Using Firefox on Glean to Record Legacy Telemetry Scalars

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

A Glean metric definition to generate a Glean API
- This will go in a metrics.yaml file likely in your component
- The specific Glean metric you will use depends on how you record your data. We have prepared this guide to help you out.
- If you already know of a Legacy Telemetry scalar definition similar to what you want, you can use ./mach gifft <Legacy Telemetry scalar name like browser.backup.enabled> to generate a Glean metric definition from that Legacy Telemetry scalar definition.
A Legacy Telemetry scalar definition, for the Glean Interface For Firefox Telemetry to mirror to
- Place it in toolkit/components/telemetry/Scalars.yaml
- Be sure to add the telemetry_mirror property to the Glean metric definition from step 1. The value to use for telemetry_mirror is a straightforward conjugation of the scalar’s category and name.

Now build Firefox.

To record data to your new scalar, use the Glean API for the Glean metric you defined in Step 1.

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

Your Legacy Telemetry scalar 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’s C++ enums, which means that testing the Legacy Telemetry scalar or seeing its value in about:telemetry requires a full, compiled build.