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Sharding

Shard Keys

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  • Shard Key Indexes
  • Missing Shard Key Fields

The shard key is either a single indexed field or multiple fields covered by a compound index that determines the distribution of the collection's documents among the cluster's shards.

MongoDB divides the span of shard key values (or hashed shard key values) into non-overlapping ranges of shard key values (or hashed shard key values). Each range is associated with a chunk, and MongoDB attempts to distribute chunks evenly among the shards in the cluster.

Diagram of the shard key value space segmented into smaller ranges or chunks.

The shard key has a direct relationship to the effectiveness of chunk distribution. See Choose a Shard Key.

Shard Key Indexes

All sharded collections must have an index that supports the shard key. The index can be an index on the shard key or a compound index where the shard key is a prefix of the index.

You cannot drop or hide an index if it is the only non-hidden index that supports the shard key.

Unique Indexes

MongoDB can enforce a uniqueness constraint on a ranged shard key index. Through the use of a unique index on the shard key, MongoDB enforces uniqueness on the entire key combination and not individual components of the shard key.

For a ranged sharded collection, only the following indexes can be unique:

  • The index on the shard key

  • A compound index where the shard key is a prefix

  • The default _id index; however, the _id index only enforces the uniqueness constraint per shard if the _id field is not the shard key.

Important

Sharded clusters only enforce the uniqueness constraint on _id fields across the cluster when the _id field is also the shard key.

If the _id field is not the shard key or if it is only the prefix to the shard key, the uniqueness constraint only applies to the shard that stores the document. This means that two or more documents can have the same _id value, provided they occur on different shards.

For example, consider a sharded collection with shard key {x: 1} that spans two shards A and B. Because the _id key is not the shard key, the collection could have a document with _id value 1 in shard A and another document with _id value 1 in shard B.

In cases where the _id field is not the shard key, MongoDB expects applications to enforce the uniqueness of _id values across the shards.

The unique index constraints mean that:

  • For a to-be-sharded collection, you cannot shard the collection if the collection has multiple unique indexes unless the shard key is the prefix for all the unique indexes.

  • For an already-sharded collection, you cannot create unique indexes on other fields unless the shard key is included as the prefix.

  • A unique index stores a null value for a document missing the indexed field; that is a missing index field is treated as another instance of a null index key value. For more information, see Missing Document Field in a Unique Single-Field Index.

To enforce uniqueness on the shard key values, pass the unique parameter as true to the sh.shardCollection() method:

  • If the collection is empty, sh.shardCollection() creates the unique index on the shard key if such an index does not already exist.

  • If the collection is not empty, you must create the index first before using sh.shardCollection().

Although you can have a unique compound index where the shard key is a prefix, if using unique parameter, the collection must have a unique index that is on the shard key.

You cannot specify a unique constraint on a hashed index.

To maintain uniqueness on a field that is not your shard key, see Unique Constraints on Arbitrary Fields.

Missing Shard Key Fields

Documents in sharded collections can be missing the shard key fields. To set missing shard key fields, see Set Missing Shard Key Fields.

Chunk Range and Missing Shard Key Fields

Missing shard key fields fall within the same chunk range as shard keys with null values. For example, if the shard key is on the fields { x: 1, y: 1 }, then:

Document Missing Shard Key
Falls into Same Range As

{ x: "hello" }

{ x: "hello", y: null }

{ y: "goodbye" }

{ x: null, y: "goodbye" }

{ z: "oops" }

{ x: null, y: null }

Read/Write Operations and Missing Shard Key Fields

To target documents with missing shard key fields, you can use the { $exists: false } filter condition on the shard key fields. For example, if the shard key is on the fields { x: 1, y: 1 }, you can find the documents with missing shard key fields by running this query:

db.shardedcollection.find( { $or: [ { x: { $exists: false } }, { y: { $exists: false } } ] } )

If you specify a null equality match filter condition (e.g. { x: null }), the filter matches both those documents with missing shard key fields and those with shard key fields set to null.

Some write operations, such as a write with an upsert specification, require an equality match on the shard key. In those cases, to target a document that is missing the shard key, include another filter condition in addition to the null equality match. For example:

{ _id: <value>, <shardkeyfield>: null } // _id of the document missing shard key

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