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Indexes
An index in Elasticsearch is like a table in a relational database. It has a mapping which defines the fields in the index, which are grouped by multiple type. An index is a logical namespace which maps to one or more primary shards and can have zero or more replica shards.
Learn more about indexing in Elasticsearch here.
With eXo Platform and Elasticsearch, an index is dedicated to each application (Wiki, Calendar, Documents...). All the application data (for example wiki application data: wiki, wiki page, wiki attachment) will be indexed in the same index.
Sharding
A shard is a single Lucene instance. It is a low-level worker unit which is managed automatically by Elasticsearch.
Learn more about Sharding in Elasticsearch here.
In eXo Platform with Elasticsearch:
Sharding will only be used for horizontal scalability.
eXo Platform does not use routing policies to route documents or documents type to a specific shard.
The default number of shards is 5: the default value of Elasticsearch.
This value is configurable per index by setting the parameter shard.number in the constructor parameters of the connectors.
Replicas
Each index can be replicated over the Elasticsearch cluster.
The default number of replicas is 1 (the default value of Elasticsearch) which means one replica for each primary shard.
This value is configurable per index by setting the parameter replica.number in the constructor parameters of the connectors.