The master server receives all updates and all changes (such as inserts, updates, or deletes) are made against a single master server. Changes made on the master are distributed to all the slave servers which service all query requests from the clients. This division of labor enables Solr to scale to provide adequate responsiveness to queries against large search volumes.
The master server tracks the models, metadata, permissions, and content where as the slave server only tracks the models.
The figure below shows a Solr configuration using index replication. The master server's index is replicated on the slaves.
The master-slave replication requires non-SSL communication between the master server and the slave server.