Solr Collection vs Cores

I struggle with understanding the difference between collections and cores. If I understand it correctly, cores are multiple indexes. Collection consists of cores, so essentially they share the same logic in separation, i.e. separate cores and collections have separate end-points.

I have the following scenario. I create a backend for cloud service for several online shops. Each shop has a set of products, to which customers can add reviews. I want to index static data (product information) separately from dynamic information(reviews) so I can improve performance.

How can I best separate in Solr???

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

1. Collection per server is called a core.
2. Collection is same as an index.
3. One Solr server can have many cores.
4. 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).
5. 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.

This explains the use of cores and collections.

Single instance

When dealing with a single solr instance you query to cores.

The admin UI of a single Solr instance has no collection selector:

Solr Cloud

When dealing with Solr Cloud you query to collections. The collections are organized in different cores (replicas, shards) on different solr instances.

The admin UI of a Solr Cloud instance has a collection and core selector. But cores are technically instances, here:

From the Solr docs:

> Usage: solr create [-c name] [-d confdir] [-n configName] [-shards #] > [-replicationFactor #] [-p port] [-V] > > Create a core or collection depending on whether Solr is running in > standalone (core) or SolrCloud mode (collection). In other words, > this action detects which mode Solr is running in, and then takes
> the appropriate action (either create_core or create_collection).