The last topic to cover in this chapter is site search. The core Drupal server comes with a built-in search engine. This engine can index all of the content on a site, allowing visitors to search for content.
Because it stores all of its information in the database, each Drupal site in a multi-site setup can have its own search engine.
But what if we want to provide a single search instance that can search content across all of our sites? In this case, when a user searches on our fabled books.local site, that user would see looks.local and cooks.local content as well.
This particular form of cross-site searching cannot be done with the built-in search engine. However, since Drupal's search is pluggable, we can use an add-on module to provide this service.
The most popular search engine alternative for Drupal 7 is the Apache Solr module (http://drupal.org/project/apachesolr). This module relies on an external Apache Solr server (http://lucene.apache.org/solr/) to...