Status: Experimental, Unless otherwise specified.
The OpenTelemetry project aims to provide backwards compatibility with the OpenCensus project in order to ease migration of instrumented codebases.
This functionality will be provided as a bridge layer implementing the OpenCensus API using the OpenTelemetry 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 upstream OpenTelemetry API.
The OpenCensus 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. It is expected that application owners will begin the migration process towards OpenTelemetry via the shim and adding new telemetry information via OpenTelemetry. Slowly, libraries and integrations will also migrate towards opentelemetry until the shim is no longer necessary.
For example, an application may have traces today of the following variety:
|-- Application - Configured OpenCensus --------------------------------- | |-- gRPC -> Using OpenCensus to generate Trace A --------- | |-- Application -> Using OpenCensus to generate a sub Trace B-- |
In this case, the application should be able to update its outer layer to OpenTelemetry, without having to wait for all downstream dependencies to have updated to OpenTelemetry (or deal with incompatibilities therein). The Application also doesn’t need to rewrite any of its own instrumentation.
|-- Application - Configured Otel w/ OpenCensus Shim ------------------- | |-- gRPC -> Using OpenCensus to generate Trace A --------- | |-- Application -> Using OpenCensus to generate a sub Trace B-- |
Next, the application can update its own instrumentation in a piecemeal fashion:
|-- Application - Configured Otel w/ OpenCensus Shim ---------------------- | |-- gRPC -> Using OpenCensus to generate Trace A --------- | |-- Application -> Using OpenTelemetry to generate a sub Trace B-- |
This layer of Otel -> OpenCensus -> Otel tracing can be thought of as the “OpenTelemetry sandwich” problem, and is the key motivating factor for this specification.
Finally, the Application would update all usages of OpenCensus to OpenTelemetry.
|-- Application - Configured Otel standalone ----------------------------- | |-- gRPC -> Using Otel to generate Trace A --------- | |-- 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.
OpenTelemetry<->OpenCensus compatibility has the following goals:
- OpenCensus has no hard dependency on OpenTelemetry
- Minimal changes to OpenCensus for implementation
- Easy for users to use, ideally no change to their code
Additionally, for tracing there are the following goals:
- Maintain parent-child span relationship between applications and libraries
- Maintain span link relationships between applications and libraries
Status: Experimental, Feature Freeze
OpenTelemetry will provide an OpenCensus-Trace-Shim component that can be added as a dependency to ensure compatibility with OpenCensus.
This component MUST be an optional dependency.
Creating Spans in OpenCensus
When the shim is in place, all OpenCensus Spans MUST be sent through an
Tracer as specified for the OpenTelemetry API.
This mechanism SHOULD be seamless to the user in languages that allow discovery and auto injection of dependencies.
Methods on Spans
All specified methods in OpenCensus will delegate to the underlying
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 using the OpenCensus <-> OpenTelemetry bridge.
- In OpenCensus, there is no specification when parent spans can be specified on a child span. OpenTelemetry specifies that parent spans must be specified during span creation. This leads to some issues with OpenCensus APIs that allowed flexible specification of parent spans post-initialization.
- Links added to spans after the spans are created. This is not supported in OpenTelemetry, therefore OpenCensus spans that have links added to them after creation will be mapped to OpenTelemetry spans without the links.
- OpenTelemetry specifies that samplers are attached to an entire Trace provider while OpenCensus allows custom samplers per span.
- TraceFlags in both OpenCensus and OpenTelemetry only specify the single
sampledflag (OpenTelemetry, OpenCensus). Some OpenCensus APIs support “debug” and “defer” tracing flags in addition to “sampled”. In this case, the OpenCensus bridge will do its best to support and translate unspecified flags into the closest OpenTelemetry equivalent.
The shim will provide an OpenCensus
PropagationComponent implementation which
maps OpenCenus binary and text propagation to OpenTelemetry context.
This adapter MUST use an OpenTelemetry
TextMapPropagator to implement the
This adapter SHOULD use configured OpenTelemetry
TextMapPropagator on the
TraceProvider for text format propagation.
This adapter MUST provide a default
OpenTelemetry defines a global TextMapPropogator, OpenCensus SHOULD use this
This adapter SHOULD use a contributed OpenTelemetry
B3Propagator for the
B3 text format.
OpenCensus Binary Context
This adapter MUST provide an implementation of OpenCensus
write OpenCensus binary format using OpenTelemetry’s context. This
implementation may be drawn from OpenCensus if applicable.
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.
OpenCensus specifies the following HTTP Attributes:
|OpenCensus Name||OpenTelemetry Name||Comments|
|key-name change only|
|N/A||Pass through ok|
Metrics / Stats
OpenTelemetry will provide an OpenCensus-Metrics-Shim component which implements the OpenTelemetry MetricProducer interface. When Produce() is invoked, the shim collects metrics from the OpenCensus global state, converts the metrics to an OpenTelemetry metrics batch, and returns.
- This component MUST be an optional dependency
- MUST NOT require OpenTelemetry to be included in OpenCensus API distributions
- SHOULD NOT require OpenCensus to depend on OpenTelemetry at runtime
- MUST require few or no changes to OpenCensus
- MUST be compatible with push and pull exporters
- MUST support Gauges, Counters, Cumulative Histograms, and Summaries
- Is NOT REQUIRED to support Gauge Histograms
- MUST support exemplar span context in language that provide utilities for recording span context in exemplars
The shim MUST discard the resource attached to OpenCensus metrics, and insert the resource provided during initialization, or fall back to the the default OpenTelemetry resource.
Metric attributes SHOULD be mapped following semantic convention mappings.
The shim MUST add an instrumentation scope name and version which identifies the shim.
The shim can be passed as an option to an OpenTelemetry MetricReader when configuring the OpenTelemetry SDK. This enables the bridge to work with both push and pull metric exporters.
- OpenTelemetry does not support OpenCensus’ GaugeHistogram type; these metrics MUST be dropped when using the bridge.
- OpenTelemetry does not currently support context-based attributes (tags).
- OpenTelemetry does not support OpenCensus’ SumOfSquaredDeviation field; this is dropped when using the bridge.