MongoDB\Driver\ReadConcern controls the level of isolation for read operations for replica sets and replica set shards. This option requires the WiredTiger storage engine and MongoDB 3.2 or later.
A linearizable read avoids returning reads from a
stale primary (one that has already been superseded
by a new primary but doesn't know it yet). It is important to note
that this read concern level does not by itself produce linearizable
reads; they must be issued in conjunction with
MongoDB\Driver\WriteConcern::MAJORITY writes to the
same document(s) in order to be linearizable.
Queries using this read concern will return the node's most recent copy of the data. This provides no guarantee that the data has been written to a majority of the nodes (i.e. it may be rolled back).
This is the default read concern level.
Queries using this read concern will return the node's most recent copy of the data confirmed as having been written to a majority of the nodes (i.e. the data cannot be rolled back).