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 shard your single index over multiple Solr nodes, and then share the querying load over many Solr nodes. The ability to scale wide is a hallmark of modern scalable Internet systems, and Solr shares this.

Arguably the biggest feature in Solr 4, SolrCloud provides a self-managing cluster of Solr servers (also known as nodes) to meet your scaling and near real-time search demands. SolrCloud is conceptually quite simple, and setting it up to test is fairly straightforward. The challenge typically is keeping all of the moving pieces in sync over time as your data set grows and you add and remove nodes.