## Plugging in search engines

The new federated information search in Help system uses the notion of search engine types and search engines. An engine type is a meta-engine from which a number of concrete search engines can be created by parameterization.

New engine types are contributed through the org.eclipse.help.ui.searchEngine:

>     <extension point="org.eclipse.help.ui.searchEngine">
>     	<engineType
>     		scopeFactory="com.example.xyz.XYZScopeFactory"
>     		label="XYZ Search"
>     		class="com.example.xyz.search.XYZSearch"
>     		icon="icons/etool16/xyzsearch.gif"
>     		pageClass="com.example.xyz.search.XYZSearchPage"
>     		id="com.example.xyz.XYZSearch">
>     		<description>
>     			Instances of XYZ Search search the XYZ site.
>     		</description>
>     	</engineType>

This extension point is used to plug in search participants in the information search. Each search engine can be configured individually. When search is initiated, each search engine is executed as a background job, and the results are collated in the help view immediately under the query.

Search engines defined here will not automatically show up as federated search participants until engine product binding is established, unless `productId` attribute is left undefined. For engines that define it, only those bound to a particular product will show up when that product is running.

Search engines can simply compose a URL and provide only one hit containing that URL as `href`. Popular search engines for which API support requires license can be plugged in like this. On the other end of the spectrum, search engines can communicate with the server and receive individual hits with information like label, href, short description, score etc. Local help engine can produce hits this way.

Regardless of the search mechanism, engines can provide various search scope settings using JFace preference pages. These pages are shown when 'Advanced Settings' link is followed from the Help view. In addition to root preference pages defined with the engine, additional preference sub-pages can be plugged in for more advanced settings.

Scope settings are loaded and stored using `IPreferenceStore` objects. Scope settings for all engines are grouped together under a named scope set. When first opened, default scope set ('Default') is created, but users can define more scope sets and flip between them.

Since federated search support is part of `org.eclipse.help.base` plug-in, a factory is needed to create search scope objects from the data in the preference store. Clients that plug in scope preference pages are required to plug in scope factories as well.

Engines defined in this extension point do not show up in the UI by default. What is shown there is a concrete instance of a search engine that can be individually modified. Products can pre-configure the help system with a number of instances of the registered engine types, possibly parameterized to perform in a desired way. In addition, users can add their own instances of the registered engines and configure them to their liking:

>     <engine
>     	enabled="true"
>     	engineTypeId="com.example.xyz.search.XYZSearch"
>     	id="com.example.xyz.XYZSearch"
>     	label="XYZ Search">
>     </engine>
>     <engine
>     	enabled="true"
>     	engineTypeId="org.eclipse.help.ui.web"
>     	id="org.eclipse.sdk.Eclipse"
>     	label="%search.Eclipse.label">
>     	<description>
>     		%search.Eclipse.desc
>     	</description>
>     	<param
>     		name="url"
>     		value="http://eclipse.org/search/search.cgi?q={expression}&amp;ul=&amp;ps=20&amp;m=all">
>     	</param>
>     </engine>