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.
SpacetimeDB is a real-time database system that enables developers to upload application logic directly into the database as modules, eliminating the need for traditional backend servers. Clients connect directly to the database using language-specific SDKs (Rust, C#, TypeScript, C++, Unreal) and receive real-time updates when data changes. The system features a Rust-based core server, WebSocket-based synchronization, ACID transactions, and a code generation system that creates language bindings from module definitions. Notable features include schema evolution capabilities, scheduled procedures, and row-level security. The entire backend of MMORPG BitCraft Online runs as a single SpacetimeDB module, demonstrating its capacity for complex, high-performance applications.
The OpenCog AtomSpace is a hypergraph-based knowledge representation system serving as the core component for Artificial General Intelligence (AGI) research. It provides an in-memory database for storing immutable, uniquely-typed atoms (nodes and links) with mutable key-value stores attached for dynamic data. The system supports executable graphs through the Atomese language, features a sophisticated pattern matching and query engine, includes type inheritance and intersection capabilities, and offers multi-language bindings (Scheme, Python). Architecture-wise, it follows a modular design with clear separation between atom management, value systems, execution frameworks, and query processing. The codebase demonstrates strong engineering practices with factory patterns, handle-based memory management, and thread-safe frame stacks for evaluation contexts.