What are replica sets and how do they provide high availability?
Assesses fundamental understanding of MongoDB conventions, runtime behavior, and memory/performance considerations.
Hiring managers look for precision, avoidance of ambiguous jargon, and ability to explain trade-offs under real production conditions.
A replica set is a group of nodes holding the same data: one primary and two or more secondaries. All writes go to the primary. Secondaries replicate the oplog and can serve reads when the client uses a secondary read preference.
If the primary fails, the remaining members hold an election and promote a secondary. A majority of voting members must be available to elect a primary, so an odd number of members, typically three, is recommended, or an arbiter can break ties. Elections usually complete in seconds.
Replication is asynchronous by default, so a secondary can lag. Read concern majority with write concern majority gives reads that reflect acknowledged writes. Linearizable read concern is stronger but slower. Monitor replication lag and rehearse failover, and remember that a network partition can leave the old primary isolated until it steps down.
Candidate Response Strategy & Interview Tips
- Start with a concise one-sentence summary: Deliver a direct, confident answer first before expanding into nuances.
- Demonstrate real-world trade-offs: Discuss where this approach excels and when you would avoid it in production systems.
- Discuss complexity & edge cases: Proactively explain time/space complexity or boundary conditions (null values, scale limits).
- Prepare for interviewer follow-ups: Technical hiring panels frequently probe deeper into concurrency, backward compatibility, or alternative libraries.