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When you index a document, it is stored on a single primary shard. How doesElasticsearch know which shard a document belongs to? When we create a newdocument, how does it know whether it should store that document on shard 1 orshard 2?
The process can"t be random, since we may need to retrieve the document in thefuture. In fact, it is determined by a very simple formula:
shard = hash(routing) % number_of_primary_shards
The routing value is an arbitrary string, which defaults to the document"s_id but can also be set to a custom value. This routing string is passedthrough a hashing function to generate a number, which is divided by thenumber of primary shards in the index to return the remainder. The remainderwill always be in the range 0 to number_of_primary_shards - 1, and givesus the number of the shard where a particular document lives.
This explains why the number of primary shards can only be set when an indexis created and never changed: if the number of primary shards ever changed inthe future, all previous routing values would be invalid and documents wouldnever be found.
All document APIs (get, index, delete, bulk, update and mget)accept a routing parameter that can be used to customize the document-to-shard mapping. A custom routing value could be used to ensure that all relateddocuments -- for instance all the documents belonging to the same user -- arestored on the same shard. We discuss in detail why you may want to do this in<>.