Once you've optimized Solr running on a single server, and reached the point of diminishing returns for optimizing further, the next step is to split the querying load over multiple slave instances of Solr. The ability to scale horizontally is a hallmark of modern scalable Internet systems, and Solr shares this.
Introduced in Solr 1.4 is an HTTP-based replication strategy that is tightly integrated into Solr. Configuration is done through the already familiar
solrconfig.xml, and replication encompasses transferring both data as well as configuration files such as
onfig.xml and ...