Logging Library SDK Specification
This document contains the first cut from the OTEP0150
This specification defines how the OpenTelemetry Logging Library SDK exposes its functionality to authors of extensions to language-specific 3rd party logging libraries and to end users that want to produce logs in the OpenTelemetry manner.
The specification defines SDK elements that to some extent mirror the OpenTelemetry Trace SDK. This ensures uniformity and consistency of the OpenTelemetry specification and of the implementations across traces and logs. For additional clarity the definitions in this document refer to the Trace analogs where appropriate.
Many existing logging libraries have some sort of extension mechanism that allows to customize how log records are encoded and delivered to their destinations (for example Appender in Log4j or Core in zapcore). The OpenTelemetry Logging Library SDK is intended to be used by such extensions to emit logs in OpenTelemetry formats.
Note: The functionality that this document describes is an SDK package. A logging-related API package may be added in the future if we decide to have an end-user callable logging API. Until then the functions and methods callable from this SDK package are intended to be used by Logging Libraries only and are NOT intended to be used by the end user and will NOT be exposed in the OpenTelemetry API package.
Logging Library SDK contains the following components:
- Get LogEmitter. Accepts the instrumentation scope name and version and returns a LogEmitter associated with the instrumentation scope.
LogEmitterProvider can be configured at startup time, to be associated with a Resource and with LogProcessor/LogExporter pipeline.
Emit(LogRecord). Emits a log record. The LogRecord and the Resource and Instrumentation library associated with the LogEmitter will be converted into a readable LogData and will be pushed through the SDK and the configured LogProcessors and LogExporter. It is expected that the caller will populate trace context related fields (TraceId,SpanId,TraceFlags) if applicable before making the call. Open Question: do we need to also pass the Baggage so that log processors and exporters can use it if they see the need (see e.g. Java issue)?
Note: some languages may opt to avoid having a LogRecord data type and instead use a more idiomatic builder pattern to prepare and emit a log record (see e.g. Java discussion)
See LogRecord data model for the list of fields.
Plugin interface. Analog of SpanProcessor. Interface to hook the log record emitting action.
- Emit(LogData). Call when a log record is ready to be processed and exported.
Built-in implementations: SimpleLogProcessor, BatchLogProcessor.
Readable LogRecord data plus associated Resource and InstrumentationLibrary. Analog of SpanData.
Plugin interface. Analog of SpanExporter. Allows to implement protocol-specific exporters so that they can be plugged into OpenTelemetry SDK and support sending of log data.
- Export(batch). Exports a batch of LogData.
How to Create Log4J Style Appender
An Appender implementation can be used to allow emitting log records via OpenTelemetry Logging Library exporters. This approach is typically used for applications which are fine with changing the log transport and is one of the supported log collection approaches.
The Appender implementation will typically acquire a LogEmitter from the global LogEmitterProvider at startup time, then call LogEmitter.Emit for log records received from the application.
For languages with implicit Context, the Appender may call Context API to get the currently active Span and populate TraceId, SpanId, TraceFlags fields of the LogRecord before emitting it. The log library may also have an alternate way to inject the context into log records (e.g. MDC in Log4j).
This same approach can be also used for example for:
- Python logging library by creating a Handler.
- Go zap logging library by implementing the Core interface. Note that since there is no implicit Context in Go it is not possible to get and use the active Span.
Appenders can be created in OpenTelemetry language libraries by OpenTelemetry maintainers, or by 3rd parties for any logging library that supports a similar extension mechanism. This specification recommends each OpenTelemetry language library to include out-of-the-box Appender implementation for at least one popular logging library.
Logging to File
One of the possible approaches to emit and collect logs that OpenTelemetry supports is via intermediary files. When configuring the LogEmitterProvider, OTLP File exporter should be used to write logs to a file or stdout in either OTLP JSON or OTLP Protobuf binary format.
TODO: clarify how this functionality co-exists with the overlapping functionality in logging libraries that allow specifying how logs are written to a file.
Logging Directly to OTLP Network Destination
The approach is the same as for logging to a file, except OTLP/gRPC or OTLP/HTTP exporter implementation is used.
Implicit Context Injection
When Context is implicitly available (e.g. in Java) it may be fetched by the log library extension synchronously for every log record by calling the OpenTelemetry Context API and injecting the span context fields into the LogRecord before emitting it.
Some log libraries have mechanisms specifically tailored for injecting contextual information into log records. An example of such a mechanism is Log4j MDC. When available such mechanisms may be the preferable place to fetch the span context and inject it into the log records, since it usually allows fetching of the context to work correctly even when log records are emitted asynchronously (which otherwise can result in the incorrect implicit context being fetched.
TODO: clarify how works or doesn’t work when the log statement call site and the log appender are executed on different threads.
In languages where the Context must be provided explicitly (e.g. Go) the end user must capture the context and explicitly pass it to the logging subsystem in order for trace context to be recorded in Log records.
Support for OpenTelemetry for logging libraries in these languages typically can be implemented in the form of logger wrappers that can capture the context once, when the span is created and then use the wrapped logger to execute log statements in a normal way. The wrapper will be responsible for injecting the captured context in the log records.
This specification does not define how exactly it is achieved since the actual mechanism depends on the language and the particular logging library used. In any case the wrappers are expected to make use of the Trace Context API to get the current active span.
See an example of how it can be done for zap logging library for Go.
LogExporter implementations can be plugged into OpenTelemetry Logging Library to send logs via custom protocols.
OTLP/gRPC, OTLP/HTTP, OTLP/File log exporters are provided with OpenTelemetry Logging Library out of the box.
LogProcessor implementations can be plugged into the OpenTelemetry Logging Library to have custom processing of logs before they are exported.
Simple and Batch processors should be provided by the OpenTelemetry Logging Library out of the box.