Read Preference

Read Preference Modes

The following table summarizes the read preference modes.

• For all modes, a member is available when it responds to the client's periodic monitoring checks, which confirm that the member is reachable over the network and able to respond to requests.

• For all modes except primary, a member is eligible if it satisfies the read preference's maxStalenessSeconds option and any specified tag set lists.

• For all modes except primary, the localThresholdMS connection string option sets the local threshold that the client uses to calculate the latency window when choosing among eligible members.

For detailed description of the read preference modes, see Read Preference Modes.

Behavior

• All read preference modes except primary may return stale data because secondaries replicate operations from the primary in an asynchronous process. [1] Ensure that your application can tolerate stale data if you choose to use a non-primary mode.

• Read preference does not affect the visibility of data. Clients can see the results of writes before they are acknowledged or have propagated to a majority of replica set members. For details, see Read Isolation, Consistency, and Recency.

• Read preference does not affect causal consistency. The causal consistency guarantees provided by causally consistent sessions for read operations with "majority" read concern and write operations with "majority" write concern hold across all members of the MongoDB deployment.

Read Preference Modes

primary

All read operations use only the current replica set primary. [1] This is the default read mode. If the primary is unavailable, read operations produce an error or throw an exception.

The primary read preference mode is not compatible with read preference modes that use tag set lists or maxStalenessSeconds. If you specify tag set lists or a maxStalenessSeconds value with primary, the driver will produce an error.

Transactions that contain read operations must use read preference primary. All operations in a given transaction must route to the same member.

primaryPreferred

Operations read from the primary member of the set if it is available. If the primary is unavailable, such as during failover situations, operations read from secondary members that satisfy the read preference's maxStalenessSeconds and tag set lists.

When the primaryPreferred read preference includes a maxStalenessSeconds value and there is no primary from which to read, the client estimates how stale each secondary is by comparing the secondary's last write to that of the secondary with the most recent write. The client then directs the read operation to a secondary whose estimated lag is less than or equal to maxStalenessSeconds.

When the read preference includes a tag set list (an array of tag sets) and there is no primary from which to read, the client attempts to find secondary members with matching tags (trying the tag sets in order until a match is found). If matching secondaries are found, the client selects a random secondary from the nearest group of matching secondaries. If no secondaries have matching tags, the read operation produces an error.

When the read preference includes a maxStalenessSeconds value and a tag set list, the client filters by staleness first and then by the specified tags.

Read operations using the primaryPreferred mode may return stale data. Use the maxStalenessSeconds option to avoid reading from secondaries that the client estimates are overly stale.

In sharded clusters that enable the ingress request rate limiter, shard nodes that are shedding load can return errors labeled SystemOverloadedError.

For clusters that have overloadAwareServerSelectionEnabled set to true, when a primary responds with a retryable error labeled with SystemOverloadedError, the router may temporarily route reads to eligible secondaries instead of the overloaded server. This applies to any read preference that can select multiple servers. By default, overloadAwareServerSelectionEnabled is set to false.

secondary

Operations read only from the secondary members of the set. If no secondaries are available, then this read operation produces an error or exception.

Most replica sets have at least one secondary, but there are situations where there may be no available secondary. For example, a replica set with a primary, a secondary, and an arbiter may not have any secondaries if a member is in recovering state or unavailable.

When the secondary read preference includes a maxStalenessSeconds value, the client estimates how stale each secondary is by comparing the secondary's last write to that of the primary. The client then directs the read operation to a secondary whose estimated lag is less than or equal to maxStalenessSeconds. If there is no primary, the client uses the secondary with the most recent write for the comparison.

When the read preference includes a tag set list (an array of tag sets), the client attempts to find secondary members with matching tags (trying the tag sets in order until a match is found). If matching secondaries are found, the client selects a random secondary from the nearest group of matching secondaries. If no secondaries have matching tags, the read operation produces an error.

When the read preference includes a maxStalenessSeconds value and a tag set list, the client filters by staleness first and then by the specified tags.

Read operations using the secondary mode may return stale data. Use the maxStalenessSeconds option to avoid reading from secondaries that the client estimates are overly stale.

secondaryPreferred

Operations read data from eligible secondary members of the replica set. If no eligible secondaries are available, operations read from the primary member.

When the secondaryPreferred read preference includes a maxStalenessSeconds value, the client estimates how stale each secondary is by comparing the secondary's last write to that of the primary. The client then directs the read operation to a secondary whose estimated lag is less than or equal to maxStalenessSeconds. If there is no primary, the client uses the secondary with the most recent write for the comparison. If there are no secondaries with estimated lag less than or equal to maxStalenessSeconds, the client directs the read operation to the replica set's primary.

When the read preference includes a tag set list (an array of tag sets), the client attempts to find secondary members with matching tags (trying the tag sets in order until a match is found). If matching secondaries are found, the client selects a random secondary from the nearest group of matching secondaries. If no secondaries have matching tags, the client ignores tags and reads from the primary.

When the read preference includes a maxStalenessSeconds value and a tag set list, the client filters by staleness first and then by the specified tags.

Read operations using the secondaryPreferred mode may return stale data. Use the maxStalenessSeconds option to avoid reading from secondaries that the client estimates are overly stale.

nearest

The driver reads from a member whose network latency falls within the acceptable latency window. Reads in the nearest mode do not consider whether a member is a primary or secondary when routing read operations: primaries and secondaries are treated equivalently.

Set this mode to minimize the effect of network latency on read operations without preference for current or stale data.

When the read preference includes a maxStalenessSeconds value, the client estimates how stale each secondary is by comparing the secondary's last write to that of the primary, if available, or to the secondary with the most recent write if there is no primary. The client will then filter out any secondary whose estimated lag is greater than maxStalenessSeconds and randomly direct the read to a remaining member (primary or secondary) whose network latency falls within the acceptable latency window.

If you specify a tag set list, the client attempts to find a replica set member that matches the specified tag set lists and directs reads to an arbitrary member from among the nearest group.

When the read preference includes a maxStalenessSeconds value and a tag set list, the client filters by staleness first and then by the specified tags. From the remaining mongod instances, the client then randomly directs the read to an instance that falls within the acceptable latency window. The read preference member selection documentation describes the process in detail.

Read operations using the nearest mode may return stale data. Use the maxStalenessSeconds option to avoid reading from secondaries that the client estimates are overly stale.

Configure Read Preference

When using a MongoDB driver, you can specify the read preference using the driver's read preference API. See the driver API documentation. You can also set the read preference when connecting to the replica set or sharded cluster. For an example, see connection string.

For a given read preference, the MongoDB drivers use the same member selection logic.

When using mongosh, see cursor.readPref() and Mongo.setReadPref().

Read Preference and Transactions

Transactions that contain read operations must use read preference primary. All operations in a given transaction must route to the same member.

Additional Considerations

Consider the following points when using $merge or $out stages in an aggregation pipeline:

• Starting in MongoDB 5.0, pipelines with a $merge stage can run on replica set secondary nodes if all the nodes in the cluster have the featureCompatibilityVersion set to 5.0 or higher and the read preference allows secondary reads.

  • $merge and $out stages run on secondary nodes, but write operations are sent to the primary node.

  • Not all driver versions support $merge operations sent to the secondary nodes. For details, see the driver documentation.

• In earlier MongoDB versions, pipelines with $out or $merge stages always run on the primary node and read preference isn't considered.

For mapReduce operations, only "inline" mapReduce operations that do not write data support read preference. Otherwise, mapReduce operations run on the primary member.