exporters (e.g. the OpenTelemetry Prometheus exporter [adds type and unit suffixes](https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/metrics/data-model.md#metric-metadata-1); OpenCensus [does not](https://github.com/census-ecosystem/opencensus-go-exporter-prometheus/blob/v0.4.1/prometheus.go#L227))

This migration path groups most breaking changes into the installation of bridges. This gives
users control over introducing the initial breaking change, and makes subsequent migrations of
instrumentation (including in third-party libraries) non-breaking.

### Migration Plans

#### Migrating from the OpenCensus Agent and Protocol

Starting with a deployment of the OC agent, using the OC protocol, migrate by:

1. Deploy the OpenTelemetry collector with OpenCensus and OTLP receivers and equivalent processors and exporters.
2. **breaking**: For each workload sending the OC protocol, change to sending to the OpenTelemetry collector's OpenCensus receiver.
3. Remove the deployment of the OC Agent.
4. For each workload, migrate the application from OpenCensus to OpenTelemetry, following the guidance below, and use the OTLP exporter.

#### Migrating an Application using Bridges

Starting with an application using entirely OpenCensus instrumention for traces and metrics, it can be migrated by:

1. Migrate the exporters (SDK)
1. Install the OpenTelemetry SDK, with an equivalent exporter
1. If using an OpenCensus exporter, switch to using an OTLP exporter
2. Install equivalent OpenTelemetry resource detectors
3. Install OpenTelemetry [`W3c TraceContext`](../context/api-propagators.md#propagators-distribution) propagator, which is the equivalent of the OpenCensus' `TextFormat` propagator.
4. **breaking**: Install the metrics and trace bridges
5. Remove initialization of OpenCensus exporters
2. Migrate the instrumentation (API)
1. **breaking**: For OpenCensus instrumentation packages, migrate to the OpenTelemetry equivalent.
1. If migrating gRPC, enable the [`BinaryPropagation`](../context/api-propagators.md#propagators-distribution) propagator if the language supports it. Otherwise, enable OpenCensus `BinaryPropagation` on the OpenTelemetry gRPC instrumentation.
2. For external dependencies, wait for it to migrate to OpenTelemetry, and update the dependency.
3. For instrumentation which is part of the application, migrate it following the "library" guidance below.
3. Clean up: Remove the metrics and trace bridges

#### Migrating Libraries using OC Instrumentation

##### In-place Migration

Libraries which want a simple migration can choose to replace instrumentation in-place.

Starting with a library using OpenCensus Instrumentation:

1. Annouce to users the library's transition from OpenCensus to OpenTelemetry, and recommend users adopt OC bridges.
2. Change unit tests to use the OC bridges, and use OpenTelemetry unit testing frameworks.
3. After a notification period, migrate instrumentation line-by-line to OpenTelemetry. The notification period should be long for popular libraries.
4. Remove the OC bridge from unit tests.

##### Migration via Config

Libraries which are eager to add native OpenTelemetry instrumentation sooner, and/or want to
provide extended support for OpenCensus may choose to provide users the option to use OpenCensus
instrumentation _or_ OpenTelemetry instrumentation.

Starting with a library using OpenCensus Instrumentation:

1. Change unit tests to use the OC bridges, and use OpenTelemetry unit testing frameworks.
2. Add configuration allowing users to enable OpenTelemetry instrumentation and disable OpenCensus instrumentation.
3. Add OpenTelemetry instrumentation gated by the configuration, and tested using the same sets of unit tests.
4. After a notification period, switch to using OpenTelemetry instrumentation by default.
5. After a deprecation period, remove the option to use OpenCensus instrumentation.

## Trace Bridge

**Status**: [Experimental, Feature Freeze](../document-status.md)

The trace bridge is provided as a shim layer implementing the
[OpenCensus Trace API](https://github.com/census-instrumentation/opencensus-specs)
using the OpenTelemetry Trace API. This layer MUST NOT rely on implementation
specific details of the OpenTelemetry SDK.

