Question: How do you configure replication in MongoDB?

Answer

MongoDB replication allows your data to be duplicated across multiple servers, improving data availability and redundancy. A replica set in MongoDB is a group of mongod instances that maintain the same data set. Here's how you can configure replication in MongoDB:

1. Define Your Replica Set: Decide on the number of nodes and their roles (primary, secondary) in your replica set.

2. Configure mongod Instances: Start each MongoDB instance with the --replSet option set to the same replica set name.

mongod --port 27017 --dbpath /srv/mongodb/rs0-0 --replSet rs0
mongod --port 27018 --dbpath /srv/mongodb/rs0-1 --replSet rs0
mongod --port 27019 --dbpath /srv/mongodb/rs0-2 --replSet rs0

In this example, three instances are started on different ports and directories, but all belong to the 'rs0' replica set.

3. Initiate the Replica Set: Connect to one of your instances using the mongo shell and initiate the replica set.

rs.initiate()

4. Add Members to the Replica Set: Still in the mongo shell, add the other instances as members of the replica set.

rs.add('hostname:27018')
rs.add('hostname:27019')

Replace 'hostname' with the actual hostname or IP address of the server where your instances are running.

5. Configure Replica Set Settings (optional): You can adjust your replica set configuration by using the rs.reconfig() method. This includes setting priorities, hidden status, tags, etc.

6. Verify Configuration: Use the rs.status() command to verify that your replica set is configured correctly and all members are connected.

Important Considerations

• Write Concerns: Be mindful of write concern settings as they can impact data consistency and performance.
• Read Preferences: Configure read preferences according to your application's consistency requirements.
• Backup Strategy: Ensure you have a robust backup strategy to prevent data loss.

Replication in MongoDB enhances data availability and disaster recovery capabilities. Properly configuring your replica set is crucial for maintaining data integrity and system performance.