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 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.

Notwithstanding any global LoggerProvider, some applications may want to or have to use multiple LoggerProvider instances, e.g. to have different configuration (like LogRecordProcessors) for each (and consequently for the Loggers obtained from them), or because it’s easier with dependency injection frameworks. Thus, implementations of LoggerProvider SHOULD allow creating an arbitrary number of instances.

LoggerProvider operations

The LoggerProvider MUST provide the following functions:

• Get a Logger

Get a Logger

This API MUST accept the following parameters:

• name (required): This name SHOULD uniquely identify 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. In case an invalid name (null or empty string) is specified, a working Logger implementation MUST be returned as a fallback rather than returning null or throwing an exception, its name property SHOULD be set to an empty string, and a message reporting that the specified value is invalid SHOULD be logged. A library implementing the OpenTelemetry API may also ignore this name and return a default instance for all calls, if it does not support “named” functionality (e.g. an implementation which is not even observability-related). A LoggerProvider could also return a no-op Logger here if application owners configure the SDK to suppress telemetry produced by this library.
• 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.
• include_trace_context (optional): Specifies whether the Trace Context should automatically be passed on to the LogRecords emitted by the Logger. This SHOULD be true by default.
• attributes (optional): Specifies the instrumentation scope attributes to associate with emitted telemetry.

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.

Implementations MUST NOT require users to repeatedly obtain a Logger again with the same name+version+schema_url+include_trace_context+attributes to pick up configuration changes. This can be achieved either by allowing to work with an outdated configuration or by ensuring that new configuration applies also to previously returned Loggers.

Note: This could, for example, be implemented by storing any mutable configuration in the LoggerProvider and having Logger implementation objects have a reference to the LoggerProvider from which they were obtained. If configuration must be stored per-Logger (such as disabling a certain Logger), the Logger could, for example, do a look-up with its name+version+schema_url+include_trace_context+attributes in a map in the LoggerProvider, or the LoggerProvider could maintain a registry of all returned Loggers and actively update their configuration if it changes.

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.

Note that Loggers should not be responsible for configuration. This should be the responsibility of the LoggerProvider instead.

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 fields:

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

Usage

How to Create Log4J Style Appender

An Appender implementation can be used to allow emitting logs via 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 Appender implementation will typically acquire a Logger from the global LoggerProvider at startup time, then construct LogRecords for each log received from the application, and then call Emit LogRecord.

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.

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 log library extension 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.