Module telemetry

Description
Data Types
Function Index
Function Details

telemetry allows you to invoke certain functions whenever a particular event is emitted.

Description

telemetry allows you to invoke certain functions whenever a particular event is emitted.

For more information see the documentation for attach/4, attach_many/4 and execute/2.

Data Types

handler()

handler() = #{id := handler_id(), event_name := event_name(), function := handler_function(), config := handler_config()}

handler_function()

handler_function() = fun((event_name(), event_measurements(), event_metadata(), handler_config()) -> any())

span_function()

span_function() = fun(() -> {span_result(), event_metadata()})

Function Index

attach/4	Attaches the handler to the event.
attach_many/4	Attaches the handler to many events.
detach/1	Removes the existing handler.
execute/2	Equivalent to `execute(EventName, Measurements, #{})`.
execute/3	Emits the event, invoking handlers attached to it.
list_handlers/1	Returns all handlers attached to events with given prefix.
span/3	Emit start, and stop/exception events, invoking the handlers attached to each.

Function Details

attach/4

attach(HandlerId, EventName, Function, Config) -> ok | {error, already_exists}

HandlerId = handler_id()
EventName = event_name()
Function = handler_function()
Config = handler_config()

Attaches the handler to the event.

handler_id must be unique, if another handler with the same ID already exists the {error, already_exists} tuple is returned.

See execute/3 to learn how the handlers are invoked.

Note: due to how anonymous functions are implemented in the Erlang VM, it is best to use function captures (i.e. fun mod:fun/4 in Erlang or &Mod.fun/4 in Elixir) as event handlers to achieve maximum performance. In other words, avoid using literal anonymous functions (fun(...) -> ... end or fn ... -> ... end) or local function captures (fun handle_event/4 or &handle_event/4 ) as event handlers.

All the handlers are executed by the process dispatching event. If the function fails (raises, exits or throws) then the handler is removed. Note that you should not rely on the order in which handlers are invoked.

attach_many/4

attach_many(HandlerId, EventNames::[EventName], Function, Config) -> ok | {error, already_exists}

HandlerId = handler_id()
EventName = event_name()
Function = handler_function()
Config = handler_config()

Attaches the handler to many events.

The handler will be invoked whenever any of the events in the event_names list is emitted. Note that failure of the handler on any of these invocations will detach it from all the events in event_name (the same applies to manual detaching using detach/1).

detach/1

detach(HandlerId::handler_id()) -> ok | {error, not_found}

Removes the existing handler.

If the handler with given ID doesn't exist, {error, not_found} is returned.

execute/2

execute(EventName, Measurements) -> ok

EventName = event_name()
Measurements = event_measurements() | event_value()

Equivalent to execute(EventName, Measurements, #{}).

execute/3

execute(EventName, Measurements, Metadata) -> ok

EventName = event_name()
Measurements = event_measurements() | event_value()
Metadata = event_metadata()

Emits the event, invoking handlers attached to it.

When the event is emitted, the handler function provided to attach/4 is called with four arguments:

the event name
the map of measurements
the map of event metadata
the handler configuration given to attach/4

Best practices and conventions:

While you are able to emit messages of any event_name structure, it is recommended that you follow the the guidelines laid out in span/3 if you are capturing start/stop events.

list_handlers/1

list_handlers(EventPrefix::event_prefix()) -> [handler()]

Returns all handlers attached to events with given prefix.

Handlers attached to many events at once using attach_many/4 will be listed once for each event they're attached to. Note that you can list all handlers by feeding this function an empty list.

span/3

span(EventPrefix::event_prefix(), StartMetadata::event_metadata(), SpanFunction::span_function()) -> span_result()

Emit start, and stop/exception events, invoking the handlers attached to each.

When this function is called, 2 events will be emitted via execute/3. Those events will be one of the following pairs:

EventPrefix ++ [start] and EventPrefix ++ [stop]
EventPrefix ++ [start] and EventPrefix ++ [exception]

However, note that in case the current processes crashes due to an exit signal of another process, then none or only part of those events would be emitted. Below is a breakdown of the measurements and metadata associated with each individual event.

A default span context is added to event metadata under the

telemetry_span_context` key if none is provided by
  the user in the `StartMetadata`. This context is useful for tracing libraries to identify unique
  executions of span events within a process to match start, stop, and exception events. Users
  should ensure this value is unique within the context of a process at a minimum if overriding this key and
  that the same value is provided to both `StartMetadata` and `StopMetadata`.
 
  For `telemetry

events denoting the start of a larger event, the following data is provided:

Event:
```
  EventPrefix ++ [start]
```

Measurements:

  #{
    % The current system time in native units from
    % calling: erlang:system_time()
    system_time => integer()
  }

Metadata:

  #{
    telemetry_span_context => term(),
    % User defined metadata
    ...
  }

For telemetry events denoting the stop of a larger event, the following data is provided:

Event:
```
  EventPrefix ++ [stop]
```

Measurements:

  #{
    % The current monotonic time minus the start monotonic time in native units
    % by calling: erlang:monotonic_time() - start_monotonic_time
    duration => integer()
  }

Metadata:

  #{
    % An optional error field if the stop event is the result of an error
    % but not necessarily an exception. Additional user defined metadata can
    % also be added here.
    error => term(),
    telemetry_span_context => term(),
    ...
  }

For telemetry events denoting an exception of a larger event, the following data is provided:

Event:
```
  EventPrefix ++ [exception]
```

Measurements:

  #{
    % The current monotonic time minus the start monotonic time in native units
    % derived by calling: erlang:monotonic_time() - start_monotonic_time
    duration => integer()
  }

Metadata:

  #{
    kind => throw | error | exit,
    reason => term(),
    stacktrace => list(),
    telemetry_span_context => term(),
    % User defined metadata from the start event
     ...
  }

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