Replica Set Maintenance

Introduction

A MongoDB Replica Set is a group of mongod processes that maintain the same data set. Replica sets provide redundancy and high availability, making them the standard deployment architecture for production databases.

Maintaining a replica set involves monitoring node health, managing failovers, and upgrading versions without downtime.

Primary and Secondary Nodes

In a replica set, there is exactly one Primary node that receives all write operations. The other nodes are Secondaries that replicate the primary's oplog (operations log) and apply the operations to their own data sets.

If the primary node crashes, the secondary nodes hold an election and automatically promote a new primary within seconds.

Adding and Removing Members

You can easily scale your replica set horizontally by adding new members.

Connect to the primary node via mongosh and run:

// Add a new node
rs.add( { host: "mongodb3.example.net:27017" } )

// Remove a node
rs.remove("mongodb3.example.net:27017")

Maintenance Modes

When you need to perform server maintenance (like upgrading the OS or MongoDB version), you should step down the primary node to force an election, ensuring your application experiences zero downtime.

// Force the primary to step down and become a secondary
rs.stepDown()

You can then take the node offline, upgrade it, and bring it back up. It will automatically catch up with the new primary using the oplog.

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