* <<micrometer-observation-default-meter-handler, DefaultMeterObservationHandler>>

[[micrometer-observation-default-meter-handler]]

=== DefaultMeterObservationHandler

Micrometer provides `DefaultMeterObservationHandler`, an `ObservationHandler` implementation that automatically creates metrics from Observations.

When registered on an `ObservationRegistry`, it translates Observation lifecycle events into the following meter types:

* **Timer** (`<observation-name>`) -- Created on `stop`. Records the duration between `start` and `stop`. Tagged with all low cardinality key values and an `error` tag (set to the exception class name if an error was recorded, or `none` otherwise).

* **LongTaskTimer** (`<observation-name>.active`) -- Created on `start`. Tracks the duration of in-progress observations. Stopped when the observation is stopped. Only tagged with low cardinality key values that are available at the time of `start`.

* **Counter** (`<observation-name>.<event-name>`) -- Incremented when an event is signaled via `Observation.event(Event)`. Tagged with all low cardinality key values available at the time of the event.

NOTE: Only low cardinality key values are used as meter tags. High cardinality key values are not included in meters because they can cause a cardinality explosion in the metrics backend.

WARNING: Since the `LongTaskTimer` is created in the `onStart` method, it can only contain tags that are available by that time. Any low cardinality key values added after calling `start` on the `Observation` will not be included as tags on the `LongTaskTimer`.

The following example shows how to register `DefaultMeterObservationHandler` and the metrics it creates:

[source,java,subs=+attributes]

If you do not need the `LongTaskTimer` (for example, to reduce the number of created meters), you can disable it using `IgnoredMeters`:

[source,java,subs=+attributes]

@Test

void default_meter_observation_handler() {

// tag::default_meter_handler[]

MeterRegistry meterRegistry = new SimpleMeterRegistry();

ObservationRegistry registry = ObservationRegistry.create();

// Register DefaultMeterObservationHandler to create metrics from observations

registry.observationConfig().observationHandler(new DefaultMeterObservationHandler(meterRegistry));

// Perform an observation

Observation observation = Observation.createNotStarted("my.operation", registry)

.lowCardinalityKeyValue("region", "us-east-1");

observation.start(); // LongTaskTimer "my.operation.active" starts here

observation.event(Observation.Event.of("my.event")); // Counter

// "my.operation.my.event"

// incremented

observation.stop(); // Timer "my.operation" and LongTaskTimer stop

// Verify metrics were created:

// Timer: my.operation (with tags: region=us-east-1, error=none)

assertThat(meterRegistry.get("my.operation").timer().count()).isEqualTo(1);

// Counter: my.operation.my.event (with tags: region=us-east-1)

assertThat(meterRegistry.get("my.operation.my.event").counter().count()).isEqualTo(1);

// end::default_meter_handler[]

}

@Test

void default_meter_observation_handler_ignore_long_task_timer() {

// tag::default_meter_handler_ignore_ltt[]

MeterRegistry meterRegistry = new SimpleMeterRegistry();

ObservationRegistry registry = ObservationRegistry.create();

// You can disable the LongTaskTimer if you don't need it

registry.observationConfig()

.observationHandler(new DefaultMeterObservationHandler(meterRegistry,

DefaultMeterObservationHandler.IgnoredMeters.LONG_TASK_TIMER));

Observation.createNotStarted("my.operation", registry)

.lowCardinalityKeyValue("region", "us-east-1")

.start()

.stop();

// Timer is created, but no LongTaskTimer

assertThat(meterRegistry.get("my.operation").timer().count()).isEqualTo(1);

assertThat(meterRegistry.find("my.operation.active").longTaskTimer()).isNull();

// end::default_meter_handler_ignore_ltt[]

}