read_preferences – Utilities for choosing which member of a replica set to read from.

Utilities for choosing which member of a replica set to read from.

class pymongo.read_preferences.``Primary

Primary read preference.

• When directly connected to one mongod queries are allowed if the server is standalone or a replica set primary.
• When connected to a mongos queries are sent to the primary of a shard.

• When connected to a replica set queries are sent to the primary of the replica set.

• document

  Read preference as a document.

• mode

  The mode of this read preference instance.

• name

  The name of this read preference.

class pymongo.read_preferences.``PrimaryPreferred(tag_sets=None, max_staleness=-1, hedge=None)

PrimaryPreferred read preference.

• When directly connected to one mongod queries are allowed to standalone servers, to a replica set primary, or to replica set secondaries.
• When connected to a mongos queries are sent to the primary of a shard if available, otherwise a shard secondary.
• When connected to a replica set queries are sent to the primary if available, otherwise a secondary.

Changed in version 3.11: Added hedge parameter.

• document

  Read preference as a document.

• hedge

  The read preference hedge parameter.

  A dictionary that configures how the server will perform hedged reads. It consists of the following keys:

  • enabled: Enables or disables hedged reads in sharded clusters.

  Hedged reads are automatically enabled in MongoDB 4.4+ when using a nearest read preference. To explicitly enable hedged reads, set the enabled key to true:

  >>> Nearest(hedge={'enabled': True})

  To explicitly disable hedged reads, set the enabled key to False:

  >>> Nearest(hedge={'enabled': False})

  New in version 3.11.

• max_staleness

  The maximum estimated length of time (in seconds) a replica set secondary can fall behind the primary in replication before it will no longer be selected for operations, or -1 for no maximum.

• min_wire_version

  The wire protocol version the server must support.

  Some read preferences impose version requirements on all servers (e.g. maxStalenessSeconds requires MongoDB 3.4 / maxWireVersion 5).

  All servers’ maxWireVersion must be at least this read preference’s min_wire_version, or the driver raises ConfigurationError.

• mode

  The mode of this read preference instance.

• mongos_mode

  The mongos mode of this read preference.

• name

  The name of this read preference.

• tag_sets

  Set tag_sets to a list of dictionaries like [{‘dc’: ‘ny’}] to read only from members whose dc tag has the value "ny". To specify a priority-order for tag sets, provide a list of tag sets: [{'dc': 'ny'}, {'dc': 'la'}, {}]. A final, empty tag set, {}, means “read from any member that matches the mode, ignoring tags.” MongoReplicaSetClient tries each set of tags in turn until it finds a set of tags with at least one matching member.

  See also

  Data-Center Awareness

class pymongo.read_preferences.``Secondary(tag_sets=None, max_staleness=-1, hedge=None)

Secondary read preference.

• When directly connected to one mongod queries are allowed to standalone servers, to a replica set primary, or to replica set secondaries.
• When connected to a mongos queries are distributed among shard secondaries. An error is raised if no secondaries are available.
• When connected to a replica set queries are distributed among secondaries. An error is raised if no secondaries are available.

Changed in version 3.11: Added hedge parameter.

• document

  Read preference as a document.

• hedge

  The read preference hedge parameter.

  A dictionary that configures how the server will perform hedged reads. It consists of the following keys:

  • enabled: Enables or disables hedged reads in sharded clusters.

  Hedged reads are automatically enabled in MongoDB 4.4+ when using a nearest read preference. To explicitly enable hedged reads, set the enabled key to true:

  >>> Nearest(hedge={'enabled': True})

  To explicitly disable hedged reads, set the enabled key to False:

  >>> Nearest(hedge={'enabled': False})

  New in version 3.11.

• max_staleness

  The maximum estimated length of time (in seconds) a replica set secondary can fall behind the primary in replication before it will no longer be selected for operations, or -1 for no maximum.

• min_wire_version

  The wire protocol version the server must support.

  Some read preferences impose version requirements on all servers (e.g. maxStalenessSeconds requires MongoDB 3.4 / maxWireVersion 5).

  All servers’ maxWireVersion must be at least this read preference’s min_wire_version, or the driver raises ConfigurationError.

• mode

  The mode of this read preference instance.

• mongos_mode

  The mongos mode of this read preference.

• name

  The name of this read preference.

• tag_sets

  Set tag_sets to a list of dictionaries like [{‘dc’: ‘ny’}] to read only from members whose dc tag has the value "ny". To specify a priority-order for tag sets, provide a list of tag sets: [{'dc': 'ny'}, {'dc': 'la'}, {}]. A final, empty tag set, {}, means “read from any member that matches the mode, ignoring tags.” MongoReplicaSetClient tries each set of tags in turn until it finds a set of tags with at least one matching member.

  See also

  Data-Center Awareness

class pymongo.read_preferences.``SecondaryPreferred(tag_sets=None, max_staleness=-1, hedge=None)

SecondaryPreferred read preference.

• When directly connected to one mongod queries are allowed to standalone servers, to a replica set primary, or to replica set secondaries.
• When connected to a mongos queries are distributed among shard secondaries, or the shard primary if no secondary is available.
• When connected to a replica set queries are distributed among secondaries, or the primary if no secondary is available.

Changed in version 3.11: Added hedge parameter.

• document

  Read preference as a document.

• hedge

  The read preference hedge parameter.