More specifically, the intention is to allow OpenCensus instrumentation to be
recorded using OpenTelemetry. This Shim Layer MUST NOT publicly expose any
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|-- Application -> Using OpenTelemetry to generate a sub Trace B-- |
```

OpenCensus supports two primary types of telemetry: Traces and Stats (Metrics).
Compatibility for these is defined separately.

> The overriding philosophy for compatibility is that OpenCensus instrumented
> libraries and applications need make *no change* to their API usage in order
> to use OpenTelemetry. All changes should be solely configuration / setup.
## Goals

OpenTelemetry<->OpenCensus compatibility has the following goals:

1. OpenCensus has no hard dependency on OpenTelemetry
2. Minimal changes to OpenCensus for implementation
3. Easy for users to use, ideally no change to their code

Additionally, for tracing there are the following goals:

1. Maintain parent-child span relationship between applications and libraries
2. Maintain span link relationships between applications and libraries

## Trace

**Status**: [Experimental, Feature Freeze](../document-status.md)
### Requirements

OpenTelemetry will provide an OpenCensus-Trace-Shim component that can be
added as a dependency to ensure compatibility with OpenCensus.
The OpenTelemetry<->OpenCensus trace bridge has the following requirements:

This component MUST be an optional dependency.
* OpenCensus has no hard dependency on OpenTelemetry
* Minimal changes to OpenCensus for implementation
* Easy for users to use, ideally no change to their code
* Maintain parent-child span relationship between applications and libraries
* Maintain span link relationships between applications and libraries
* This component MUST be an optional dependency.

### Creating Spans in OpenCensus

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All specified methods in OpenCensus will delegate to the underlying `Span` of
OpenTelemetry.

### Resources

Note: resources appear not to be usable in the "API" section of OpenCensus.

#### Known Incompatibilities

Below are listed known incompatibilities between OpenTelemetry and OpenCensus
specifications. Applications leveraging unspecified behavior from OpenCensus
that *is* specified incompatibly within OpenTelemetry are not eligible for
specifications. Applications leveraging unspecified behavior from OpenCensus
that _is_ specified incompatibly within OpenTelemetry are not eligible for
using the OpenCensus <-> OpenTelemetry bridge.

1. In OpenCensus, there is [no specification](https://github.com/census-instrumentation/opencensus-specs/blob/master/trace/Span.md#span-creation)
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"sampled". In this case, the OpenCensus bridge will do its best to support
and translate unspecified flags into the closest OpenTelemetry equivalent.

### Context Propagation

The shim will provide an OpenCensus `PropagationComponent` implementation which
maps OpenCenus binary and text propagation to OpenTelemetry context.

#### Text Context

This adapter MUST use an OpenTelemetry `TextMapPropagator` to implement the
OpenCensus `TextFormat`.

This adapter SHOULD use configured OpenTelemetry `TextMapPropagator` on the
OpenTelemetry `TraceProvider` for text format propagation.
## OpenCensus Binary Context Propagation

This adapter MUST provide a default `W3CTraceContextPropagator`. If
OpenTelemetry defines a global TextMapPropogator, OpenCensus SHOULD use this
for OpenCensus `traceContextFormat` propagation.

#### B3 Context

This adapter SHOULD use a contributed OpenTelemetry `B3Propagator` for the
B3 text format.

#### OpenCensus Binary Context
The shim will provide an OpenCensus `BinaryPropogator` implementation which
maps [OpenCenus binary trace context format](https://github.com/census-instrumentation/opencensus-specs/blob/master/encodings/BinaryEncoding.md#trace-context) to an OpenTelemetry
[SpanContext](../overview.md#spancontext).

This adapter MUST provide an implementation of OpenCensus `BinaryPropogator` to
write OpenCensus binary format using OpenTelemetry's context. This
implementation may be drawn from OpenCensus if applicable.

### Resources

Note: resources appear not to be usable in the "API" section of OpenCensus.

### Semantic Convention Mappings

Where possible, the tracing shim should provide mappings of labels to attributes
defined within the OpenTelemetry semantic convetions.

> The principle is to ensure OpenTelemetry exporters, which use these semantic
> conventions, are likely to export the correct data.
#### HTTP Attributes

OpenCensus specifies the following [HTTP Attributes](https://github.com/census-instrumentation/opencensus-specs/blob/master/trace/HTTP.md#attributes):

| OpenCensus Name | OpenTelemetry Name | Comments |
| ------------------ | ------------------ |----------------------|
| `http.host` | `http.host` | |
| `http.method` | `http.method` | |
| `http.user_agent` | `http.user_agent` | |
| `http.status_code` | `http.status_code` | |
| `http.url` | `http.url` | |
| `http.path` | `http.target` | key-name change only |
| `http.route` | N/A | Pass through ok |
The `BinaryPropagator` MUST be a [TextMapPropagator](../context/api-propagators.md#textmap-propagator)
if possible in the language. Otherwise, the `BinaryPropagator` MUST be a library,
which is expected to be used by OpenTelemetry gRPC instrumentation. gRPC
instrumentation in those languages SHOULD NOT enable the `BinaryPropagator` by
default, but SHOULD provide configuration to allow users to enable it.

## Metrics / Stats

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the resource provided during initialization, or fall back to the the default
OpenTelemetry resource.

### Metric Attributes

Metric attributes SHOULD be mapped following
[semantic convention mappings](#semantic-convention-mappings).

### Instrumentation Scope

The shim MUST add an instrumentation scope name and version which identifies
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