How do distributed databases ensure data availability during system failures?

Distributed databases ensure data availability during system failures primarily through replication, redundancy, and automatic failover mechanisms. By storing copies of data across multiple nodes or servers, these systems can continue operating even if some components fail. For example, a database might replicate each piece of data to three nodes in different locations. If one node goes offline, requests are redirected to the remaining nodes that hold the replicated data. This approach minimizes downtime and ensures users can still access or modify data without interruption. Tools like Apache Cassandra use a “replication factor” to control how many copies exist, allowing developers to balance availability with storage costs.

Another key method involves consensus protocols like Raft or Paxos, which help nodes agree on the current state of data during failures. These protocols enable a distributed database to maintain consistency even when some nodes are unreachable. For instance, in a Raft-based system, a leader node coordinates updates and replicates changes to follower nodes. If the leader fails, followers quickly elect a new leader to take over, ensuring operations continue. This automatic failover process is critical for high availability. Databases like MongoDB use similar logic in their replica sets, where a primary node handles writes, and secondary nodes step in if the primary becomes unavailable.

Finally, conflict resolution strategies and decentralized architectures further enhance availability. Systems like Amazon DynamoDB or Cassandra allow writes to any node, resolving conflicts later using techniques like “last-write-wins” or vector clocks. This ensures that even during network partitions or regional outages, the database remains accessible for both reads and writes. For example, DynamoDB’s global tables replicate data across AWS regions, so a failure in one region doesn’t affect others. By combining these approaches—replication, consensus-driven failover, and conflict resolution—distributed databases provide robust availability guarantees, even under significant hardware or network failures.