Ranged Sharding
Ranged-based sharding involves dividing data into contiguous ranges determinedby the shard key values. In this model, documents with “close” shard keyvalues are likely to be in the same chunk or shard. Thisallows for efficient queries where reads target documents within a contiguousrange. However, both read and write performance may decrease with poor shardkey selection. See Shard Key Selection.
Range-based sharding is the default sharding methodology if no other optionssuch as those required for Hashed Sharding orzones are configured.
Shard Key Selection
Ranged sharding is most efficient when the shard key displays the followingtraits:
The following image illustrates a sharded cluster using the field X
as theshard key. If the values for X
have a large range, low frequency, andchange at a non-monotonic rate, the distribution of inserts may look similarto the following:
Shard a Collection
Use the sh.shardCollection()
method, specifying the full namespaceof the collection and the target index or compound indexto use as the shard key.
- sh.shardCollection( "database.collection", { <shard key> } )
Important
Once you shard a collection, the selection of the shard key isimmutable; i.e. you cannot select a different shard key for thatcollection.
Starting in MongoDB 4.2, you can update a document’s shard key valueunless the shard key field is the immutable
_id
field. For detailson updating the shard key, see Change a Document’s Shard Key Value.
Before MongoDB 4.2, a document’s shard key field value is immutable.
Shard a Populated Collection
If you shard a populated collection:
- The sharding operation creates the initial chunk(s) to cover theentire range of the shard key values. The number of chunks createddepends on the configured chunk size.
- After the initial chunk creation, the balancer migrates these initialchunks across the shards as appropriate as well as manages the chunkdistribution going forward.
Shard an Empty Collection
If you shard an empty collection:
- With no zones and zone ranges specified for theempty or non-existing collection:
- The sharding operation creates a single empty chunk to cover theentire range of the shard key values.
- After the initial chunk creation, the balancer migrates the initialchunk across the shards as appropriate as well as manages the chunkdistribution going forward.
- With zones and zone ranges specified for theempty or a non-existing collection (Available starting in MongoDB4.0.3),
- The sharding operation creates empty chunks for the defined zoneranges as well as any additional chunks to cover the entire rangeof the shard key values and performs an initial chunk distributionbased on the zone ranges. This initial creation and distribution ofchunks allows for faster setup of zoned sharding.
- After the initial distribution, the balancer manages the chunkdistribution going forward.