Using Firefox on Glean to Record Legacy Telemetry Events

To record Legacy Telemetry events using Glean APIs, you need these two things:

1. A Glean event definition to generate a Glean API

   • This will go in a metrics.yaml file likely in your component

   • If you’re familiar with Telemetry events, you might like to read this short section about how Glean events are different.

   • The Glean event metric docs has an example definition you might find helpful.

   • If you already know of a Legacy Telemetry event definition similar to what you want, you can use ./mach gifft <Legacy Telemetry event name like readermode.view> to generate a Glean event metric definition from that Legacy Telemetry event definition.

2. A Legacy Telemetry event definition, for the Glean Interface For Firefox Telemetry to mirror to

   • Use ./mach event-into-legacy <Glean event metric name like privacy.sanitize.dialog_open> to generate this automatically.

   • Place it in toolkit/components/telemetry/Events.yaml

   • Be sure to add the telemetry-mirror property to the Glean event definition from step 1. You can follow the instructions in the output from ./mach event-into-legacy, or this guide for determining the Legacy Telemetry event’s enum name manually.

Now build Firefox.

To record your new event, use the Glean record(...) API.

To test your new event, use the Glean testGetValue() API.

Your Legacy Telemetry event will appear in about:telemetry when your code is triggered as confirmation this is all working as you expect.

Artifact Build Support

Firefox on Glean supports artifact builds, so you can instrument and test your Glean instrumentation with an artifact build. However, mirroring requires use of the Legacy Telemetry event’s C++ enum, which means that testing the Legacy Telemetry event or seeing its value in about:telemetry requires a full, compiled build.