Custom pings

Applications can define metrics that are sent in custom pings. Unlike the built-in pings, custom pings are sent explicitly by the application.

This is useful when the scheduling of the built-in pings (metrics, baseline and events) are not appropriate for your data. Since the timing of the submission of custom pings is handled by the application, the measurement window is under the application's control.

This is especially useful when metrics need to be tightly related to one another, for example when you need to measure the distribution of frame paint times when a particular rendering backend is in use. If these metrics were in different pings, with different measurement windows, it is much harder to do that kind of reasoning with much certainty.

Defining a custom ping

Custom pings must be defined in a pings.yaml file, placed in the same directory alongside your app's metrics.yaml file.

For example, to define a custom ping called search specifically for search information:

$schema: moz://mozilla.org/schemas/glean/pings/2-0-0

search:
  description: >
    A ping to record search data.
  include_client_id: false
  notification_emails:
    - CHANGE-ME@example.com
  bugs:
    - http://bugzilla.mozilla.org/123456789/
  data_reviews:
    - http://example.com/path/to/data-review

Refer to the pings YAML registry format for a full reference on the pings.yaml file structure.

Sending metrics in a custom ping

To send a metric on a custom ping, you add the custom ping's name to the send_in_pings parameter in the metrics.yaml file.

Ping metadata must be loaded before sending!

After defining a custom ping, before it can be used for sending data, its metadata must be loaded into your application or library.

For example, to define a new metric to record the default search engine, which is sent in a custom ping called search, put search in the send_in_pings parameter. Note that it is an error to specify a ping in send_in_pings that does not also have an entry in pings.yaml.

search.default:
  name:
    type: string
    description: >
      The name of the default search engine.
    send_in_pings:
      - search

If this metric should also be sent in the default ping for the given metric type, you can add the special value default to send_in_pings:

    send_in_pings:
      - search
      - default

The glean.restarted event

For custom pings that contain event metrics, the glean.restarted event is injected by Glean on every application restart that may happen during the pings measurement window.

Only applies to the Glean JavaScript SDK

The behavior described in this section only applies to custom pings sent by the Glean JavaScript SDK. Follow the implementation of this feature on the other SDKs through Bug 1716725.

Event timestamps throughout application restarts

Event timestamps are always calculated relative to the first event in a ping. The first event will always have timestamp 0 and subsequent events will have timestamps corresponding to the elapsed amount of milliseconds since that first event.

That is also the case for events recorded throughout restarts.

Example

In the below example payload, there were two events recorded on the first application run. The first event is timestamp 0 and the second event happens one second after the first one, so it has timestamp 1000.

The application is restarted one hour after the first event and a glean.restarted event is recorded, timestamp 3600000. Finally, an event is recorded during the second application run two seconds after restart, timestamp 3800000.

{
  ...
  "events": [
    {
      "timestamp": 0,
      "category": "examples",
      "name": "event_example",
    },
    {
      "timestamp": 1000,
      "category": "examples",
      "name": "event_example"
    },
    {
      "timestamp": 3600000,
      "category": "glean",
      "name": "restarted"
    },
    {
      "timestamp": 3800000,
      "category": "examples",
      "name": "event_example"
    },
  ]
}

Caveat: Handling decreasing time offsets

For events recorded in a single application run, Glean relies on a monotonically increasing timer to calculate event timestamps, while for calculating the time elapsed between application runs Glean has to rely on the computer clock, which is not necessarily monotonically increasing.

In the case that timestamps in between application runs are not monotonically increasing, Glean will take the value of the previous timestamp and add one millisecond, thus guaranteeing that timestamps are always increasing.

Checking for decreasing time offsets between restarts

When this edge case is hit, Glean records an InvalidValue error for the glean.restarted metric. This metric may be consulted at analysis time. It is sent in the same ping where the error happened.

In the below example payload, the first and second application runs go exactly like in the example above.

The only difference is that when the restart happens, the offset between the absolute time of the first event and the absolute time of the restart is not enough to keep the timestamps increasing. That may happen for many reasons, such as a change in timezones or simply a manual change in the clock by the user.

In this case, Glean will ignore the incorrect timestamp and add one millisecond to the last timestamp of the previous run, in order to keep the monotonically increasing nature of the timestamps.

{
  ...
  "events": [
    {
      "timestamp": 0,
      "category": "examples",
      "name": "event_example",
    },
    {
      "timestamp": 1000,
      "category": "examples",
      "name": "event_example"
    },
    {
      "timestamp": 1001,
      "category": "glean",
      "name": "restarted"
    },
    {
      "timestamp": 3001,
      "category": "examples",
      "name": "event_example"
    },
  ]
}