php mongodb类 setreadpreference,PHP - Manual: MongoDB\Driver\ReadPreference (官方文档)

MongoDB\Driver\ReadPreference::RP_PRIMARY

All operations read from the current replica set primary. This is the

default read preference for MongoDB.

MongoDB\Driver\ReadPreference::RP_PRIMARY_PREFERRED

In most situations, operations read from the primary but if it is

unavailable, operations read from secondary members.

MongoDB\Driver\ReadPreference::RP_SECONDARY

All operations read from the secondary members of the replica set.

MongoDB\Driver\ReadPreference::RP_SECONDARY_PREFERRED

In most situations, operations read from secondary members but if no

secondary members are available, operations read from the primary.

MongoDB\Driver\ReadPreference::RP_NEAREST

Operations read from member of the replica set with the least network

latency, irrespective of the member's type.

MongoDB\Driver\ReadPreference::NO_MAX_STALENESS

The default value for the "maxStalenessSeconds"

option is to specify no limit on maximum staleness, which means that the

driver will not consider a secondary's lag when choosing where to

direct a read operation.

MongoDB\Driver\ReadPreference::SMALLEST_MAX_STALENESS_SECONDS

The minimum value for the "maxStalenessSeconds" option

is 90 seconds. The driver estimates secondaries' staleness by

periodically checking the latest write date of each replica set member.

Since these checks are infrequent, the staleness estimate is coarse.

Thus, the driver cannot enforce a max staleness value of less than 90

seconds.