content_image/788925.jpg
Important: Update operations that affect a single document must include the shard key or the _id field. Updates that affect multiple documents are more efficient in some situations if they have the shard key , but can be broadcast to all shards. 

If the value of the shard key increases or decreases with every insert, all insert operations target a single shard. As a result, the capacity of a single shard becomes the limit for the insert capacity of the sharded cluster. 

For more information, see Sharded Cluster Tutorials (page 690) and Bulk Write Operations (page 87). 

## Write Operations on Replica Sets 

In replica sets , all write operations go to the set’s primary , which applies the write operation then records the oper- ations on the primary’s operation log or oplog . The oplog is a reproducible sequence of operations to the data set. Secondary members of the set are continuously replicating the oplog and applying the operations to themselves in an asynchronous process. 

<img src='content_image/788925.jpg'>

Large volumes of write operations, particularly bulk operations, may create situations where the secondary members have difficulty applying the replicating operations from the primary at a sufficient rate: this can cause the secondary’s state to fall behind that of the primary. Secondaries that are significantly behind the primary present problems for 

