OpenSearch¶

OpenSearch is a distributed search and analytics engine.

Main concepts¶

Documents are units that store information, and are stored in json format. An index is a collection of documents.

Search are done on one or more nodes. Nodes need a lot of memory and disk. In cluster, there is a node which is elected as cluster manager, and orchestrates cluster-level operations.

OpenSearch splits indexes into shards. Each shard is actually a full Lucene index. Limit shard size to 10–50 GB. A shard may be either a primary (original) shard or a replica (copy) shard. OpenSearch distributes replica shards to different nodes than their corresponding primary shards.

Doing a search, OpenSearch matches the words in the query to the words in the documents, and each document has a relevance score. Individual words in a search query are called search terms.

For relevance score, the term frequency is used, combined with the inverse document frequency which measure the number of document in which the word occurs. OpenSearch uses the BM25 ranking algorithm to calculate document relevance scores.