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Bug 1852098 - Move and update Use Counter documentation r=emilio
Differential Revision: https://phabricator.services.mozilla.com/D193247
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@ -16,3 +16,4 @@ These linked pages contain design documents for the DOM implementation in Gecko.
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ioutils_migration
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fedcm
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streams
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use-counters
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@ -0,0 +1,165 @@
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============
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Use Counters
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============
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Use counters are used to report statistics on how much a given web platform feature is used across the Web.
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Supported features include:
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* WebIDL methods and attributes (getters and setters are reported separately) for pages, documents, and workers,
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* CSS properties (including properties that aren't in the web platform, but we're interested in),
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* Deprecated DOM operations,
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* Other things like SVG filters and APIs specifically unsupported in Private Browsing Mode,
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via custom use counters.
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Adding a new Use Counter
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========================
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How you add a new use counter is different depending on what kind of web platform feature you're instrumenting.
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The one constant is that you must run ``./mach gen-use-counter-metrics``
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after adding or removing a use counter.
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(Why this is a manual step and not part of the build is explained in
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`the implementation bug 1852098 <https://bugzilla.mozilla.org/show_bug.cgi?id=1852098#c11>`_.)
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WebIDL Methods and Attributes
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-----------------------------
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Use counters for WebIDL Methods and Attributes are added manually by editing
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:searchfox:`UseCounters.conf <dom/base/UseCounters.conf>` or, for workers,
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:searchfox:`UseCountersWorker.conf <dom/base/UseCountersWorker.conf>`, and
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by annotating the WebIDL Method or Attribute with the ``[UseCounter]``
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extended attribute.
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(Why you must write this in two places is because generating things from
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bindings codegen and ensuring all the dependencies were correct proved to be
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rather difficult)
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Then run ``./mach gen-use-counter-metrics`` and build as normal.
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CSS Properties
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--------------
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Use counters for CSS properties are automatically generated for every property Gecko supports.
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To add a use counter for a CSS property that isn't supported by Gecko,
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add it to :searchfox:`counted_unknown_properties.py <servo/components/style/properties/counted_unknown_properties.py>`.
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Then run ``./mach gen-use-counter-metrics`` and build as normal.
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Deprecated DOM operations
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-------------------------
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Use counters for deprecated DOM operations are declared in
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:searchfox:`nsDeprecatedOperationList.h <dom/base/nsDeprecatedOperationList.h>`.
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To add a use counter for a deprecated DOM operation, you'll add an invocation of the
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``DEPRECATED_OPERATION(DeprecationReference)`` macro.
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The provided parameter must have the same value of the deprecation note added to the *IDL* file.
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See `bug 1860635 <https://bugzilla.mozilla.org/show_bug.cgi?id=1860635>`_ for a sample
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deprecated operation.
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Then run ``./mach gen-use-counter-metrics`` and build as normal.
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Custom use counters
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-------------------
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Custom use counters are for counting per-page, per-document, or per-worker
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uses of web platform features that can't be handled directly through WebIDL annotation.
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For example, the use of specific SVG filters isn't a WebIDL method or attribute,
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but was still an aspect of the web platform of interest.
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To add a custom use counter, define it in
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:searchfox:`UseCounters.conf <dom/base/UseCounters.conf>` or, for workers,
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:searchfox:`UseCountersWorker.conf <dom/base/UseCountersWorker.conf>`
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by following the instructions in the file.
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Broadly, you'll be writing a line like ``custom feBlend uses the feBlend SVG filter``.
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Then, by running the build as normal, an enum in ``enum class UseCounter``
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will be generated for your use counter, which you should pass to
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``Document::SetUseCounter()`` when it's used.
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``Document::SetUseCounter()`` is very cheap,
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so do not be afraid to call it every time the feature is used.
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Take care to craft the description appropriately.
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It will be appended to "Whether a document " or "Whether a shared worker ",
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so write only the ending.
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The processor scripts
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=====================
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The definition files are processed during the build to generate C++ headers
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included by web platform components (e.g. DOM) that own the features to be tracked.
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The definition files are also processed during ``./mach gen-use-counter-metrics``
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to generate :searchfox:`use_counter_metrics.yaml <dom/base/use_counter_metrics.yaml>`
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which generates the necessary Glean metrics for recording and reporting use counter data.
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gen-usecounters.py
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------------------
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This script is called by the build system to generate:
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- the ``UseCounterList.h`` header for the WebIDL, out of the definition files.
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- the ``UseCounterWorkerList.h`` header for the WebIDL, out of the definition files.
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usecounters.py
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--------------
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Contains methods for parsing and transforming use counter definition files,
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as well as the mechanism that outputs the Glean use counter metrics definitions.
