What is a distributed SQL database?

A distributed SQL database is a relational database system that spreads data across multiple servers or nodes, combining traditional SQL features with distributed architecture. Unlike single-node databases, it handles storage, processing, and query execution across a cluster of machines. This approach allows the database to scale horizontally—adding more nodes to increase capacity—while maintaining SQL’s familiar querying capabilities and ACID (Atomicity, Consistency, Isolation, Durability) transactions. For example, a distributed SQL database might partition a large table across nodes based on a sharding key, such as user IDs, ensuring each node holds a subset of data while the system acts as a single logical database to applications.

The technical foundation of distributed SQL databases relies on consensus protocols like Raft or Paxos to synchronize data across nodes and ensure consistency. Each node can process read and write requests, with the system automatically routing queries to the appropriate nodes. For writes, the database replicates data to multiple nodes to prevent data loss and enable fault tolerance. For instance, Google Spanner uses atomic clocks and GPS to synchronize time across globally distributed nodes, enabling strong consistency even at scale. Similarly, CockroachDB uses a peer-to-peer architecture where nodes share metadata to route requests efficiently, avoiding bottlenecks. These systems abstract the complexity of distribution, allowing developers to interact with them using standard SQL syntax and tools.

Developers use distributed SQL databases when they need scalability beyond single-node limits without sacrificing transactional guarantees. For example, an e-commerce platform handling millions of transactions globally could use a distributed SQL database to ensure inventory updates are consistent across regions while serving low-latency queries. Unlike NoSQL databases that often prioritize availability over consistency, distributed SQL systems maintain ACID compliance even during network partitions. Features like automatic sharding, load balancing, and failover simplify operations, reducing the need for manual intervention. Tools like YugabyteDB or Amazon Aurora (when configured for cross-region replication) exemplify this balance, offering compatibility with PostgreSQL or MySQL while adding distributed capabilities. This makes distributed SQL databases a practical choice for applications requiring both scale and reliability.