MongoDB Replication and Replica Sets Flashcards

What is the minimum recommended number of members in a MongoDB replica set for high availability?

Answer: 3

MongoDB recommends a minimum of 3 members for high availability. With 3 members, the replica set can tolerate the failure of one member while still maintaining a majority (2 of 3) required to elect a new primary and continue accepting writes.

What is the role of the oplog in MongoDB replication?

Answer: A capped collection on the primary that records all write operations for secondaries to replicate

The oplog (operations log) is a special capped collection in the local database that records all write operations in an idempotent form. Secondary members tail the primary's oplog and replay operations to stay in sync with the primary.

How does a MongoDB replica set elect a new primary?

Answer: Members vote and a candidate must receive a majority of votes

MongoDB replica set elections use a majority voting protocol. A candidate primary must receive votes from a majority of all voting members (not just available members). This prevents split-brain scenarios in network partitions.

What is a MongoDB arbiter in a replica set?

Answer: A member that holds no data but participates in elections to break ties

An arbiter holds no data and cannot become primary. Its sole purpose is to vote in elections to break ties when an even number of data-bearing members are present. Arbiters use minimal resources but introduce a potential single point of failure.

What is the write concern { w: 'majority' } in MongoDB?

Answer: Writes are acknowledged after being written to a majority of voting replica set members

{ w: 'majority' } ensures MongoDB acknowledges a write only after it has been written to a majority of the replica set's voting members and is durable. This guarantees the write survives the failure of any single member, providing strong durability guarantees.

What happens during a replica set failover when the primary becomes unavailable?

Answer: An election is held among eligible secondaries to elect a new primary

When a primary becomes unavailable, eligible secondary members initiate an election. The secondary with the most up-to-date oplog and sufficient priority that can reach a majority of voting members will be elected as the new primary, typically within seconds.