Search at Etsy has grown significantly over the years. In January of 2009 we started using Solr for search. We used the standard master-slave configuration for our search servers with replication.
All of the changes to the search index are written to the master server. The slaves are read-only copies of master which serve production traffic. The search index is replicated by copying files from the master server to the slave servers. The slave servers poll the master server for updates, and when there are changes to the search index the slave servers will download the changes via HTTP. Our search indexes have grown from 2 GB to over 28 GB over the past 2 years, and copying the index from the master to the slave nodes became a problem.
Brilliant use of BitTorrent to solve a difficult problem.