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+=======
+Scalars
+=======
+
+Historically we started to overload our histogram mechanism to also collect scalar data,
+such as flag values, counts, labels and others.
+The scalar measurement types are the suggested way to collect that kind of scalar data.
+We currently only support recording of scalars from the parent process.
+The serialized scalar data is submitted with the :doc:`main pings <../data/main-ping>`.
+
+The API
+=======
+Scalar probes can be managed either through the `nsITelemetry interface <https://dxr.mozilla.org/mozilla-central/source/toolkit/components/telemetry/nsITelemetry.idl>`_
+or the `C++ API <https://dxr.mozilla.org/mozilla-central/source/toolkit/components/telemetry/Telemetry.h>`_.
+
+JS API
+------
+Probes in privileged JavaScript code can use the following functions to manipulate scalars:
+
+.. code-block:: js
+
+  Services.telemetry.scalarAdd(aName, aValue);
+  Services.telemetry.scalarSet(aName, aValue);
+  Services.telemetry.scalarSetMaximum(aName, aValue);
+
+  Services.telemetry.keyedScalarAdd(aName, aKey, aValue);
+  Services.telemetry.keyedScalarSet(aName, aKey, aValue);
+  Services.telemetry.keyedScalarSetMaximum(aName, aKey, aValue);
+
+These functions can throw if, for example, an operation is performed on a scalar type that doesn't support it
+(e.g. calling scalarSetMaximum on a scalar of the string kind). Please look at the `code documentation <https://dxr.mozilla.org/mozilla-central/search?q=regexp%3ATelemetryScalar%3A%3A(Set%7CAdd)+file%3ATelemetryScalar.cpp&redirect=false>`_ for
+additional information.
+
+C++ API
+-------
+Probes in native code can use the more convenient helper functions declared in `Telemetry.h <https://dxr.mozilla.org/mozilla-central/source/toolkit/components/telemetry/Telemetry.h>`_:
+
+.. code-block:: cpp
+
+    void ScalarAdd(mozilla::Telemetry::ScalarID aId, uint32_t aValue);
+    void ScalarSet(mozilla::Telemetry::ScalarID aId, uint32_t aValue);
+    void ScalarSet(mozilla::Telemetry::ScalarID aId, const nsAString& aValue);
+    void ScalarSet(mozilla::Telemetry::ScalarID aId, bool aValue);
+    void ScalarSetMaximum(mozilla::Telemetry::ScalarID aId, uint32_t aValue);
+
+    void ScalarAdd(mozilla::Telemetry::ScalarID aId, const nsAString& aKey, uint32_t aValue);
+    void ScalarSet(mozilla::Telemetry::ScalarID aId, const nsAString& aKey, uint32_t aValue);
+    void ScalarSet(mozilla::Telemetry::ScalarID aId, const nsAString& aKey, bool aValue);
+    void ScalarSetMaximum(mozilla::Telemetry::ScalarID aId, const nsAString& aKey, uint32_t aValue);
+
+The YAML definition file
+========================
+Scalar probes are required to be registered, both for validation and transparency reasons,
+in the `Scalars.yaml <https://dxr.mozilla.org/mozilla-central/source/toolkit/components/telemetry/Scalars.yaml>`_
+definition file.
+
+The probes in the definition file are represented in a fixed-depth, two-level structure:
+
+.. code-block:: yaml
+
+    # The following is a group.
+    a.group.hierarchy:
+      a_probe_name:
+        kind: uint
+        ...
+      another_probe:
+        kind: string
+        ...
+      ...
+    group2:
+      probe:
+        kind: int
+        ...
+
+Group and probe names need to follow a few rules:
+
+- they cannot exceed 40 characters each;
+- group names must be alpha-numeric + ``.``, with no leading/trailing digit or ``.``;
+- probe names must be alpha-numeric + ``_``, with no leading/trailing digit or ``_``.
+
+A probe can be defined as follows:
+
+.. code-block:: yaml
+
+    a.group.hierarchy:
+      a_scalar:
+        bug_numbers:
+          - 1276190
+        description: A nice one-line description.
+        expires: never
+        kind: uint
+        notification_emails:
+          - telemetry-client-dev@mozilla.com
+
+Required Fields
+---------------
+
+- ``bug_numbers``: A list of unsigned integers representing the number of the bugs the probe was introduced in.
+- ``description``: A single or multi-line string describing what data the probe collects and when it gets collected.
+- ``expires``: The version number in which the scalar expires, e.g. "30"; a version number of type "N" and "N.0" is automatically converted to "N.0a1" in order to expire the scalar also in the development channels. A telemetry probe acting on an expired scalar will print a warning into the browser console. For scalars that never expire the value ``never`` can be used.
+- ``kind``: A string representing the scalar type. Allowed values are ``uint``, ``string`` and ``boolean``.
+- ``notification_emails``: A list of email addresses to notify with alerts of expiring probes. More importantly, these are used by the data steward to verify that the probe is still useful.
+
+Optional Fields
+---------------
+
+- ``cpp_guard``: A string that gets inserted as an ``#ifdef`` directive around the automatically generated C++ declaration. This is typically used for platform-specific scalars, e.g. ``ANDROID``.
+- ``release_channel_collection``: This can be either ``opt-in`` (default) or ``opt-out``. With the former the scalar is submitted by default on pre-release channels; on the release channel only if the user opted into additional data collection. With the latter the scalar is submitted by default on release and pre-release channels, unless the user opted out.
+- ``keyed``: A boolean that determines whether this is a keyed scalar. It defaults to ``False``.
+
+String type restrictions
+------------------------
+To prevent abuses, the content of a string scalar is limited to 50 characters in length. Trying
+to set a longer string will result in an error and no string being set.
+
+Keyed Scalars
+-------------
+Keyed scalars are collections of one of the available scalar types, indexed by a string key that can contain UTF8 characters and cannot be longer than 70 characters. Keyed scalars can contain up to 100 keys. This scalar type is for example useful when you want to break down certain counts by a name, like how often searches happen with which search engine.
+
+Keyed scalars should only be used if the set of keys are not known beforehand. If the keys are from a known set of strings, other options are preferred if suitable, like categorical histograms or splitting measurements up into separate scalars.
+
+The processor scripts
+=====================
+The scalar definition file is processed and checked for correctness at compile time. If it
+conforms to the specification, the processor scripts generate two C++ headers files, included
+by the Telemetry C++ core.
+
+gen-scalar-data.py
+------------------
+This script is called by the build system to generate the ``TelemetryScalarData.h`` C++ header
+file out of the scalar definitions.
+This header file contains an array holding the scalar names and version strings, in addition
+to an array of ``ScalarInfo`` structures representing all the scalars.
+
+gen-scalar-enum.py
+------------------
+This script is called by the build system to generate the ``TelemetryScalarEnums.h`` C++ header
+file out of the scalar definitions.
+This header file contains an enum class with all the scalar identifiers used to access them
+from code through the C++ API.