How do you handle failover in document databases?

Document databases handle failover through replication and automatic leader election processes. In most document databases like MongoDB or Couchbase, data is replicated across multiple nodes in a cluster or replica set. One node acts as the primary (leader), handling write operations, while others serve as secondaries (followers) that replicate data from the primary. If the primary node fails, the system automatically promotes a secondary to become the new primary, ensuring minimal downtime. This process is designed to maintain availability and consistency without manual intervention.

The failover mechanism typically relies on heartbeat signals and consensus protocols. For example, MongoDB uses a heartbeat system where nodes periodically check each other’s status. If the primary stops responding, the remaining nodes initiate an election to select a new primary. This election process often follows a protocol like Raft, which ensures a majority of nodes agree on the new leader. During this transition, clients may experience brief unavailability for write operations, but read operations can often continue using secondary nodes. Once the new primary is elected, the database updates client connections to route writes to the new leader. Some systems, like Couchbase, also allow for manual failover triggers or predefined priorities to influence which secondary becomes primary.

Developers must configure their document databases properly to ensure reliable failover. For instance, a replica set in MongoDB requires at least three nodes to avoid split-brain scenarios and ensure a majority vote during elections. Network partitions or misconfigured timeouts can interfere with failover, so tuning heartbeat intervals and election timeouts is critical. Additionally, applications should implement retry logic to handle transient errors during failover. For example, MongoDB drivers can automatically detect primary changes and reroute requests. Testing failover scenarios (e.g., simulating node outages) is essential to validate the setup. While automatic failover works for most cases, some systems offer tools like Couchbase’s “Failover” command for controlled maintenance or disaster recovery, allowing administrators to gracefully remove a failed node and rebalance data.