Mycat is an open-source database middleware proxy for MySQL, implementing the MySQL protocol to provide sharding, read/write splitting, global tables, XA transactions, and multi-database backend support (MySQL, PostgreSQL, MongoDB, SequoiaDB). The system routes SQL statements to appropriate backend shards based on configurable partition algorithms, merges result sets, and manages connection pooling, caching, and distributed sequence generation. Architecture is a Java NIO-based server with frontend and backend connection handlers, an SQL parser (Druid extended), a routing engine, and a management interface. The codebase is large (1200+ files) and shows significant engineering investment but suffers from heavy technical debt: mixed Chinese and English comments, many hardcoded values, duplicate boilerplate response classes, deprecated internal APIs, and an aging dependency footprint (Java 8, old Druid version, sun.misc.Unsafe usage). The project appears to be in maintenance mode with limited recent contributions, and the configuration/synchronization via ZooKeeper adds operational complexity.