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Targeted Operations All insert() operations target to one shard. 

All single update() (including upsert operations) and remove() operations must target to one shard. 

Important: All update() and remove() operations for a sharded collection that specify the justOne or multi: false option must include the shard key or the _id field in the query specification. update() and remove() operations specifying justOne or multi: false in a sharded collection without the shard key or the _id field return an error. 

For queries that include the shard key or portion of the shard key, mongos can target the query at a specific shard or set of shards. This is the case only if the portion of the shard key included in the query is a prefix of the shard key. For example, if the shard key is: 

{ a: 1, b: 1, c: 1 } 

The mongos program can route queries that include the full shard key or either of the following shard key prefixes at a specific shard or set of shards: 

{ a: 1 } { a: 1, b: 1 } 

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Depending on the distribution of data in the cluster and the selectivity of the query, mongos may still have to contact 

