bridge-api.md

Logs Bridge API

Status: Stable

Table of Contents

• LoggerProvider
  • LoggerProvider operations
    • Get a Logger
• Logger
  • Logger operations
    • Emit a LogRecord
• Optional and required parameters
• Concurrency requirements
• Artifact Naming
• Usage
  • How to Create a Log4J Log Appender
  • Implicit Context Injection
  • Explicit Context Injection

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)

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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.

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 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 a LogRecord

Emit a LogRecord

The effect of calling this API is to emit a LogRecord to the processing pipeline.

The API MUST accept the following parameters:

• Timestamp
• Observed Timestamp. If unspecified the implementation SHOULD set it equal to the current time.
• The Context associated with the LogRecord. The API MAY implicitly use the current Context as a default behavior.
• 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 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 NOT explicitly setting Context when calling emit a LogRecord.

Some log libraries have mechanisms specifically tailored for injecting contextual information info logs, such as MDC in Log4j. When available, it may be preferable to use these mechanisms to set the Context. A log appender can then fetch the Context and explicitly set it when calling emit a LogRecord. This allows the correct Context to be included even when log records are emitted asynchronously, which can otherwise lead the Context to be incorrect.

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 order for TraceContext to be recorded in LogRecords 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. The log appender must take this Context and explicitly set it when calling emit a LogRecord.

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 logs.

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.