How SolrCloud Works

The following sections cover provide general information about how various SolrCloud features work. To understand these features, it’s important to first understand a few key concepts that relate to SolrCloud.

If you are already familiar with SolrCloud concepts and basic functionality, you can skip to the section covering SolrCloud Configuration and Parameters.

Key SolrCloud Concepts

A SolrCloud cluster consists of some "logical" concepts layered on top of some "physical" concepts.

Logical Concepts

  • A Cluster can host multiple Collections of Solr Documents.
  • A collection can be partitioned into multiple Shards, which contain a subset of the Documents in the Collection.
  • The number of Shards that a Collection has determines:
    • The theoretical limit to the number of Documents that Collection can reasonably contain.
    • The amount of parallelization that is possible for an individual search request.

Physical Concepts

  • A Cluster is made up of one or more Solr Nodes, which are running instances of the Solr server process.
  • Each Node can host multiple Cores.
  • Each Core in a Cluster is a physical Replica for a logical Shard.
  • Every Replica uses the same configuration specified for the Collection that it is a part of.
  • The number of Replicas that each Shard has determines:
    • The level of redundancy built into the Collection and how fault tolerant the Cluster can be in the event that some Nodes become unavailable.
    • The theoretical limit in the number concurrent search requests that can be processed under heavy load.
Make sure the DNS resolution in your cluster is stable, i.e., for each live host belonging to a Cluster the host name always corresponds to the same specific IP and physical node. For example, in clusters deployed on AWS this would require setting preserve_hostname: true in /etc/cloud/cloud.cfg. Changing DNS resolution of live nodes may lead to unexpected errors. See SOLR-13159 for more details.