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Data Review
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===========
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The concept of a Use Counter data collection
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(being a web platform feature which has the number of pages, documents, workers
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(of various types), or other broad category of web platform API surfaces that
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*use* it recorded and reported by a data collection mechanism (like Glean))
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was approved for opt-out collection in all products using Gecko and Glean in
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`bug 1852098 <https://bugzilla.mozilla.org/show_bug.cgi?id=1852098>`_.
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As a result,
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if you are adding new use counter data collections for WebIDL methods or attributes,
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deprecated operations, or CSS properties:
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you almost certainly don't need a data collection review.
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If you are adding a custom use counter, you might need a data collection review.
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The criteria for whether you do or not is whether the custom use counter you're adding
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can fall under
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`the over-arching data collection review request <https://bugzilla.mozilla.org/show_bug.cgi?id=1852098>`_.
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For example: a custom use counter for an SVG filter? Clearly a web platform feature being counted.
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A custom use counter that solely increments when you visit a social media website?
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Doesn't seem like it'd be covered, no.
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If unsure, please ask on
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`the #data-stewards channel on Matrix <https://chat.mozilla.org/#/room/#data-stewards:mozilla.org>`_.
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The Data
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========
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Use Counters are, as of Firefox 121, collected using Glean as
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``counter`` metrics on the "use-counters" ping.
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They are in a variety of metrics categories of ``use.counter.X``
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which you can browse on
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`the Glean Dictionary <https://dictionary.telemetry.mozilla.org/apps/firefox_desktop?page=1&search=use.counter>`_.
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The dictionary also contains information about how to view the data.
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Interpreting the data
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---------------------
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A use counter on its own is minimally useful, as it is solely a count of how many
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(pages, documents, workers of a specific type, other web platform API surfaces)
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a given part of the web platform was used on.
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Knowing a feature was encountered ``0`` times across all of Firefox would be useful to know.
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(If you wanted to remove something).
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Knowing a feature was encountered *more than* ``0`` times would be useful.
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(If you wanted to argue against removing something).
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But any other number of, say, pages using a web platform feature is only useful
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in context with how many total pages were viewed.
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Thus, each use counter has in its description a name of another counter
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-- a denominator -- to convert the use counter into a usage rate.
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Using pages as an example, knowing the CSS property ``overflow``
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is used on ``1504`` pages is... nice. I guess.
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But if you sum up ``use.counters.top_level_content_documents_destroyed``
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to find that there were only ``1506`` pages loaded?
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That's a figure we can do something with.
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We can order MDN search results by popularity.
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We can prioritize performance efforts in Gecko to focus on the most-encountered features.
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We can view the popularity over time and see when we expect we'll be able to deprecate and remove the feature.
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This is why you'll more likely encounter use counter data expressed as usage rates.
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@ -18,7 +18,6 @@ The current data collection possibilities include:
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* :doc:`events` can record richer data on individual occurrences of specific actions
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* :doc:`Measuring elapsed time <measuring-time>`
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* :doc:`Custom pings <custom-pings>`
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* :doc:`Use counters <use-counters>` measure the usage of web platform features
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* :doc:`Experiment annotations <experiments>`
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* :doc:`Remote content uptake <uptake>`
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* :doc:`WebExtension API <webextension-api>` can be used in privileged webextensions
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@ -1,105 +0,0 @@
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============
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Use Counters
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============
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Use counters are used to report Telemetry statistics on whether individual documents
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use a given WebIDL method or attribute (getters and setters are reported separately), CSS
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property, or deprecated DOM operation. Custom use counters can also be
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defined to test frequency of things that don't fall into one of those
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categories.
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As of Firefox 65 the collection of Use Counters is enabled on all channels.
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The API
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=======
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The process to add a new use counter is different depending on the type feature that needs
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to be measured. In general, for each defined use counter, two separate boolean histograms are generated:
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- one describes the use of the tracked feature for individual documents and has the ``_DOCUMENT`` suffix;
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- the other describes the use of the same thing for top-level pages (basically what we think of as a *web page*) and has the ``_PAGE`` suffix.
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Using two histograms is particularly suited to measure how many sites would be affected by
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removing the tracked feature.
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Example scenarios:
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- Site *X* triggers use counter *Y*. We report "used" (true) in both the ``_DOCUMENT`` and ``_PAGE`` histograms.
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- Site *X* does not trigger use counter *Y*. We report "unused" (false) in both the ``_DOCUMENT`` and ``_PAGE`` histograms.
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- Site *X* has an iframe for site *W*. Site *W* triggers use counter *Y*, but site *X* does not. We report one "used" and one "unused" in the individual ``_DOCUMENT`` histogram and one "used" in the top-level ``_PAGE`` histogram.
