View Source Semantic Conventions for Events

Status: Experimental

This document describes the characteristics of standalone Events that are represented
in the data model by LogRecords.

Semantically, an Event is a named occurrence at an instant in time. It signals that
"this thing happened at this time" and provides additional specifics about the occurrence.
Examples of Events might include things like uncaught exceptions, button clicks, user logout,
network connection severed, etc.

In OpenTelemetry, Events are implemented as a specific type of LogRecord that conforms to
the conventions included here, and Events
have their own API.
The API abstracts away knowledge of LogRecord so that users only deal with Event
semantics.

In addition to a required name, an Event may contain a payload (body) of any type permitted
by the LogRecord body.
In its implementation, the Event payload (body) will constitute the Body of the LogRecord.
Like all other OpenTelemetry signals, an Event has optional attribute metadata that helps describe
the event context.

Over time, some Events will be specified by OpenTelemetry and will have documented payload structure,
field semantics, and stability and requirement levels. Other events may be user-defined and carry
bespoke user semantics. When an Event name exists in the semantic conventions, its payload
structure and semantics will also be defined.

Event definition

Attribute	Type	Description	Examples	Requirement Level	Stability
event.name	string	Identifies the class / type of event. [1]	browser.mouse.click; device.app.lifecycle	Required

[1]: Event names are subject to the same rules as attribute names. Notably, event names are namespaced to avoid collisions and provide a clean separation of semantics for events in separate domains like browser, mobile, and kubernetes.

General event semantics

• An event MUST have an event.name attribute that uniquely identifies the event.
• It MAY have standard
  attributes that provide additional context about the event.
• It MAY contain a payload (body) that describes the specific details of the
  named event.
• The event name uniquely identifies event structure / type of the payload (body)
  and the set of attributes.
• The payload (body) MAY contain any type supported by the OpenTelemetry data
  model for the log body
  and the semantic conventions will define the expected structure of the payload
  (body) for the event.
• The payload (body) SHOULD be used to represent the structure of the event.

Recommendations for defining events:

• Use the payload (body) to represent the details of the event instead of a
  collection of standard
  attributes.
• Events SHOULD be generated / produced / recorded using the
  Event API
  to ensure that the event is created using the configured SDK instance.
  • The Event API is not yet available in all OpenTelemetry SDKs.
  • TODO: Add deep link to the compliance matrix of the Event API
    when it exists.
• It's NOT RECOMMENDED to prefix the payload (body) fields with the event.name to
  avoid redundancy and to keep the event definition clean.
• The events SHOULD document their semantic conventions including event name,
  attributes, and the payload.

Recommendations on using attributes vs. body fields:

• If the field should be comparable across every type of record, it should be an attribute.
• If the field is specific to the event itself, then it should be a body field.
• Unless the same event.name exists on two events, anything in two event bodies is not comparable to each other.

Event payload (body)

• Common attribute naming conventions and registry
  requirements don't apply to event payload fields.
• The definition for OpenTelemetry defined events supports describing
  individual fields (Body Fields)
  • The fields are unique to the named event (event.name) and different events
    may use the same field name to represent different data, due to the unique
    nature of the event.
• The fields MAY reference / inherit details from the attribute registry
  attributes and provide additional details specific to the event, including
  providing an alias (shorter) name for the attribute.

External event compatibility

When recording events from an existing system as OpenTelemetry Events, it is
possible that the existing system does not have the equivalent of a name or
requires multiple fields to identify the structure of the events. In such cases,
OpenTelemetry recommends using a combination of one or more fields as the name
such that the name identifies the event structurally. It is also recommended that
the event names have low-cardinality, so care must be taken to use fields
that identify the class of Events but not the instance of the Event.