Solr Collection vs Cores
From the SolrCloud Documentation
Collection: A single search index.
Shard: A logical section of a single collection (also called Slice). Sometimes people will talk about "Shard" in a physical sense (a manifestation of a logical shard)
Replica: A physical manifestation of a logical Shard, implemented as a single Lucene index on a SolrCore
Leader: One Replica of every Shard will be designated as a Leader to coordinate indexing for that Shard
SolrCore: Encapsulates a single physical index. One or more make up logical shards (or slices) which make up a collection.
Node: A single instance of Solr. A single Solr instance can have multiple SolrCores that can be part of any number of collections.
Cluster: All of the nodes you are using to host SolrCores.
So basically a Collection (Logical group) has multiple cores (physical indexes).
Also, check the discussion
Core
In Solr, a core is composed of a set of configuration files, Lucene index files, and Solr’s
transaction log.
a Solr core is a uniquely named, managed, and configured index running in a Solr server; a Solr server can host one or more cores. A core is typically used to separate documents that have different schemas
collection
Solr also uses the term collection, which only has meaning in the context
of a Solr cluster in which a single index is distributed across multiple servers.
SolrCloud introduces the concept of a collection, which extends the concept of a uniquely
named, managed, and configured index to one that is split into shards and distributed
across multiple servers.
As per my understanding:
In distributed search,
Collection is a logical index spread across multiple servers. Core is that part of server which runs one collection.
In non-distributed search,
Single server running the Solr can have multiple collections and each of those collection is also a core. So collection and core are same if search is not distributed.
Summary
- Collection per server is called a core.
- Collection is same as an index.
- One Solr server can have many cores.
- Collection is a logical index (Example usage for multiple collections: Say two teams in same group are not big enough to justify a full Solr server of their own. But they also do not want to mix their data in a single index. They can then create separate collections/indexes which will keep their data separate).
- Its better to use a separate Solr Cloud rather than create collections if the data for a collection is big enough (not sure, comments please?)
Single instance
On a single instance, Solr has something called a SolrCore that is essentially a single index. If you want multiple indexes, you create multiple SolrCores.
Solr Cloud
With SolrCloud, a single index can span multiple Solr instances. This means that a single index can be made up of multiple SolrCore's on different machines. We call all of these SolrCores that make up one logical index a collection.
A collection is a essentially a single index that spans many SolrCore's, both for index scaling as well as redundancy. If you wanted to move your 2 SolrCore Solr setup to SolrCloud, you would have 2 collections, each made up of multiple individual SolrCores.
From Solr Wiki:
Collections are made up of one or more shards. Shards have one or more replicas. Each replica is a core. A single collection represents a single logical index.