An Apache Solr instance can run as a single core or multicore; it is a client server model. A Solr core is nothing but the running instance of a Solr index along with its configuration. Earlier, Apache Solr had a single core that in turn limited the consumers to run Solr on one application, through a single schema and configuration file. Later, support for creating multiple cores was added. With this support one can now run one Solr instance for multiple schemas and configurations with unified administrations. You can run Solr in multicore with the following command:

java -Dsolr.solr.home=multicore -jar start.jar

Apache Solr is composed of multiple modules, some of them being separate projects in themselves. Let's understand the different components of the Apache Solr architecture. The following diagram depicts the Apache Solr conceptual architecture:

Apache Solr can run in a master-slave mode. Index replicator is responsible for distributing indexes across multiple...