Zones

In sharded clusters, you can create zones of sharded data based on the shard key. You can associate each zone with one or more shards in the cluster. A shard can associate with any number of non-conflicting zones. In a balanced cluster, MongoDB migrates chunks covered by a zone only to those shards associated with the zone.

Some common deployment patterns where zones can be applied are as follows:

  • Isolate a specific subset of data on a specific set of shards.
  • Ensure that the most relevant data reside on shards that are geographically closest to the application servers.
  • Route data to shards based on the hardware / performance of the shard hardware.

The following image illustrates a sharded cluster with three shards and two zones. The A zone represents a range with a lower boundary of 1 and an upper bound of 10. The B zone represents a range with a lower boundary of 10 and an upper boundary of 20. Shards Alpha and Beta have the A zone. Shard Beta also has the B zone. Shard Charlie has no zones associated with it. The cluster is in a steady state and no chunks violate any of the zones.

Diagram of data distribution based on zones in a sharded cluster

Behavior and Operations

Ranges

Each zone covers one or more ranges of shard key values. Each range a zone covers is always inclusive of its lower boundary and exclusive of its upper boundary.

Zones cannot share ranges, nor can they have overlapping ranges.

Balancer

The balancer attempts to evenly distribute a sharded collection’s chunks across all shards in the cluster.

For each chunk marked for migration, the balancer checks each possible destination shard for any configured zones. If the chunk range falls into a zone, the balancer migrates the chunk into a shard inside that zone. Chunks that do not fall into a zone can exist on any shard in the cluster and are migrated normally.

During balancing rounds, if the balancer detects that any chunks violate the configured zones for a given shard, the balancer migrates those chunks to a shard where no conflict exists.

After configuring a zone with a shard key range and associating it with a shard or shards, the cluster may take some time to migrate the affected data. This depends on the division of chunks and the current distribution of data in the cluster. When balancing is complete, reads and writes for documents in a given zone are routed only to the shard or shards inside that zone.

Once configured, the balancer respects zones during future balancing rounds.

Shard Key

You must use fields contained in the shard key when defining a new range for a zone to cover. If using a compound shard key, the range must include the prefix of the shard key.

For example, given a shard key { a : 1, b : 2, c : 3 }, creating or updating a zone to cover values of b requires including a as the prefix. Creating or updating a zone to covers values of c requires including a and b as the prefix.

You cannot create zones using fields not included in the shard key. For example, if you wanted to use zones to partition data based on geographic location, the shard key would need at least one field that contained geographic data.

When choosing a shard key for a collection, consider what fields you might want to use for configuring zones. After sharding, you cannot change the shard key. See Choosing a Shard Key for considerations in choosing a shard key.

Hashed Shard Keys and Zones

When using zones on a hashed shard key, each zone covers the hashed shard key values. Given a shard key { a : 1 } and a zone alpha with a lower bound of 1 and an upper bound of 5, the bounds represent the hashed value of a, and not the actual value. Therefore, there is no guarantee that MongoDB routes documents where a has a value of 1 to 5 to zone alpha. MongoDB routes any document where the hashed shard key value falls within the range of 1 or 5 to a shard inside zone alpha.

In general, a zone covering a sequential range of hashed shard key values may exhibit unexpected behavior.

It is possible create a zone which covers the entire range of shard key values using minkey and maxkey to guarantee that MongoDB restricts all the data for a specific collection to the shard or shards in that zone.

Shard Zone Boundaries

Zone ranges are always inclusive of the lower boundary and exclusive of the upper boundary.