[lttng-docs.git] / contents / understanding-lttng / core-concepts / event.md

---
id: event
---

An _event_, in LTTng's realm, is a term often used metonymically,
having multiple definitions depending on the context:

  1. When tracing, an event is a _point in space-time_. Space, in a
     tracing context, is the set of all executable positions of a
     compiled application by a logical processor. When a program is
     executed by a processor and some instrumentation point, or
     _probe_, is encountered, an event occurs. This event is accompanied
     by some contextual payload (values of specific variables at this
     point of execution) which may or may not be recorded.
  2. In the context of a recorded trace file, the term _event_ implies
     a _recorded event_.
  3. When configuring a tracing session, _enabled events_ refer to
     specific rules which could lead to the transfer of actual
     occurring events (1) to recorded events (2).

The whole [Core concepts](#doc-core-concepts) section focuses on the
third definition. An event is always registered to _one or more_
channels and may be enabled or disabled at will per channel. A disabled
event never leads to a recorded event, even if its channel is enabled.

An event (3) is enabled with a few conditions that must _all_ be met
when an event (1) happens in order to generate a recorded event (2):

  1. A _probe_ or group of probes in the traced application must be
     executed.
  2. **Optionally**, the probe must have a log level matching a
     log level range specified when enabling the event.
  3. **Optionally**, the occurring event must satisfy a custom
     expression, or _filter_, specified when enabling the event.
Commit	Line	Data
5e0cbfb0 PP	1	---
	2	id: event
	3	---
	4
	5	An _event_, in LTTng's realm, is a term often used metonymically,
	6	having multiple definitions depending on the context:
	7
	8	1. When tracing, an event is a _point in space-time_. Space, in a
	9	tracing context, is the set of all executable positions of a
	10	compiled application by a logical processor. When a program is
	11	executed by a processor and some instrumentation point, or
	12	_probe_, is encountered, an event occurs. This event is accompanied
	13	by some contextual payload (values of specific variables at this
	14	point of execution) which may or may not be recorded.
	15	2. In the context of a recorded trace file, the term _event_ implies
	16	a _recorded event_.
	17	3. When configuring a tracing session, _enabled events_ refer to
	18	specific rules which could lead to the transfer of actual
	19	occurring events (1) to recorded events (2).
	20
0c9c0c96	21	The whole [Core concepts](#doc-core-concepts) section focuses on the
5e0cbfb0 PP	22	third definition. An event is always registered to _one or more_
5e0cbfb0 PP	23	channels and may be enabled or disabled at will per channel. A disabled
47bfcb75	24	event never leads to a recorded event, even if its channel is enabled.
5e0cbfb0 PP	25
	26	An event (3) is enabled with a few conditions that must _all_ be met
	27	when an event (1) happens in order to generate a recorded event (2):
	28
	29	1. A _probe_ or group of probes in the traced application must be
	30	executed.
	31	2. Optionally, the probe must have a log level matching a
	32	log level range specified when enabling the event.
	33	3. Optionally, the occurring event must satisfy a custom
	34	expression, or _filter_, specified when enabling the event.