What are replica set members?

The minimum recommended configuration for a replica set is a three member replica set with three data-bearing members: one primary and two secondary members.

The members of a replica set are:

1
Primary
2
Secondary
3
Arbiter

Primary

1
The primary is the only member that receives all write operations.
2
MongoDB applies write operations on the primary and then records the operations on the primary's oplog. Secondary members replicate this log and apply the operations to their data sets.
3
All members of the replica set can accept read operations. by default, an application directs its read operations to the primary member.
4
The replica set can have at most one primary. [2] If the current primary becomes unavailable, an election determines the new primary.

Secondary

1
Secondaries replicate operations from the primary to maintain an identical data set.
2
To replicate data, a secondary applies operations from the primary's oplog to its own data set in an asynchronous process.
3
A replica set can have one or more secondaries.
4
Although clients cannot write data to secondaries, clients can read data from secondary members.
5
A secondary can become a primary. If the current primary becomes unavailable, the replica set holds an election to choose which of the secondaries becomes the new primary.

Arbiter

1
An arbiter participates in elections for primary but an arbiter does not have a copy of the data set and cannot become a primary.
2
An arbiter has exactly 1 election vote. By default an arbiter has priority 0.

Difficulty: 5/10

Topics: replica set members, member roles, failover

Scenario Questions

0-2 years experience

1
If you need to add a new secondary to an existing replica set to increase read capacity, what steps would you take and what role does that member play?
2
During a simple deployment, you notice that one member is showing state PRIMARY while another shows SECONDARY. What does that tell you about the replica set members?

2-5 years experience

1
Your application is experiencing intermittent read errors after a network partition. How would you investigate which replica set member is causing the issue?
2
You need to reconfigure a replica set to add an arbiter without downtime. Explain why you would choose an arbiter and what impact it has on the member composition.

5-8 years experience

1
At scale, you have a replica set spread across three data centers. How do you decide the distribution of voting members and secondaries to balance latency and fault tolerance?
2
During a rolling upgrade, a secondary member fails to catch up and remains in RECOVERING state. What are the possible reasons related to its member configuration, and how would you resolve it?

8+ years experience

1
Your organization is migrating from multiple independent replica sets to a sharded cluster with replica set shards. How would you plan the transition of member roles to ensure continuity and minimal impact on write availability?
2
When designing a global multi‑region deployment, how would you leverage replica set member configurations (e.g., priority, hidden, delayed) to meet both low‑latency reads and disaster‑recovery requirements?

Follow-up Questions

❓What would happen if you removed the primary without re-electing a new one?
❓How does adding an arbiter affect write concern and election quorum?
❓Can you describe how member priority influences which node becomes primary?