Read preference describes how MongoDB clients route read operations to members of a replica set.
By default, an application directs its read operations to the primary member in a replica set. Reading from the primary guarantees that read operations reflect the latest version of a document. However, by distributing some or all reads to secondary members of the replica set, you can improve read throughput or reduce latency for an application that does not require fully up-to-date data.
You must exercise care when specifying read preferences: modes other than primary can and willreturn stale data because the secondary queries will not include the most recent write operations to the replica set’s primary.
Read operations to a replica set. Default read preference routes the read to the primary. Read preference of nearest routes the read to the nearest member.
The following are common use cases for using non-primary read preference modes:
- Running systems operations that do not affect the front-end application.
Read preferences aren’t relevant to direct connections to a single mongod instance. However, in order to perform read operations on a direct connection to a secondary member of a replica set, you must set a read preference, such as secondary.
- Providing local reads for geographically distributed applications.
If you have application servers in multiple data centers, you may consider having a geographically distributed replica set and using a non primary read preference or the nearest. This allows the client to read from the lowest-latency members, rather than always reading from the primary.
- Maintaining availability during a failover.
Use primaryPreferred if you want an application to read from the primary under normal circumstances, but to allow stale reads from secondaries in an emergency. This provides a “read-only mode” for your application during a failover.
- All members of a replica have roughly equivalent write traffic, as a result secondaries will service reads at roughly the same rate as the primary.
- Because replication is asynchronous and there is some amount of delay between a successful write operation and its replication to secondaries, reading from a secondary can return out-of-date data.
- Distributing read operations to secondaries can compromise availability if any members of the set are unavailable because the remaining members of the set will need to be able to handle all application requests.
- For queries of sharded collections that do not include the shard key, secondaries may return stale results with missing or duplicated data because of incomplete or terminated migrations.
Sharding increases read and write capacity by distributing read and write operations across a group of machines, and is often a better strategy for adding capacity.
See Read Preference Processes for more information about the internal application of read preferences.
Read Preference Modes
New in version 2.2.
All read preference modes except primary may return stale data because secondaries replicate operations from the primary with some delay. Ensure that your application can tolerate stale data if you choose to use a non-primary mode.
MongoDB drivers support five read preference modes.
Read Preference Mode
The syntax for specifying the read preference mode is specific to the driver and to the idioms of the host language.
Read preference modes are also available to clients connecting to a sharded cluster through a mongos. The mongos instance obeys specified read preferences when connecting to the replica set that provides each shard in the cluster.
Tag sets allow you to target read operations to specific members of a replica set.
Custom read preferences and write concerns evaluate tags sets in different ways. Read preferences consider the value of a tag when selecting a member to read from. Write concerns ignore the value of a tag to when selecting a member, except to consider whether or not the value is unique.
You can specify tag sets with the following read preference modes:
Tags are not compatible with mode primary and, in general, only apply when selecting a secondarymember of a set for a read operation. However, the nearest read mode, when combined with a tag set, selects the matching member with the lowest network latency. This member may be a primary or secondary.
All interfaces use the same member selection logic to choose the member to which to direct read operations, basing the choice on read preference mode and tag sets.
For information on configuring tag sets, see the Configure Replica Set Tag Sets tutorial.