readConcern

ReadConcern is a MongoDB feature that controls the level of isolation and data freshness for read operations. It specifies how up-to-date data must be and how it is acknowledged by replica set members. ReadConcern can be set per operation or per session and applies to reads against primary and secondary members depending on the level and cluster configuration.

Common readConcern levels include local, majority, linearizable, and snapshot. Local returns data from the node’s own data set without waiting for replication, offering low latency but weaker cross-node consistency. Majority returns data that has been acknowledged by a majority of replica set members, providing stronger durability across the cluster. Linearizable reads ensure the read reflects the most recent acknowledged write by requiring participation from a primary, delivering strict, real-time consistency at the potential cost of higher latency. Snapshot is used with multi-document transactions to provide a point-in-time view, enabling repeatable reads within a transaction.

Usage and implications. ReadConcern is specified in the read command, or configured through a session for transactional reads. For example, a command may include readConcern: { level: "majority" }. In transactions, readConcern: { level: "snapshot" } is used to obtain a consistent transactional view. Not all levels are supported in every deployment or version; some levels require replica sets or transactions. ReadConcern is distinct from WriteConcern and interacts with replication and durability guarantees for reads as defined by the cluster configuration.