Hashed Sharding

Hashed sharding uses either a single field hashed index or a compound hashed index as the shard key to partition data across your sharded cluster.

Sharding on a Single Field Hashed Index

Hashed sharding provides a 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.

Diagram of the hashed based segmentation.

Hashed indexes compute the hash value of a single field as the index value; this value is used as your shard key. [1]

Sharding on a Compound Hashed Index

MongoDB includes support for creating compound indexes with a single hashed field. To create a compound hashed index, specify hashed as the value of any single index key when creating the index.

Compound hashed index compute the hash value of a single field in the compound index; this value is used along with the other fields in the index as your shard key.

Compound hashed sharding supports features like zone sharding, where the prefix (i.e. first) non-hashed field or fields support zone ranges while the hashed field supports more even distribution of the sharded data. Compound hashed sharding also supports shard keys with a hashed prefix for resolving data distribution issues related to monotonically increasing fields.

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 2 ⁵³.

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

[1]	`mongosh` provides the `convertShardKeyToHashed()` method. 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 Shard a Collection.

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

click to enlarge

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

click to enlarge

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" } )

To shard a collection on a compound hashed index, specify the full namespace of the collection and the target compound hashed index to use as the shard key:

sh.shardCollection(
  "database.collection",
  { "fieldA" : 1, "fieldB" : 1, "fieldC" : "hashed" }
)

Important

Starting in MongoDB 5.0, you can reshard a collection by changing a collection's shard key.
You can refine a shard key by adding a suffix field or fields to the existing shard key.

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

The shard collection operation can perform an initial chunk creation and distribution for empty or non-existing collections if zones and zone ranges have been defined for the collection. Initial creation and distribution of chunk allows for faster setup of zoned sharding. After the initial distribution, the balancer manages the chunk distribution going forward per usual.

Sharding Empty Collection on Single Field 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.

Sharding Empty Collection on Compound Hashed Shard Key with Hashed Field Prefix

If the compound hashed shard key has the hashed field as the prefix (i.e. the hashed field is the first field in the 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. The value of all non-hashed fields is MinKey at each split point. 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 a single zone with a range from MinKey to MaxKey specified for the empty or a non-existing collection and the presplitHashedZones option specified to sh.shardCollection():
- The sharding operation creates empty chunks for the defined zone range 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.

Sharding Empty Collection on Compound Hashed Shard Key with Non-Hashed Prefix

If the compound hashed shard key has one or more non-hashed fields as the prefix (i.e. the hashed field is not the first field in the shard key):

With no zones and zone ranges specified for the empty or non-existing collection and preSplitHashedZones is false or omitted, MongoDB does not perform any initial chunk creation or distribution when sharding the collection.
With no zones and zone ranges specified for the empty or non-existing collection and preSplitHashedZones, sh.shardCollection() / shardCollection returns an error.
With zones and zone ranges specified for the empty or a non-existing collection and the preSplitHashedZones option specified to sh.shardCollection():
- 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.
- The sharding operation further subdivides the initial chunk for each range, such that each shard in the zone is allocated an equal number of chunks.
- 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.
The defined ranges for each zone must meet certain requirements. For a description of the requirements and a complete example, see Pre-Define Zones and Zone Ranges for an Empty or Non-Existing Collection.

Drop a Hashed Shard Key Index

Starting in MongoDB 5.0.22, you can drop the index for a hashed shard key.

This can speed up data insertion for collections sharded with a hashed shard key. It can also speed up data ingestion when using mongosync.

For details, see Drop a Hashed Shard Key Index.

Tip