Read concern vs write concern vs read preference?

Definition:

Read concern, write concern and read preference control different parts of consistency and availability. Read concern controls which data a read may observe, write concern controls required acknowledgement, and read preference chooses eligible replica members. Combining them deliberately matters when an application expects to read a recent write or survive failover.

Key points:

• Read concern controls the visibility guarantees of reads.

• Write concern controls the required write acknowledgement and durability options.

• Read preference chooses where to read; secondary reads may be behind the primary.

• A secondary may be available but stale; sending reads there is not automatically safe for every business decision.

Interview point:

• Name the guarantee the user needs before choosing a setting; routing, visibility and acknowledgement are not interchangeable.