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. However, if you can divide your data on some basis (for example, the client identifier), you can use a powerful document and query distribution control mechanism—routing. In short, it allows us to choose a shard that will be used to index or search 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. Then, those documents are redistributed to the replicas. The following diagram shows a simple illustration of how indexing...