At the moment the best search engine available for Drupal is Apache Solr. There are many Drupal sites which actually use Solr to power their search. Using the configuration options of the solr integration modules, the search is already quite decent, and this is where many developers stop tweaking Solr. In some situations however, it might be necessary to tweak solr beyond the standard configuration options. Luckily this tweaking is relatively easy: almost all tweaking of Solr can be done by adding and/or altering parameters to the request to Solr. The Solr integration modules on the other hand, offer hooks to alter the parameters send to Solr. So if the search behavior should be tweaked, the question is not 'Can it be done?', but rather 'What should I send to Solr, to get this behavior?'. This blog series will explain how Solr works in the first place, and will show how to debug and optimize indexing and querying.