Read Preference
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Read preference describes how MongoDB clients route read operations to the members of a replica set.
By default, an application directs its read operations to the primary member in a replica set (i.e. read preference mode "primary"). But, clients can specify a read preference to send read operations to secondaries.
Read preference consists of the read preference mode and optionally, a tag set list, and the maxStalenessSeconds option.
Read Preference Modes
The following table summarizes the read preference modes:
Read Preference Mode | Description |
|---|---|
Default mode. All operations read from the current replica set primary. Distributed transactions that contain
read operations must use read preference | |
All operations read from the secondary members of the replica set. | |
Operations read from a random eligible replica set member, irrespective of whether that member is a primary or secondary, based on a specified latency threshold. The operation considers the following when calculating latency:
|
For detailed description of the read preference modes, see Read Preference Modes.
Behavior
All read preference modes except
primarymay 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-primarymode.Read preference does not affect the visibility of data; i.e. 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.
Warning
Secondary Reads in a Sharded Cluster with Migrations Can Miss Documents
Long-running secondary reads in a sharded cluster can miss documents if migrations are occurring.
Before deleting a chunk during chunk migration, MongoDB waits for
orphanCleanupDelaySecs, or for in-progress queries
involving the chunk to complete on the shard primary, whichever is
longer. Queries that were initially run on a node that was primary,
but continue after the node has stepped down to a secondary, will be
treated as if they were initially executed on a secondary. That is,
the server only waits for orphanDelayCleanupSecs if there are no
queries targeting the chunk on the current primary.
Queries that target the chunk and are run on secondaries may miss
documents if these queries take longer than
orphanCleanupDelaySecs.
Read Preference Modes
primaryAll 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
primaryread preference mode is not compatible with read preference modes that use tag set lists or maxStalenessSeconds. If you specify tag set lists or amaxStalenessSecondsvalue withprimary, the driver will produce an error.Distributed transactions that contain read operations must use read preference
primary. All operations in a given transaction must route to the same member.
primaryPreferredIn most situations, operations read from the primary member of the set. However, if the primary is unavailable, as is the case during failover situations, operations read from secondary members that satisfy the read preference's
maxStalenessSecondsand tag set lists.When the
primaryPreferredread 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 tomaxStalenessSeconds.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
maxStalenessSecondsvalue and a tag set list, the client filters by staleness first and then by the specified tags.Read operations using the
primaryPreferredmode may return stale data. Use themaxStalenessSecondsoption to avoid reading from secondaries that the client estimates are overly stale.
secondaryOperations 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
secondaryread 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 tomaxStalenessSeconds. 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
maxStalenessSecondsvalue and a tag set list, the client filters by staleness first and then by the specified tags.Read operations using the
secondarymode may return stale data. Use themaxStalenessSecondsoption to avoid reading from secondaries that the client estimates are overly stale.
secondaryPreferredOperations typically read data from secondary members of the replica set. If the replica set has only one single primary member and no other members, operations read data from the primary member.
When the
secondaryPreferredread 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 tomaxStalenessSeconds. 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 tomaxStalenessSeconds, 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
maxStalenessSecondsvalue and a tag set list, the client filters by staleness first and then by the specified tags.Read operations using the
secondaryPreferredmode may return stale data. Use themaxStalenessSecondsoption to avoid reading from secondaries that the client estimates are overly stale.
nearestThe driver reads from a member whose network latency falls within the acceptable latency window. Reads in the
nearestmode 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
maxStalenessSecondsand 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
maxStalenessSecondsvalue and a tag set list, the client filters by staleness first and then by the specified tags. From the remainingmongodinstances, 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
nearestmode may return stale data. Use themaxStalenessSecondsoption to avoid reading from secondaries that the client estimates are overly stale.
Tip
See also:
To learn about use cases for specific read preference settings, see Read Preference Use Cases.
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
Distributed 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
$mergestage can run on replica set secondary nodes if all the nodes in the cluster have the featureCompatibilityVersion set to5.0or higher and the read preference allows secondary reads.In earlier MongoDB versions, pipelines with
$outor$mergestages 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.
| [1] | (1, 2) In some circumstances, two nodes in a replica set
may transiently believe that they are the primary, but at most, one
of them will be able to complete writes with { w:
"majority" } write concern. The node that can complete
{ w: "majority" } writes is the current
primary, and the other node is a former primary that has not yet
recognized its demotion, typically due to a network partition.
When this occurs, clients that connect to the former primary may
observe stale data despite having requested read preference
primary, and new writes to the former primary will
eventually roll back. |