  A dictionary that configures how the server will perform hedged reads. It consists of the following keys:

  • enabled: Enables or disables hedged reads in sharded clusters.

  Hedged reads are automatically enabled in MongoDB 4.4+ when using a nearest read preference. To explicitly enable hedged reads, set the enabled key to true:

  >>> Nearest(hedge={'enabled': True})

  To explicitly disable hedged reads, set the enabled key to False:

  >>> Nearest(hedge={'enabled': False})

  New in version 3.11.

• max_staleness

  The maximum estimated length of time (in seconds) a replica set secondary can fall behind the primary in replication before it will no longer be selected for operations, or -1 for no maximum.

• min_wire_version

  The wire protocol version the server must support.

  Some read preferences impose version requirements on all servers (e.g. maxStalenessSeconds requires MongoDB 3.4 / maxWireVersion 5).

  All servers’ maxWireVersion must be at least this read preference’s min_wire_version, or the driver raises ConfigurationError.

• mode

  The mode of this read preference instance.

• mongos_mode

  The mongos mode of this read preference.

• name

  The name of this read preference.

• tag_sets

  Set tag_sets to a list of dictionaries like [{‘dc’: ‘ny’}] to read only from members whose dc tag has the value "ny". To specify a priority-order for tag sets, provide a list of tag sets: [{'dc': 'ny'}, {'dc': 'la'}, {}]. A final, empty tag set, {}, means “read from any member that matches the mode, ignoring tags.” MongoReplicaSetClient tries each set of tags in turn until it finds a set of tags with at least one matching member.

  See also

  Data-Center Awareness

class pymongo.read_preferences.``Nearest(tag_sets=None, max_staleness=-1, hedge=None)

Nearest read preference.

• When directly connected to one mongod queries are allowed to standalone servers, to a replica set primary, or to replica set secondaries.
• When connected to a mongos queries are distributed among all members of a shard.
• When connected to a replica set queries are distributed among all members.

Changed in version 3.11: Added hedge parameter.

• document

  Read preference as a document.

• hedge

  The read preference hedge parameter.

  A dictionary that configures how the server will perform hedged reads. It consists of the following keys:

  • enabled: Enables or disables hedged reads in sharded clusters.

  Hedged reads are automatically enabled in MongoDB 4.4+ when using a nearest read preference. To explicitly enable hedged reads, set the enabled key to true:

  >>> Nearest(hedge={'enabled': True})

  To explicitly disable hedged reads, set the enabled key to False:

  >>> Nearest(hedge={'enabled': False})

  New in version 3.11.

• max_staleness

  The maximum estimated length of time (in seconds) a replica set secondary can fall behind the primary in replication before it will no longer be selected for operations, or -1 for no maximum.

• min_wire_version

  The wire protocol version the server must support.

  Some read preferences impose version requirements on all servers (e.g. maxStalenessSeconds requires MongoDB 3.4 / maxWireVersion 5).

  All servers’ maxWireVersion must be at least this read preference’s min_wire_version, or the driver raises ConfigurationError.

• mode

  The mode of this read preference instance.

• mongos_mode

  The mongos mode of this read preference.

• name

  The name of this read preference.

• tag_sets

  Set tag_sets to a list of dictionaries like [{‘dc’: ‘ny’}] to read only from members whose dc tag has the value "ny". To specify a priority-order for tag sets, provide a list of tag sets: [{'dc': 'ny'}, {'dc': 'la'}, {}]. A final, empty tag set, {}, means “read from any member that matches the mode, ignoring tags.” MongoReplicaSetClient tries each set of tags in turn until it finds a set of tags with at least one matching member.

  See also

  Data-Center Awareness

class pymongo.read_preferences.``ReadPreference

An enum that defines the read preference modes supported by PyMongo.

See High Availability and PyMongo for code examples.

A read preference is used in three cases:

MongoClient connected to a single mongod:

• PRIMARY: Queries are allowed if the server is standalone or a replica set primary.
• All other modes allow queries to standalone servers, to a replica set primary, or to replica set secondaries.

MongoClient initialized with the replicaSet option:

• PRIMARY: Read from the primary. This is the default, and provides the strongest consistency. If no primary is available, raise AutoReconnect.
• PRIMARY_PREFERRED: Read from the primary if available, or if there is none, read from a secondary.
• SECONDARY: Read from a secondary. If no secondary is available, raise AutoReconnect.
• SECONDARY_PREFERRED: Read from a secondary if available, otherwise from the primary.
• NEAREST: Read from any member.

MongoClient connected to a mongos, with a sharded cluster of replica sets:

• PRIMARY: Read from the primary of the shard, or raise OperationFailure if there is none. This is the default.
• PRIMARY_PREFERRED: Read from the primary of the shard, or if there is none, read from a secondary of the shard.
• SECONDARY: Read from a secondary of the shard, or raise OperationFailure if there is none.
• SECONDARY_PREFERRED: Read from a secondary of the shard if available, otherwise from the shard primary.

• NEAREST: Read from any shard member.

• PRIMARY = Primary()

• PRIMARY_PREFERRED = PrimaryPreferred(tag_sets=None, max_staleness=-1, hedge=None)

• SECONDARY = Secondary(tag_sets=None, max_staleness=-1, hedge=None)

• SECONDARY_PREFERRED = SecondaryPreferred(tag_sets=None, max_staleness=-1, hedge=None)

• NEAREST = Nearest(tag_sets=None, max_staleness=-1, hedge=None)

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