First of all what is Apache Solr?
Apache Solr – is the advanced search platform with open primary code by Apache Lucene project.
Why Apache Solr?
The main criteria for us to choose this platform was the fact that Apache Solr is optimized for work with big loading and that replication is already in platform core and this gives big opportunities for scaling.
One of our projects made us come across a question: “What will happen if our main Solr server will suddenly stop responding? Will all search system on site die in that case?”
Sometimes it’s possible to switch on search of Drupal core (this kind of functional is in Apache Solr Search Integration module), but it can’t be done with big projects because it will create deadly load on data base – so this option is set aside at once.
As another option if main Solr-server is down the administrator sets up a new one ASAP (or makes a master-server out of slave-server) and writes a new connect in Drupal.
But there are 2 problems with that. Firstly, the administrator can’t be online 24/7, secondly in setting up a new server re-indexation of material might take several dozens of hours…