In the Choosing the right amount of shards and replicas section in this chapter, we mentioned routing as a solution for the shards on which queries will be executed on a single one. Now it's time to look closer at this functionality.
Usually, it is not important how Elasticsearch divides data into shards and which shard holds the particular document. During query time, the query will be sent to all the shards of a particular index, so the only crucial thing is to use the algorithm that spreads our data evenly so that each shard contains similar amounts of data. We don't want one shard to hold 99 percent of the data while the other shard holds the rest—it is not efficient.
The situation complicates slightly when we want to remove or add a newer version of the document. Elasticsearch must be able to determine which shard should be updated. Although it may seem troublesome, in practice, it is not a huge problem. It is enough to use the sharding algorithm, which...