Replica set architecture mental model in MongoDB - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a MongoDB replica set?

A MongoDB replica set is a group of mongod processes that maintain the same data set, providing redundancy and high availability.

beginner

What role does the primary node play in a replica set?

The primary node receives all write operations and replicates data to secondary nodes. It is the main point for data changes.

beginner

How do secondary nodes function in a replica set?

Secondary nodes replicate data from the primary node and can serve read operations if configured. They provide data redundancy.

intermediate

What happens during a primary node failure in a replica set?

The replica set automatically holds an election to select a new primary from the secondary nodes to maintain availability.

intermediate

Why is an arbiter used in a replica set?

An arbiter participates in elections to help choose a primary but does not store data, saving resources while maintaining voting balance.

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

A Handle all write operations

B Store backup data only

C Serve read operations exclusively

D Participate in elections without storing data

The primary node handles all write operations and replicates data to secondary nodes.

What does a secondary node do in a replica set?

A Only votes in elections

B Handles all write operations

C Replicates data from the primary node

D Does not participate in the replica set

Secondary nodes replicate data from the primary to maintain redundancy.

What triggers an election in a MongoDB replica set?

A Manual user command only

B Secondary node failure

C Arbiter shutdown

D Primary node failure

An election is triggered automatically when the primary node fails to select a new primary.

What is the purpose of an arbiter in a replica set?

A Participate in elections without storing data

B Store data backups

C Serve read and write operations

D Replace the primary node

Arbiters help maintain voting balance during elections but do not store data.

Which of the following is NOT true about replica sets?

A They provide data redundancy

B They require manual intervention to elect a primary

C They allow automatic failover

D They consist of primary and secondary nodes

Replica sets automatically elect a primary without manual intervention.

Explain the roles of primary, secondary, and arbiter nodes in a MongoDB replica set.

Think about who writes data, who copies it, and who helps decide leadership.

Describe what happens in a replica set when the primary node fails.

Focus on the failover process and how the system stays online.

Practice

1. What is the main purpose of a MongoDB replica set?

easy

A. To backup data manually every hour

B. To keep copies of data on multiple servers for safety and availability

C. To store data in different formats on the same server

D. To speed up queries by using multiple CPUs on one server

Solution

1. Step 1: Understand replica set function

   A replica set keeps copies of data on multiple servers to protect against failure.

2. Step 2: Compare options

   Only To keep copies of data on multiple servers for safety and availability describes this purpose correctly; others describe unrelated tasks.

3. Final Answer:

   To keep copies of data on multiple servers for safety and availability -> Option B

4. Quick Check:

   Replica set = data copies on servers [OK]

Hint: Replica sets = multiple copies for safety [OK]

Common Mistakes:

• Confusing replica sets with sharding
• Thinking replica sets speed up CPU usage
• Believing backups are manual in replica sets

2. Which of the following is the correct way MongoDB decides the primary in a replica set?

easy

A. MongoDB automatically elects a primary among members

B. The server with the most data becomes primary

C. The oldest server is always primary

D. The user manually sets the primary server

Solution

1. Step 1: Recall primary election process

   MongoDB automatically elects one server as primary to manage writes.

2. Step 2: Eliminate incorrect options

   Primary is not chosen by data size, age, or manual setting.

3. Final Answer:

   MongoDB automatically elects a primary among members -> Option A

4. Quick Check:

   Primary election = automatic [OK]

Hint: Primary is chosen automatically, not manually [OK]

Common Mistakes:

• Assuming user sets primary manually
• Thinking data size decides primary
• Believing oldest server is primary

3. Consider a replica set with one primary and two secondaries. If the primary goes down, what happens next?

medium

A. All secondaries become primary at the same time

B. The replica set stops working until the original primary returns

C. One of the secondaries is automatically elected as the new primary

D. The data is lost because there is no primary

Solution

1. Step 1: Understand failover in replica sets

   If the primary fails, MongoDB elects a new primary from secondaries automatically.

2. Step 2: Check other options

   Replica sets do not stop working, do not have multiple primaries, and do not lose data immediately.

3. Final Answer:

   One of the secondaries is automatically elected as the new primary -> Option C

4. Quick Check:

   Failover = automatic new primary [OK]

Hint: Primary failure triggers automatic new primary election [OK]

Common Mistakes:

• Thinking replica set stops on primary failure
• Believing multiple primaries exist simultaneously
• Assuming data loss happens immediately

4. You set up a replica set but notice writes are failing. Which of these could be the cause?

medium

A. All members are healthy and synced

B. The replica set has only one member

C. Secondary nodes are accepting writes

D. No primary is elected in the replica set

Solution

1. Step 1: Identify write requirements in replica sets

   Writes must go to the primary; if no primary exists, writes fail.

2. Step 2: Analyze options

   Healthy members or single member replica sets usually allow writes; secondaries do not accept writes.

3. Final Answer:

   No primary is elected in the replica set -> Option D

4. Quick Check:

   Writes need primary elected [OK]

Hint: Writes fail if no primary exists [OK]

Common Mistakes:

• Thinking secondaries accept writes
• Assuming single member replica set blocks writes
• Ignoring primary election status

5. You want to ensure your MongoDB replica set remains available even if two servers fail. What is the minimum number of members you should have in your replica set?

hard

A. 5 members

B. 4 members

C. 3 members

D. 6 members

Solution

1. Step 1: Understand majority voting in replica sets

   Replica sets use majority votes to elect primary; to tolerate 2 failures, majority must be > half of total members.

2. Step 2: Calculate minimum members

   With 5 members, majority is 3; losing 2 still leaves 3 members to form majority and keep primary.

3. Final Answer:

   5 members -> Option A

4. Quick Check:

   To survive 2 failures, need 5 members [OK]

Hint: Majority = more than half; 5 members survive 2 failures [OK]

Common Mistakes:

• Choosing 3 or 4 members which can't tolerate 2 failures
• Not considering majority voting
• Assuming even number members are better