Why Is It Hard to Horizontally Scale SQL Databases?

Scaling SQL databases horizontally is a complex task due to challenges such as maintaining ACID properties across distributed systems, handling distributed joins, managing transactions across shards, ensuring data consistency, selecting appropriate sharding strategies, and enforcing data integrity. Various strategies, including partitioning and federated databases, application-level sharding, consistent hashing, database middleware, denormalization, polyglot persistence, and geo-partitioning, can be used to mitigate these challenges. It is crucial for system designers to understand the trade-offs and choose the right strategies to design scalable SQL database systems.