H2 is a full-featured, embeddable SQL database engine written in pure Java. It operates as a JDBC driver supporting both embedded (in-process) and client-server (TCP, PostgreSQL wire protocol, web console) modes. The codebase implements a complete SQL parser, query optimizer, transaction engine with MVCC, multiple storage engines (MVStore, legacy PageStore), and a rich set of data types, functions, and compatibility modes for databases like MySQL, PostgreSQL, Oracle, and MSSQLServer. It also includes tools for backup, recovery, CSV import/export, and a web-based administration console. The architecture is modular: the `org.h2.mvstore` package provides a standalone B-tree based transactional key-value store, `org.h2.value` defines a comprehensive type system, and `org.h2.expression` offers an extensible expression and function framework. The repository is exhaustively tested with thousands of SQL scripts and unit tests covering concurrency, crash recovery, and cross-database compatibility. It is actively maintained, with recent commits and support for Java 21 virtual threads and GraalVM native-image.