How do relational databases ensure high availability?

Relational databases ensure high availability through replication, failover mechanisms, and clustering. These techniques work together to minimize downtime and maintain access to data even during hardware failures, software issues, or maintenance. By distributing data across multiple servers and automating recovery processes, relational databases can continue serving requests when individual components fail.

One core method is replication, where copies of the database (replicas) are maintained on separate servers. A primary database handles write operations, while replicas synchronize data changes and serve read requests. If the primary fails, a replica can be promoted to take over, ensuring continuity. For example, PostgreSQL uses streaming replication to keep replicas in sync with the primary, while MySQL employs semi-synchronous replication to confirm data is written to at least one replica before acknowledging a transaction. Some systems, like SQL Server AlwaysOn Availability Groups, allow synchronous replication for zero data loss during failover. Automated monitoring tools (e.g., Patroni for PostgreSQL) detect failures and trigger failovers without manual intervention, reducing downtime to seconds.

Clustering adds another layer by grouping database instances to act as a single system. In a cluster, multiple nodes share storage or coordinate via distributed consensus protocols. For instance, MySQL Group Replication uses a Paxos-based algorithm to ensure all nodes agree on transaction order, allowing any node to handle writes if the primary fails. Load balancers distribute traffic across nodes, preventing overload on any single instance. Additionally, partitioning (sharding) splits data into subsets stored on different servers, limiting the impact of a single server failure. Cloud-managed services like Amazon RDS automate many of these processes, combining multi-zone deployments with automatic backups to maintain availability during outages.

These strategies are often complemented by regular backups, incremental snapshots, and point-in-time recovery to address data corruption or human errors. While no system is fully immune to downtime, relational databases use these layered approaches to achieve 99.9%+ uptime in practice.