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+======================
+Default Search Engines
+======================
+
+Default Engine
+==============
+
+The search service specifies default search engines via the `configuration
+schema`_.
+
+Changing Defaults
+=================
+
+The default engine may change when:
+
+* The user has the default engine set and the configuration for the locale/region
+  changes.
+* The user has the default engine set and their locale/region changes to one
+  which has a different default.
+* The user chooses to set a different engine via preferences.
+* The user installs an add-on which sets its default as one of the application
+  provided engines.
+* The user installs an add-on which supplies a different engine and the user allows
+  the different engine to be set as default.
+* The user or Firefox (e.g. via blocklist) causes the default engine to be removed.
+
+Add-ons and Prompting for Default
+---------------------------------
+
+The prompt for selecting a search engine from an add-on as default is shown to
+the user on installation of the add-on. It may also be shown if an add-on is
+re-enabled, if the default engine was not changed in the meantime.
+
+The following diagram shows the full flow for search engines from add-ons:
+
+.. image:: ./search-add-on-prompts-flow.png
+    :align: center
+    :alt: Flowchart for prompting for default engine for Search Engines related to add-ons.
+
+When the Default Engine is Removed
+==================================
+
+If the default engine is removed by the user, or by Firefox in the case of a
+blocklist or for some other region, the new default engine is chosen by the
+following process.
+
+* If the default engine specified by the configuration for the user's region and locale
+  is visible, then it will be selected as default.
+* If there is another engine visible, fall back to the first engine identified
+  as a general search engine (see below).
+* If there are no other visible engines, unhide the region/locale default engine
+  from the configuration and set it as default if it is not the one being removed.
+* Otherwise, unhide the first general search engine, or the first visible engine.
+
+A general search engine is defined as one that returns general search results,
+for example Google or DuckDuckGo. A non-general search engine returns results
+for a specific area, e.g. shopping, books, dictionaries.
+
+Add-ons and App-provided Engines
+================================
+
+An add-on may set the name of the search provider in the manifest.json to be
+the name of an app-provided engine. In this case:
+
+* If the add-on is a non-authorised partner, then we set the user's default
+  engine to be the name of the app-provided engine.
+* If the add-on is from an authorised partner, then we set the users' default
+  engine to be the same as the app-provided engine, and we allow the
+  app-provided urls to be overridden with those of the add-on.
+
+If the specified engine is already default, then the add-on does
+not override the app-provided engine, and it's settings are ignored and no
+new engine is added.
+
+The list of authorised add-ons is stored in `remote settings`_ in the
+`search-default-override-allowlist bucket`_. The list
+includes records containing:
+
+* Third-party Add-on Id: The identifier of the third party add-on which will
+  override the app provided one.
+* Add-on Id to Override: The identifier of the app-provided add-on to be
+  overridden.
+* a list of the url / params that are authorised to be replaced.
+
+When an authorised add-on overrides the default, we record the add-on's id
+with the app-provided engine in the ``overriddenBy`` field. This is used
+when the engine is loaded on startup to known that it should load the parameters
+from that add-on.
+
+The ``overriddenBy`` annotation may be removed when:
+
+* The associated authorised add-on is removed, disabled or can no longer be found.
+* The user changes their default to another engine.
+
+If the ``overriddenBy`` annotation is present, but the add-on is not authorised,
+then the annotation will be maintained in case the add-on is later re-authorised.
+For example, a url is updated, but the update is performed before the allow list
+is updated.
+
+.. _configuration schema: SearchConfigurationSchema.html
+.. _remote settings: /services/settings/index.html
+.. _search-default-override-allowlist bucket: https://firefox.settings.services.mozilla.com/v1/buckets/main/collections/search-default-override-allowlist/records