MongoDB\Driver\ReadConcern::AVAILABLE-
Default for reads against secondaries when
afterClusterTimeand level are
unspecified.
The query returns the instance's most recent data. Provides no
guarantee that the data has been written to a majority of the replica set
members (i.e. may be rolled back).
For unsharded collections (including collections in a standalone
deployment or a replica set deployment), "local" and
"available" read concerns behave identically.
For a sharded cluster, "available" read concern
provides greater tolerance for partitions since it does not wait to
ensure consistency guarantees. However, a query with
"available" read concern may return orphan documents
if the shard is undergoing chunk migrations since the
"available" read concern, unlike
"local" read concern, does not contact the
shard's primary nor the config servers for updated metadata.
MongoDB\Driver\ReadConcern::LINEARIZABLE-
The query returns data that reflects all successful writes issued with a
write concern of "majority" and
acknowledged prior to the start of the read operation. For replica sets
that run with writeConcernMajorityJournalDefault set
to true, linearizable read concern returns data that will never be
rolled back.
With writeConcernMajorityJournalDefault set to
false, MongoDB will not wait for w: "majority"
writes to be durable before acknowledging the writes. As such,
"majority" write operations could possibly roll back
in the event of a loss of a replica set member.
You can specify linearizable read concern for read operations on the
primary only.
Linearizable read concern guarantees only apply if read
operations specify a query filter that uniquely identifies a single
document.
Tip
Always use maxTimeMS with linearizable read concern
in case a majority of data bearing members are unavailable.
maxTimeMS ensures that the operation does not block
indefinitely and instead ensures that the operation returns an error if
the read concern cannot be fulfilled.
Linearizable read concern requires MongoDB 3.4.
MongoDB\Driver\ReadConcern::LOCAL-
Default for reads against primary if level is
unspecified and for reads against secondaries if level
is unspecified but afterClusterTime is specified.
The query returns the instance's most recent data. Provides no
guarantee that the data has been written to a majority of the replica set
members (i.e. may be rolled back).
MongoDB\Driver\ReadConcern::MAJORITY-
The query returns the instance's most recent data acknowledged as
having been written to a majority of members in the replica set.
To use read concern level of "majority", replica sets
must use WiredTiger storage engine and election protocol version 1.
MongoDB\Driver\ReadConcern::SNAPSHOT-
Read concern "snapshot" is available for
multi-document transactions, and starting in MongoDB 5.0, certain read
operations outside of multi-document transactions.
If the transaction is not part of a causally consistent session, upon
transaction commit with write concern "majority", the
transaction operations are guaranteed to have read from a snapshot of
majority-committed data.
If the transaction is part of a causally consistent session, upon
transaction commit with write concern "majority", the
transaction operations are guaranteed to have read from a snapshot of
majority-committed data that provides causal consistency with the
operation immediately preceding the transaction start.
Outside of multi-document transactions, read concern
"snapshot" is available on primaries and secondaries
for the following read operations: find,
aggregate, and distinct (on
unsharded collections). All other read commands prohibit
"snapshot".