By default, Elasticsearch will try to distribute your documents evenly among all the shards of the index. However, that's not always the desired situation. In order to retrieve the documents, Elasticsearch must query all the shards and merge the results. What if we could divide our data on some basis (for example, the client identifier) and use that information to put data with the same properties in the same place in the cluster. Elasticsearch allows us to do that by exposing a powerful document and query distribution control mechanism routing. In short, it allows us to choose a shard to be used to index or search the data.
During indexing operations, when you send a document for indexing, Elasticsearch looks at its identifier to choose the shard in which the document should be indexed. By default, Elasticsearch calculates the hash value of the document's identifier and, on the basis of that, it puts the document in one of the available primary shards...