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Deprecated DOM operations
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-------------------------
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Use counters for deprecated DOM operations are declared in the `nsDeprecatedOperationList.h <https://searchfox.org/mozilla-central/source/dom/base/nsDeprecatedOperationList.h>`_ file. The counters are
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registered through the ``DEPRECATED_OPERATION(DeprecationReference)`` macro. The provided
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parameter must have the same value of the deprecation note added to the *IDL* file.
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See this `changeset <https://hg.mozilla.org/mozilla-central/rev/e30a357b25f1>`_ for a sample
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deprecated operation.
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CSS Properties
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~~~~~~~~~~~~~~
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Use counters for CSS properties are generated for every property Gecko supports automatically, and are counted via StyleUseCounters (`Rust code <https://searchfox.org/mozilla-central/rev/7ed8e2d3d1d7a1464ba42763a33fd2e60efcaedc/servo/components/style/use_counters/mod.rs>`_, `C++ code <https://searchfox.org/mozilla-central/rev/7ed8e2d3d1d7a1464ba42763a33fd2e60efcaedc/dom/base/Document.h#5077>`_).
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The UseCounters registry
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------------------------
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Use counters for WebIDL methods/attributes are registered in the `UseCounters.conf <https://searchfox.org/mozilla-central/source/dom/base/UseCounters.conf>`_ file. The format of this file is very strict. Each line can be:
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1. a blank line
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2. a comment, which is a line that begins with ``//``
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3. one of four possible use counter declarations:
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* ``method <IDL interface name>.<IDL operation name>``
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* ``attribute <IDL interface name>.<IDL attribute name>``
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* ``custom <any valid identifier> <description>``
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Custom use counters
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~~~~~~~~~~~~~~~~~~~
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The <description> for custom counters will be appended to "When a document " or "When a page ", so phrase it appropriately. For instance, "constructs a Foo object" or "calls Document.bar('some value')". It may contain any character (including whitespace). Custom counters are incremented when SetUseCounter(eUseCounter_custom_MyName) is called on a Document object.
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WebIDL methods and attributes
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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Additionally to having a new entry added to the `UseCounters.conf <https://searchfox.org/mozilla-central/source/dom/base/UseCounters.conf>`_ file, WebIDL methods and attributes must have a ``[UseCounter]`` extended attribute in the Web IDL file in order for the counters to be incremented.
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Both additions are required because generating things from bindings codegen and ensuring all the dependencies are correct would have been rather difficult.
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The processor script
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====================
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The definition files are processed twice:
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- once to generate two C++ headers files, included by the web platform components (e.g. DOM) that own the features to be tracked;
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- the other time by the Telemetry component, to generate the histogram definitions that make the collection system work.
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.. note::
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The histograms that are generated out of use counters are set to *never* expire and are collected from Firefox release. Note that before Firefox 65 they were only collected on pre-release.
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gen-usecounters.py
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------------------
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This script is called by the build system to generate:
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- the ``UseCounterList.h`` header for the WebIDL, out of the definition files.
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Interpreting the data
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=====================
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The histogram as accumulated on the client only puts values into the 1 bucket, meaning that
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the use counter directly reports if a feature was used but it does not directly report if
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it isn't used.
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The values accumulated within a use counter should be considered proportional to
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``CONTENT_DOCUMENTS_DESTROYED`` and ``TOP_LEVEL_CONTENT_DOCUMENTS_DESTROYED`` (see
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`here <https://searchfox.org/mozilla-central/rev/1a973762afcbc5066f73f1508b0c846872fe3952/dom/base/Document.cpp#15059-15081>`__). The difference between the values of these two histograms
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and the related use counters below tell us how many pages did *not* use the feature in question.
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For instance, if we see that a given session has destroyed 30 content documents, but a
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particular use counter shows only a count of 5, we can infer that the use counter was *not*
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used in 25 of those 30 documents.
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Things are done this way, rather than accumulating a boolean flag for each use counter,
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to avoid sending histograms for features that don't get widely used. Doing things in this
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fashion means smaller telemetry payloads and faster processing on the server side.
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Version History
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---------------
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- Firefox 65:
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- Enable Use Counters on release channel (`bug 1477433 <https://bugzilla.mozilla.org/show_bug.cgi?id=1477433>`_)
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@ -74,8 +74,6 @@ Most of our data collection happens through :doc:`scalars <../collection/scalars
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Both scalars & histograms allow recording by keys. This allows for more flexible, two-level data collection.
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Other collections can build on top of scalars & histograms. An example is :doc:`use counters <../collection/use-counters>`, which submit web feature usage through histograms.
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We also collect :doc:`environment data <../data/environment>`. This consists of mostly scalar values that capture the “working environment” a Firefox session lives in, and includes e.g. data on hardware, OS, add-ons and some settings. Any data that is part of the "working environment", or needs to split :doc:`subsessions <../concepts/sessions>`, should go into it.
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Rich data
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