MongoDB Documentation, Release 3.0.4 10.2.3 Sharded Cluster Behavior These documents address the distribution of data and queries to a sharded cluster as well as specific security and availability considerations for sharded clusters. Shard Keys (page 674) MongoDB uses the shard key to divide a collection’s data across the cluster’s shards. Sharded Cluster High Availability (page 676) Sharded clusters provide ways to address some availability concerns. Sharded Cluster Query Routing (page 678) The cluster’s routers, or mongos instances, send reads and writes to the relevant shard or shards. Tag Aware Sharding (page 682) Tags associate specific ranges of shard key values with specific shards for use in managing deployment patterns. The shard key determines the distribution of the collection’s documents among the cluster’s shards . The shard key is either an indexed field or an indexed compound field that exists in every document in the collection. MongoDB partitions data in the collection using ranges of shard key values. Each range, or chunk , defines a non- overlapping range of shard key values. MongoDB distributes the chunks, and their documents, among the shards in the cluster. Shard Keys When a chunk grows beyond the chunk size (page 688), MongoDB attempts to split the chunk into smaller chunks, always based on ranges in the shard key. Shard keys are immutable and cannot be changed after insertion. See the system limits for sharded cluster for more information. The index on the shard key cannot be a multikey index (page 491). New in version 2.4. Hashed shard keys use a hashed index (page 521) of a single field as the shard key to partition data across your sharded cluster. Chapter 10. Sharding 674 Hashed Shard Keys Considerations