======================
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