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Event basics #265

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75 changes: 75 additions & 0 deletions text/0265-event-vision.md
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# Event Basics

## Motivation

The introduction of Events has been contentious, so we want to document and agree on a few basics.

### What are OpenTelemetry Events?

OpenTelemetry Events are a type of OpenTelemetry Log that requires an event name and follows a specific structure implied by that event name.

They are a core concept in OpenTelemetry Semantic Conventions.

### OTLP

Since OpenTelemetry Events are a type of OpenTelemetry Log, they share the same OTLP log data structure and pipeline.

### API

OpenTelemetry should provide a (user-facing) Log API that includes the capability to emit OpenTelemetry Events.
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### Interoperability with Generic Logging Libraries

It should be possible to send OpenTelemetry Logs from the OpenTelemetry Log API to a generic logging library (e.g., Log4j).
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This allows users to integrate OpenTelemetry Logs into an existing (non-OpenTelemetry) log stream.

It should also be possible to bypass the OpenTelemetry Log API entirely and emit OpenTelemetry Logs (including Events)
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directly via existing language-specific logging libraries.
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OpenTelemetry will recommend that
[instrumentation libraries](https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/glossary.md#instrumentation-library)
use the OpenTelemetry Log API to emit OpenTelemetry Events rather than using a generic logging library. This recommendation aims to provide users with a simple and consistent
onboarding experience that avoids mixing approaches.

OpenTelemetry will also recommend that application developers use the OpenTelemetry Log API to emit OpenTelemetry Events instead of using a generic
logging library, as this helps prevent accidentally emitting logs that lack an event name or are unstructured.

Recommending the OpenTelemetry Log API for emitting OpenTelemetry Events, rather than using a generic logging library, contributes to a clearer overall
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OpenTelemetry API story. This ensures a unified approach with first-class user-facing APIs for traces, metrics, and events,
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all suitable for direct use in native instrumentation.

### Relationship to Span Events

Events are intended to replace Span Events in the long term.
Span Events will be deprecated to signal that users should prefer Events.
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Interoperability between Events and Span Events will be defined in the short term.

### SDK

This refers to the existing OpenTelemetry Log SDK.

## Trade-offs and mitigations

TODO
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## Prior art and alternatives

TODO
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## Open questions

* How to support routing logs from the Log API to a language-specific logging library
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while simultaneously routing logs from the language-specific logging library to an OpenTelemetry Logging Exporter?
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* How do log bodies interoperate with generic logging libraries?
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* How do event bodies interoperate with Span Events?

## Future possibilities
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* The Event API will likely need an `IsEnabled` function based on severity level, scope name, and event name.
* Ergonomic improvements to make it more attractive as a replacement for generic logging APIs.
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* Capturing raw metric events as opposed to aggregating and emitting them as OpenTelemetry Metric data
(e.g. [opentelemetry-specification/617](https://github.com/open-telemetry/opentelemetry-specification/issues/617)).
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* Capturing raw span events as opposed to aggregating and emitting them as OpenTelemetry Span data
(e.g. a [streaming SDK](https://github.com/search?q=repo%3Aopen-telemetry%2Fopentelemetry-specification+%22streaming+sdk%22&type=issues)).
* Capturing events and computing metrics from them.