Replica Set Failover and Recovery in MongoDB

Introduction

MongoDB's Replica Sets provide a high availability solution by maintaining multiple copies of data across different servers. Understanding failover and recovery processes is crucial for ensuring data integrity and availability.

Key Concepts

Definitions

Replica Set: A group of MongoDB servers that maintain the same data set.
Primary Node: The node that receives all write operations.
Secondary Node: Nodes that replicate the primary's data and can take over if the primary fails.
Heartbeat: A signal sent between members of the replica set to check their statuses.

Failover Process

Failover occurs when the primary node becomes unavailable. The following steps typically take place:

Heartbeat signals from the primary to the secondaries stop.
Secondaries initiate an election to choose a new primary.
The new primary is elected based on the priority and data freshness.
Clients are redirected to the new primary for write operations.

Note: The failover process generally takes a few seconds, but it can vary based on the configuration and the network.

Flowchart of Failover Process


        graph TD;
            A[Primary Node Down] --> B[Heartbeat Loss Detected];
            B --> C{Is Quorum Achieved?};
            C -->|Yes| D[Elect New Primary];
            C -->|No| E[Wait for Heartbeat];
            D --> F[New Primary Available];
            F --> G[Client Redirect];

Recovery Process

Once the failed primary node is back online, it undergoes a recovery process:

The node starts up and connects to the replica set.
It becomes a secondary and begins to replicate data from the primary.
Once fully caught up, it can be re-elected as a primary if necessary.

Warning: Ensure that the recovered node does not have an outdated data set that could lead to inconsistencies.

Best Practices

Regularly monitor the health of your replica set.
Configure appropriate replica set member priorities.
Test failover scenarios to ensure smooth transitions.
Maintain an odd number of voting members to avoid split-brain situations.
Implement backup strategies for critical data.

FAQ

What happens to writes during a failover?

During a failover, writes may be queued until a new primary is elected. Applications should be designed to handle these scenarios gracefully.

Can a secondary become primary?

Yes, a secondary can become primary if it is elected during the failover process and has the most up-to-date data.

How can I force a failover?

You can use the command

rs.stepDown()

while connected to the primary node to force it to step down.