How do distributed databases perform load balancing?

Distributed databases perform load balancing by spreading data and queries across multiple nodes to prevent bottlenecks and ensure efficient resource use. This is typically achieved through three main strategies: data partitioning, replication, and dynamic request routing. Each approach addresses different aspects of balancing computational, storage, and network loads while maintaining performance and availability.

First, data partitioning (or sharding) divides the database into smaller chunks distributed across nodes. For example, a database might split user records by geographic region, with each node handling a specific region. To balance uneven access patterns, systems like MongoDB use automatic shard balancing, where chunks of data are migrated between nodes if one shard becomes overloaded. Partitioning ensures that no single node handles all requests, but it requires a mechanism to track data location, such as a coordinator service or consistent hashing. Consistent hashing, used by Apache Cassandra, minimizes data movement when nodes are added or removed by assigning data to a hash ring, ensuring only adjacent nodes are affected during rebalancing.

Second, replication allows multiple copies of data to exist across nodes, enabling read queries to be distributed. In a leader-follower setup (e.g., PostgreSQL streaming replication), writes go to the leader, while followers handle read traffic. Load balancers or client-side libraries can route read requests to the least busy follower using metrics like latency or connection count. Some systems, like Amazon Aurora, extend this by offloading compute-intensive tasks (e.g., query processing) to read replicas, reducing the leader’s load. Replication also improves fault tolerance, as traffic can reroute to replicas if a node fails.

Finally, dynamic adjustments and automated tools help maintain balance as workloads change. For instance, Redis Cluster uses a gossip protocol to detect overloaded nodes and redirect traffic. Cloud-based databases like DynamoDB scale horizontally by automatically splitting partitions and redistributing data when throughput limits are reached. These systems often combine monitoring (e.g., tracking CPU or query latency) with policies to trigger rebalancing. Developers can configure thresholds to control when and how adjustments occur, ensuring minimal disruption during scaling. Together, these methods allow distributed databases to adapt to fluctuating demands while maintaining consistent performance.