Kronotop is a distributed database system built on FoundationDB that provides Redis-compatible (RESP2/RESP3) interfaces for multiple data models including key-value (stash), document (bucket with BSON storage and querying), and sorted sets (zmap). It features ACID transactions, secondary indexing (including vector search via HNSW), clustering, sharding, replication, and namespace isolation for multi-tenancy. The architecture is modular with clear separation between network handling (Netty), protocol processing, service layers (transaction, session, data services), and storage (volume-based segment store). Key entry points include the server entry point (KronotopInstanceStarter), CLI (kronotop-cli), and control tool (kronotop-ctl). The system relies on external dependencies such as FoundationDB, Netty, Jackson, Lettuce, Guava, JVector, and Typesafe Config. The codebase demonstrates strong engineering practices with reusable components like versionstamp utilities, retry mechanisms, and protocol handling.
ArcadeDB is a high-performance, multi-model database system supporting graph, document, key-value, time-series, vector, and full-text data models. It exposes multiple query languages (SQL, Cypher, Gremlin, GraphQL) and wire protocols (HTTP, Bolt, PostgreSQL, MongoDB, gRPC). The architecture is modular: a core engine (storage, indexing, query execution) with pluggable server plugins for protocols, replication (Raft-based HA), and observability. It features advanced indexing (LSM-tree, hash, vector quantization, sparse vectors, full-text BM25), time-series with PromQL and downsampling, and graph analytical views with CSR compression. Python bindings allow embedded use. The codebase demonstrates strong engineering with extensive use of interfaces, design patterns, comprehensive test coverage (including issue-specific regression tests), and automated CI/CD with performance benchmarks. It is actively maintained with low technical debt and a clean architecture.
This repository contains the Virtuoso Open-Source multi-model database server, primarily written in C, with extensive client libraries in Java, C#, and PHP. In production, it functions as a relational database, RDF triple store, SPARQL query endpoint, XML processor, and application server (VSPX). The architecture is a monolithic server process with plugin extensions for hosting languages (CLR, Java, PHP) and connectivity via ODBC, JDBC, OLE DB, and ADO.NET. Key entry points include the Virtuoso server binary (built from libsrc/Wi/ and libsrc/Dk/), ODBC driver (binsrc/virtodbc/), JDBC Type 4 driver (libsrc/JDBCDriverType4/), and .NET client (binsrc/VirtuosoClient.Net/). Data flows include SQL parsing and execution via the Wi module, RDF/SPARQL query processing, and WebDAV/HTTP file management through the built-in web server. External dependencies include PCRE, zlib, HTML Tidy (libsrc/Tidy/), and curl. The repository also contains a suite of web applications (ODS) for blogging, bookmarks, calendar, wiki, etc., built on a common SQL-based framework, and a comprehensive test suite covering performance benchmarks (BSBM, LUBM, TPC-D/H). Despite its breadth, the codebase shows signs of age: multiple duplicate Jena driver versions, a legacy Ajax toolkit (OAT), and hardcoded paths in test scripts. It remains actively maintained with regular commits and issue triage.