MongoDB Documentation, Release 3.0.4 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). 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. Write Operations on Replica Sets 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 Chapter 3. MongoDB CRUD Operations 84