MongoDB Replication and Sharding Flashcards

What is a replica set in MongoDB?

Answer: A group of mongod instances maintaining the same data set for redundancy

A replica set is a group of MongoDB instances that maintain the same data, providing high availability through automatic failover.

What is the role of the primary node in a MongoDB replica set?

Answer: Receives all write operations and records changes to the oplog

The primary is the only member that receives write operations; it records all changes in its oplog, which secondaries replicate.

How many votes are needed for a primary election in a MongoDB replica set?

Answer: A simple majority of all voting members

A replica set member wins a primary election by receiving votes from a majority of all voting members, preventing split-brain scenarios.

What is an arbiter in a MongoDB replica set?

Answer: A voting member that holds no data and only participates in elections

An arbiter participates in primary elections to maintain an odd number of votes but does not replicate data, using minimal resources.

What is the oplog in MongoDB replication?

Answer: A capped collection that records all data-modifying operations on the primary

The oplog (operations log) is a special capped collection on the primary that records every write operation; secondaries replicate these entries to stay synchronized.

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

Answer: A majority of voting replica set members have acknowledged the write

w: 'majority' means the write is acknowledged only after a majority of voting members have applied it, ensuring durability even if the primary fails.