Logs Bridge API

Status: Experimental

Note: this document defines a log backend API. The API is not intended to be called by application developers directly. It is provided for logging library authors to build log appenders, which use this API to bridge between existing logging libraries and the OpenTelemetry log data model.

The Logs Bridge API consist of these main classes:

• LoggerProvider is the entry point of the API. It provides access to Loggers.
• Logger is the class responsible for emitting logs as LogRecords.

graph TD
    A[LoggerProvider] -->|Get| B(Logger)
    B -->|Emit| C(LogRecord)

LoggerProvider

Loggers can be accessed with a LoggerProvider.

In implementations of the API, the LoggerProvider is expected to be the stateful object that holds any configuration.

Normally, the LoggerProvider is expected to be accessed from a central place. Thus, the API SHOULD provide a way to set/register and access a global default LoggerProvider.

LoggerProvider operations

The LoggerProvider MUST provide the following functions:

• Get a Logger

Get a Logger

This API MUST accept the following parameters:

• name: This name uniquely identifies the instrumentation scope, such as the instrumentation library (e.g. io.opentelemetry.contrib.mongodb), package, module or class name. If an application or library has built-in OpenTelemetry instrumentation, both Instrumented library and Instrumentation library may refer to the same library. In that scenario, the name denotes a module name or component name within that library or application.

• version (optional): Specifies the version of the instrumentation scope if the scope has a version (e.g. a library version). Example value: 1.0.0.

• schema_url (optional): Specifies the Schema URL that should be recorded in the emitted telemetry.

• attributes (optional): Specifies the instrumentation scope attributes to associate with emitted telemetry. This API MUST be structured to accept a variable number of attributes, including none.

• include_trace_context (optional): Specifies whether the Trace Context should automatically be passed on to the LogRecords emitted by the Logger. If include_trace_context is not specified, it SHOULD be true by default.

Loggers are identified by name, version, and schema_url fields. When more than one Logger of the same name, version, and schema_url is created, it is unspecified whether or under which conditions the same or different Logger instances are returned. It is a user error to create Loggers with different include_trace_context or attributes but the same identity.

The term identical applied to Loggers describes instances where all identifying fields are equal. The term distinct applied to Loggers describes instances where at least one identifying field has a different value.

The effect of associating a Schema URL with a Logger MUST be that the telemetry emitted using the Logger will be associated with the Schema URL, provided that the emitted data format is capable of representing such association.

Logger

The Logger is responsible for emitting LogRecords.

Logger operations

The Logger MUST provide functions to:

Emit LogRecord

Emit a LogRecord to the processing pipeline.

This function MAY be named logRecord.

Parameters:

• logRecord - the LogRecord to emit.

LogRecord

The API emits LogRecords using the LogRecord data model.

A function receiving this as an argument MUST be able to set the following parameters:

• Timestamp
• Observed Timestamp
• Context that contains the TraceContext
• Severity Number
• Severity Text
• Body
• Attributes

All parameters are optional.

Optional and required parameters

The operations defined include various parameters, some of which are marked optional. Parameters not marked optional are required.

For each optional parameter, the API MUST be structured to accept it, but MUST NOT obligate a user to provide it.

For each required parameter, the API MUST be structured to obligate a user to provide it.

Concurrency requirements

For languages which support concurrent execution the Logs Bridge APIs provide specific guarantees and safeties.

LoggerProvider - all methods are safe to be called concurrently.

Logger - all methods are safe to be called concurrently.

Artifact Naming

The Logs Bridge API is not intended to be called by application developers directly, and SHOULD include documentation that discourages direct use. However, in the event OpenTelemetry were to add a user facing API, the Logs Bridge API would be a natural starting point. Therefore, Log Bridge API artifact, package, and class names MUST NOT include the terms “bridge”, “appender”, or any other qualifier that would prevent evolution into a user facing API.

Usage

How to Create a Log4J Log Appender

A log appender implementation can be used to can be used to bridge logs into the Log SDK OpenTelemetry LogRecordExporters. 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 log appender implementation will typically acquire a Logger from the global LoggerProvider at startup time, then call Emit LogRecord for LogRecords received from the application.

Implicit Context Injection and Explicit Context Injection describe how an Appender injects TraceContext into LogRecords.

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.

Log 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.

Implicit Context Injection

When Context is implicitly available (e.g. in Java) the Appender can rely on automatic context propagation by obtaining a Logger with include_trace_context=true.

Some log libraries have mechanisms specifically tailored for injecting contextual information into logs, such as MDC in Log4j. When available such mechanisms may be the preferable place to fetch the TraceContext and inject it into the LogRecord, 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.

Explicit Context Injection

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 TraceContext to be recorded in LogRecords.

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 LogRecords.

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.