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.

See also

This module is compatible with both the synchronous and asynchronous PyMongo APIs.

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.

Note

When a MongoClient is first created reads will be routed to an available secondary until the primary of the replica set is discovered.

Parameters:

  • tag_sets (Optional[_TagSets]) – The tag_sets to use if the primary is not available.

  • max_staleness (int) – (integer, in seconds) The maximum estimated length of time a replica set secondary can fall behind the primary in replication before it will no longer be selected for operations. Default -1, meaning no maximum. If it is set, it must be at least 90 seconds.

  • hedge (Optional[_Hedge]) – The hedge to use if the primary is not available.

Changed in version 3.11: Added hedge parameter.

property document: dict[str, Any]

Read preference as a document.

property hedge: Mapping[str, Any] | None

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})

Added in version 3.11.

property max_staleness: int

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.

property min_wire_version: int

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.

property mode: int

The mode of this read preference instance.

property mongos_mode: str

The mongos mode of this read preference.

property name: str

The name of this read preference.

property tag_sets: Sequence[Mapping[str, Any]]

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.” MongoClient tries each set of tags in turn until it finds a set of tags with at least one matching member. For example, to only send a query to an analytic node:

Nearest(tag_sets=[{"node":"analytics"}])

Or using SecondaryPreferred:

SecondaryPreferred(tag_sets=[{"node":"analytics"}])

.. seealso:: `Data-Center Awareness
    <https://www.mongodb.com/docs/manual/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.

Parameters:

  • tag_sets (Optional[_TagSets]) – The tag_sets for this read preference.

  • max_staleness (int) – (integer, in seconds) The maximum estimated length of time a replica set secondary can fall behind the primary in replication before it will no longer be selected for operations. Default -1, meaning no maximum. If it is set, it must be at least 90 seconds.

  • hedge (Optional[_Hedge]) – The hedge for this read preference.

Changed in version 3.11: Added hedge parameter.

property document: dict[str, Any]

Read preference as a document.

property hedge: Mapping[str, Any] | None

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})

Added in version 3.11.

property max_staleness: int

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.

property min_wire_version: int

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.

property mode: int

The mode of this read preference instance.

property mongos_mode: str

The mongos mode of this read preference.

property name: str

The name of this read preference.

property tag_sets: Sequence[Mapping[str, Any]]

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.” MongoClient tries each set of tags in turn until it finds a set of tags with at least one matching member. For example, to only send a query to an analytic node:

Nearest(tag_sets=[{"node":"analytics"}])

Or using SecondaryPreferred:

SecondaryPreferred(tag_sets=[{"node":"analytics"}])

.. seealso:: `Data-Center Awareness
    <https://www.mongodb.com/docs/manual/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.

Note

When a MongoClient is first created reads will be routed to the primary of the replica set until an available secondary is discovered.

Parameters:

  • tag_sets (Optional[_TagSets]) – The tag_sets for this read preference.

  • max_staleness (int) – (integer, in seconds) The maximum estimated length of time a replica set secondary can fall behind the primary in replication before it will no longer be selected for operations. Default -1, meaning no maximum. If it is set, it must be at least 90 seconds.

  • hedge (Optional[_Hedge]) – The hedge for this read preference.

Changed in version 3.11: Added hedge parameter.

property document: dict[str, Any]

Read preference as a document.

property hedge: Mapping[str, Any] | None

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})

Added in version 3.11.

property max_staleness: int

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.

property min_wire_version: int

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.

property mode: int

The mode of this read preference instance.

property mongos_mode: str

The mongos mode of this read preference.

property name: str

The name of this read preference.

property tag_sets: Sequence[Mapping[str, Any]]

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.” MongoClient tries each set of tags in turn until it finds a set of tags with at least one matching member. For example, to only send a query to an analytic node:

Nearest(tag_sets=[{"node":"analytics"}])

Or using SecondaryPreferred:

SecondaryPreferred(tag_sets=[{"node":"analytics"}])

.. seealso:: `Data-Center Awareness
    <https://www.mongodb.com/docs/manual/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.

Parameters:

  • tag_sets (Optional[_TagSets]) – The tag_sets for this read preference.

  • max_staleness (int) – (integer, in seconds) The maximum estimated length of time a replica set secondary can fall behind the primary in replication before it will no longer be selected for operations. Default -1, meaning no maximum. If it is set, it must be at least 90 seconds.

  • hedge (Optional[_Hedge]) – The hedge for this read preference.

Changed in version 3.11: Added hedge parameter.

property document: dict[str, Any]

Read preference as a document.

property hedge: Mapping[str, Any] | None

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})

Added in version 3.11.

property max_staleness: int

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.

property min_wire_version: int

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.

property mode: int

The mode of this read preference instance.

property mongos_mode: str

The mongos mode of this read preference.

property name: str

The name of this read preference.

property tag_sets: Sequence[Mapping[str, Any]]

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.” MongoClient tries each set of tags in turn until it finds a set of tags with at least one matching member. For example, to only send a query to an analytic node:

Nearest(tag_sets=[{"node":"analytics"}])

Or using SecondaryPreferred:

SecondaryPreferred(tag_sets=[{"node":"analytics"}])

.. seealso:: `Data-Center Awareness
    <https://www.mongodb.com/docs/manual/data-center-awareness/>`_
class pymongo.read_preferences.ReadPreference

An enum that defines some commonly used read preference modes.

Apps can also create a custom read preference, for example:

Nearest(tag_sets=[{"node":"analytics"}])

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()
Parameters:

selection (Selection)

Return type:

Selection

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

selection (Selection)

Return type:

Selection

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

selection (Selection)

Return type:

Selection

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

selection (Selection)

Return type:

Selection

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

selection (Selection)

Return type:

Selection