Solr is the popular, blazing fast open source enterprise search platform from the Apache Lucene project. Its major features include powerful full-text search, hit highlighting, faceted search, dynamic clustering, database integration, rich document (e.g., Word, PDF) handling, and geospatial search. Solr is highly scalable, providing distributed search and index replication, and it powers the search and navigation features of many of the world's largest internet sites.
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Source: http://lucene.apache.org/solr/
We can configure it to work for search for multiple drupal sites by 2 simple steps:
Create a folder inside the solr directory as : site_sitename (Ex: site_donors)
Copy the conf folder inside solr directory to site_sitename
Modify the solr.xml file that has been modified to take care of multicore search, by adding the following lines inside the core tag:
<cores adminPath="/admin/cores">
...
<core name="sitename" instanceDir="site_sitename" />
</cores>Add the apachesolr module inside the drupal site and enable it. You would need to download SolrPhpClient inside the apache solr module. README.txt inside apachesolr is a good resource to read.
Inside the apachesolr settings page(admin/settings/apachesolr), in the Solr Path field add: /solr/sitename. If the port name and Solr Host Name is mentioned correctly, it should work great.
PS: Restart apache solr server for the changes to take place