Navigation

Hashed Sharding

Hashed sharding uses a hashed index to partition data across your shared cluster. Hashed indexes compute the hash value of a single field as the index value; this value is used as your shard key. [1]

Diagram of the hashed based segmentation.

Hashed sharding provides more even data distribution across the sharded cluster at the cost of reducing Targeted Operations vs. Broadcast Operations. Post-hash, documents with “close” shard key values are unlikely to be on the same chunk or shard - the mongos is more likely to perform Broadcast Operations to fulfill a given ranged query. mongos can target queries with equality matches to a single shard.

Tip

MongoDB automatically computes the hashes when resolving queries using hashed indexes. Applications do not need to compute hashes.

Warning

MongoDB hashed indexes truncate floating point numbers to 64-bit integers before hashing. For example, a hashed index would store the same value for a field that held a value of 2.3, 2.2, and 2.9. To prevent collisions, do not use a hashed index for floating point numbers that cannot be reliably converted to 64-bit integers (and then back to floating point). MongoDB hashed indexes do not support floating point values larger than 253.

To see what the hashed value would be for a key, see convertShardKeyToHashed().

[1]Starting in version 4.0, the mongo shell provides the method convertShardKeyToHashed(). This method uses the same hashing function as the hashed index and can be used to see what the hashed value would be for a key.

Hashed Sharding Shard Key

The field you choose as your hashed shard key should have a good cardinality, or large number of different values. Hashed keys are ideal for shard keys with fields that change monotonically like ObjectId values or timestamps. A good example of this is the default _id field, assuming it only contains ObjectID values.

To shard a collection using a hashed shard key, see Deploy Sharded Cluster using Hashed Sharding.

Hashed vs Ranged Sharding

Given a collection using a monotonically increasing value X as the shard key, using ranged sharding results in a distribution of incoming inserts similar to the following:

Diagram of poor shard key distribution due to monotonically increasing or decreasing shard key

Since the value of X is always increasing, the chunk with an upper bound of maxKey receives the majority incoming writes. This restricts insert operations to the single shard containing this chunk, which reduces or removes the advantage of distributed writes in a sharded cluster.

By using a hashed index on X, the distribution of inserts is similar to the following:

Diagram of hashed shard key distribution

Since the data is now distributed more evenly, inserts are efficiently distributed throughout the cluster.

Shard the Collection

Use the sh.shardCollection() method, specifying the full namespace of the collection and the target hashed index to use as the shard key.

sh.shardCollection( "database.collection", { <field> : "hashed" } )

Shard a Populated Collection

If you shard a populated collection using a hashed shard key:

  • The sharding operation creates the initial chunk(s) to cover the entire range of the shard key values. The number of chunks created depends on the configured chunk size.
  • After the initial chunk creation, the balancer migrates these initial chunks across the shards as appropriate as well as manages the chunk distribution going forward.

Shard an Empty Collection

If you shard an empty collection using a hashed shard key:

  • With no zones and zone ranges specified for the empty or non-existing collection:
    • The sharding operation creates empty chunks to cover the entire range of the shard key values and performs an initial chunk distribution. By default, the operation creates 2 chunks per shard and migrates across the cluster. You can use numInitialChunks option to specify a different number of initial chunks. This initial creation and distribution of chunks allows for faster setup of sharding.
    • After the initial distribution, the balancer manages the chunk distribution going forward.
  • With zones and zone ranges specified for the empty or a non-existing collection (Available starting in MongoDB 4.0.3),
    • The sharding operation creates empty chunks for the defined zone ranges as well as any additional chunks to cover the entire range of the shard key values and performs an initial chunk distribution based on the zone ranges. This initial creation and distribution of chunks allows for faster setup of zoned sharding.
    • After the initial distribution, the balancer manages the chunk distribution going forward.