How do you do replication in MongoDB?

Q: How do you do replication in MongoDB?

Learn the answer to "How do you do replication in MongoDB?" with detailed explanations, code examples, and best practices on DeployU.

The Scenario

You are a database administrator at a social media company. You are responsible for a MongoDB database that is critical to the business.

You need to set up replication to a secondary database to provide high availability and disaster recovery.

The Challenge

Explain how you would set up replication in MongoDB. What is a replica set, and what are the different types of nodes in a replica set?

Wrong Approach

A junior engineer might not be aware of replication. They might try to solve this problem by just taking periodic backups of the database, which would not provide high availability.

Addresses symptoms, not root cause

Right Approach

A senior engineer would know that replication is a critical part of database administration. They would be able to explain what a replica set is and would have a clear plan for how to set up replication for a production database.

Step 1: Understand What a Replica Set Is

A replica set is a group of MongoDB servers that maintain the same data set. A replica set provides redundancy and high availability.

Step 2: The Different Types of Nodes

Node Type	Description
Primary	The primary node is the only node that can accept write operations.
Secondary	The secondary nodes replicate the data from the primary node. They can be used for read operations.
Arbiter	An arbiter does not store any data, but it participates in elections to choose a new primary.

Step 3: Set Up a Replica Set

Here’s how we can set up a simple replica set with one primary and two secondaries:

1. Start the MongoDB servers:

Start three MongoDB servers on different machines.

2. Initiate the replica set:

Connect to one of the servers and initiate the replica set.

rs.initiate({
    _id: "myReplicaSet",
    members: [
        { _id: 0, host: "host1:27017" },
        { _id: 1, host: "host2:27017" },
        { _id: 2, host: "host3:27017" }
    ]
})

3. Check the status of the replica set:

You can check the status of the replica set by running the rs.status() command.

Failover

If the primary node goes down, the remaining nodes will hold an election to choose a new primary. This process is automatic and ensures that the database is always available.

Systematic, production-ready debugging

Practice Question

You have a three-node replica set and you want to be able to perform reads from the secondary nodes. What should you do?

Questions