What is MongoDB's read concern?

Read concern controls the consistency and isolation of data returned by read operations — which version of data is visible to a read. Read concern levels: local (default): returns the most recent data on the queried server. For primaries: data may not be replicated to majority. May be rolled back if primary fails before replication. available: same as local for replica sets; for sharded clusters, returns data from local shard even if rebalancing. May return orphaned data. majority: returns data acknowledged by a majority of replica set members — data will not be rolled back. Requires enableMajorityReadConcern (default true). Might return slightly older data than "local." linearizable: most strict — reads reflect all successful majority-acknowledged writes before the read. Slower — may wait for replication. Only for primary reads. snapshot: for transactions — reads from a consistent snapshot taken at the start of the transaction. Available in multi-document transactions and "majority" committed. Choosing read concern: local — high performance, slight staleness acceptable; majority — strong consistency, data won't be rolled back; linearizable — strongest, real-time read; snapshot — use in transactions. Setting: db.orders.find({}).readConcern("majority"). Combine write concern and read concern for strong consistency: write with w:"majority" and read with rc:"majority".