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Clarify how log appender use Scope name and attributes
Resolves open-telemetry#3562 I moved the recommendations for appenders to a separate file named supplementary-guidelines.md. Similar to other signals this file will contain non-normative recommendations.
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| # Supplementary Guidelines | ||
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| Note: this document is NOT a spec, it is provided to support the Logs | ||
| [API](./bridge-api.md) and [SDK](./sdk.md) specifications, it does NOT add any | ||
| extra requirements to the existing specifications. | ||
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| <details> | ||
| <summary>Table of Contents</summary> | ||
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| <!-- Re-generate TOC with `markdown-toc --no-first-h1 -i` --> | ||
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| <!-- toc --> | ||
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| - [Usage](#usage) | ||
| * [How to Create a Log4J Log Appender](#how-to-create-a-log4j-log-appender) | ||
| * [Implicit Context Injection](#implicit-context-injection) | ||
| * [Explicit Context Injection](#explicit-context-injection) | ||
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| <!-- tocstop --> | ||
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| </details> | ||
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| ## Usage | ||
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| ### How to Create a Log4J Log Appender | ||
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| A [log appender](../glossary.md#log-appender--bridge) implementation can be used | ||
| to bridge logs into the [Log SDK](./sdk.md) | ||
| OpenTelemetry [LogRecordExporters](sdk.md#logrecordexporter). This approach is | ||
| typically used for applications which are fine with changing the log transport | ||
| and is [one of the supported](README.md#direct-to-collector) log collection | ||
| approaches. | ||
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| The log appender implementation will typically acquire a [Logger](#logger) from the | ||
| global [LoggerProvider](#loggerprovider) at startup time, then | ||
| call [Emit LogRecord](#emit-a-logrecord) for `LogRecord`s received from the | ||
| application. | ||
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| [Implicit Context Injection](#implicit-context-injection) | ||
| and [Explicit Context Injection](#explicit-context-injection) describe how an | ||
| Appender injects `TraceContext` into `LogRecord`s. | ||
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|  | ||
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| This same approach can be also used for example for: | ||
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| - 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. | ||
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| 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. | ||
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| ### Logger Name | ||
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| If the logging library has a concept that is similar to OpenTelemetry's | ||
| definition of the [Instrumentation Scope's](../glossary.md#instrumentation-scope) | ||
| name then the appender's implementation should use that value as the | ||
| name parameter when [obtaining the Logger](./bridge-api.md#get-a-logger). | ||
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| This is for example applicable to: | ||
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| - Logger name in [Log4J](https://javadoc.io/doc/org.apache.logging.log4j/log4j-api/latest/org.apache.logging.log4j/org/apache/logging/log4j/Logger.html) | ||
| - Channel name in [Monolog](https://github.com/Seldaek/monolog/blob/main/doc/01-usage.md#leveraging-channels) | ||
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| Appender's should avoid setting other attributes of the Instrumentation Scope. | ||
| Doing so may result in different appenders setting different attributes on the | ||
| same Instrumentation Scope, obtained with the same identity of the | ||
| [Logger](./bridge-api.md#get-a-logger), which according to the specification | ||
| is an error. | ||
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| ### Context | ||
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| #### Implicit Context Injection | ||
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| 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](#emit-a-logrecord). | ||
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| 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](#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. | ||
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| TODO: clarify how works or doesn't work when the log statement call site and the | ||
| log appender are executed on different threads. | ||
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| #### Explicit Context Injection | ||
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| In order for `TraceContext` to be recorded in `LogRecord`s 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](#emit-a-logrecord). | ||
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| 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. | ||
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| 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. | ||
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| See | ||
| [an example](https://docs.google.com/document/d/15vR7D1x2tKd7u3zaTF0yH1WaHkUr2T4hhr7OyiZgmBg/edit#heading=h.4xuru5ljcups) | ||
| of how it can be done for zap logging library for Go. |