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By default, clients reads from a replica set’s primary; however, clients can specify a read preference to direct read operations to other members. For example, clients can configure read preferences to read from secondaries or from nearest member to:
reduce latency in multi-data-center deployments,
improve read throughput by distributing high read-volumes (relative to write volume),
perform backup operations, and/or
allow reads until a new primary is elected.
Read operations from secondary members of replica sets may not reflect the current state of the primary. Read preferences that direct read operations to different servers may result in non-monotonic reads.
Changed in version 3.6: Starting in MongoDB 3.6, clients can use causally consistent sessions, which provides various guarantees, including monotonic reads.
You can configure the read preference on a per-connection or per-operation basis. For more information on read preference or on the read preference modes, see Read Preference and Read Preference Modes.
In replica sets, all write operations go to the set’s primary. The primary applies the write operation and records the operations 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 continuously replicate the oplog and apply the operations to themselves in an asynchronous process.
Sharded clusters allow you to partition a data set among a cluster of mongod instances in a way that is nearly transparent to the application. For an overview of sharded clusters, see the Sharding section of this manual.
For a sharded cluster, applications issue operations to one of the mongos instances associated with the cluster.
Read operations on sharded clusters are most efficient when directed to a specific shard. Queries to sharded collections should include the collection’s shard key. When a query includes a shard key, the mongos can use cluster metadata from the config database to route the queries to shards.
If a query does not include the shard key, the mongos must direct the query to all shards in the cluster. These scatter gather queries can be inefficient. On larger clusters, scatter gather queries are unfeasible for routine operations.
For replica set shards, read operations from secondary members of replica sets may not reflect the current state of the primary. Read preferences that direct read operations to different servers may result in non-monotonic reads.
Starting in MongoDB 3.6,
Clients can use causally consistent sessions, which provides various guarantees, including monotonic reads.
All members of a shard replica set, not just the primary, maintain the metadata regarding chunk metadata. This prevents reads from the secondaries from returning orphaned data if not using read concern "available". In earlier versions, reads from secondaries, regardless of the read concern, could return orphaned documents.
For sharded collections in a sharded cluster, the mongos directs write operations from applications to the shards that are responsible for the specific portion of the data set. The mongos uses the cluster metadata from the config database to route the write operation to the appropriate shards.
MongoDB partitions data in a sharded collection into ranges based on the values of the shard key. Then, MongoDB distributes these chunks to shards. The shard key determines the distribution of chunks to shards. This can affect the performance of write operations in the cluster.
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.