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===========================
Telemetry review guidelines
===========================
General guidelines for reviewing changes
========================================
These are the general principles we follow when reviewing changes.
- *Be constructive.* Both reviewers and patch authors should be allies that aim to get the change landed together.
- *Consider the impact.* We deliver critical data that is processed and used by many systems and people. Any disruption should be planned and intentional.
- *Be diligent.* All changes should be tested under typical conditions.
- *Know your limits.* Defer to other peers or experts where sensible.
Main considerations for any change
========================================
For any change, these are the fundamental questions that we always need satisfactory answers for.
- Does this have a plan?
- Is there a specific need to do this?
- Does this change need `a proposal <https://github.com/mozilla/Fx-Data-Planning/blob/master/process/ProposalProcess.md>`_ first?
- Do we need to announce this before we do this? (e.g. for `deprecations <https://github.com/mozilla/Fx-Data-Planning/blob/master/process/Deprecation.md>`_)
- Does this involve the right people?
- Does this change need input from... A data engineer? A data scientist?
- Does this change need data review?
- Is this change complete?
- Does this change have sufficient test coverage?
- Does this change have sufficient documentation?
- Do we need follow-ups?
- Do we need to file validation bugs? Or other follow-up bugs?
- Do we need to communicate this to our users? Or other groups?
Additional considerations
=========================
Besides the basic considerations above, these are additional detailed considerations that help with reviewing changes.
Considerations for all changes
------------------------------
- Follow our standards and best practices.
- Firefox Desktop:
- :ref:`The Mozilla coding style <Coding style>`
- Mobile:
- Does this impact performance significantly?:
- Don't delay application initialisation (unless absolutely necessary).
- Don't ever block on network requests.
- Make longer tasks async whenever feasible.
- Does this affect products more broadly than expected?
- Consider all our platforms: Windows, Mac, Linux, Android.
- Consider all our products: Firefox, Fenix, Glean.
- Does this fall afoul of common architectural failures?
- Prefer APIs that take non-String types unless writing a parser.
- Sanity checking:
- How does this behave in a release build? Have you tested this?
- Does this change contain test coverage? We require test coverage for every new code, changes and bug fixes.
- Does this need documentation updates?
- To the :ref:`in-tree docs <Telemetry>`?
- To the `firefox-data-docs <https://docs.telemetry.mozilla.org/>`_ (`repository <https://github.com/mozilla/firefox-data-docs>`_)?
- To the `glean documentation <https://github.com/mozilla-mobile/android-components/tree/master/components/service/glean>`_?
- Following up:
- Do we have a validation bug filed yet?
- Do all TODOs have follow-up bugs filed?
- Do we need to communicate this to our users?
- fx-team (Firefox staff)
- Do we need to communicate this to other groups?
- Data engineering, data science, data stewards, ...?
Consider the impact on others
-----------------------------
- Could this change break upstream pipeline jobs?
- Does this change the format of outgoing data in an unhandled way?
- E.g. by adding, removing, or changing the type of a non-metric payload field.
- Does this require ping schema updates?
- Does this break jobs that parse the registry files for metrics? (Scalars.yaml, metrics.yaml, etc.)
- Do we need to involve others?
- Changes to data formats, ping contents, ping semantics etc. require involving a data engineer.
- Changes to any outgoing data that is in active use require involving the stakeholders (e.g. data scientists).
Considerations for Firefox Desktop
----------------------------------
- For profiles:
- How does using different profiles affect this?
- How does switching between profiles affect this?
- What happens when users switch between different channels?
- Footguns:
- Does this have side-effects on Fennec? (Unified Telemetry is off there, so behavior is pretty different.)
- Is your code gated on prefs, build info, channels? Tests should cover that.
- If test is gated on isUnified, code should be too (and vice-versa)
- Test for the other case
- Any code using `new Date()` should get additional scrutiny
- Code using `new Date()` should be using Policy so it can be mocked
- Tests using `new Date()` should use specific dates, not the current one
- How does this impact Build Faster support/Artifact builds/Dynamic builtin scalars or events? Will this be testable by others on artifact builds?
- We work in the open: Does the change include words that might scare end users?
- How does this handle client id resets?
- How does this handle users opting out of data collection?