The Physics of Databases (Part 2): The "Two-Layer" Secret to Navigating the CAP Theorem
Introduction In Part-1 , we explored how the physical storage engine (B-Trees vs. LSM-Trees) dictates your primary key strategy and single-node performance. But when you scale a database across multiple machines or global regions, the physical disk is only half the battle. One of the biggest mistakes engineers make is confusing the storage engine with the distributed protocol . If both Apache Cassandra and Google Cloud Spanner use LSM-Trees underneath, why is Cassandra eventually consistent while Spanner is strictly consistent? To choose the right database, you must evaluate the Two-Layer Problem . 1. The Two-Layer Database Architecture A distributed database is actually built of two completely separate architectural layers. Layer 1: The Local Storage Engine (The Disk) The Goal: Write bytes to a specific SSD as fast as mathematically possible. The Tech: B-Trees (PostgreSQL, MySQL) or LSM-Trees (Cassandra, Spanner, DynamoDB). This layer has absolutely no concept of "Consistency...