OrientDB is a multi-model NoSQL database management system supporting document, graph, object, and relational data models within a single engine. It provides ACID transactions, a SQL-like query language, distributed clustering with automatic sharding and replication, full-text and spatial indexing via Lucene, and a Gremlin graph traversal API. The system is implemented in Java and organized into modular components: a core storage engine with pluggable index structures (SB-Tree, hash, Lucene), a binary client-server protocol, an HTTP REST API, and a web-based administration studio (AngularJS). It includes enterprise features such as backup/restore, metrics monitoring, and security auditing. The architecture follows a plugin-based server model, allowing extensions for authentication, indexing, and custom commands. External dependencies include Dropwizard Metrics, Lucene, Gremlin/TinkerPop, and various compression libraries.
Galera is a synchronous multi-master replication middleware for transactional databases, primarily MySQL and MariaDB, implemented as a wsrep provider shared library (libgalera_smm.so). The codebase provides total-order multicast group communication via a layered protocol stack (GCS/gcomm), conflict detection through certification of write-sets, incremental state transfer (IST) with a multi-tier cache (gcache), and an arbitrator daemon (garbd) for quorum maintenance. Architecturally, it is structured as a set of cooperating C++ libraries: galerautils (memory allocators, configuration, threading, hashing), gcs (group communication service with membership and flow control), gcomm (transport abstraction with EVS/PC/GMCast protocols), galera (replication engine with certification and write-set handling), and gcache (write-set caching in memory, ring buffer, and memory-mapped pages). The code is well-tested with unit tests (Check framework), regression tests, and integration test scripts. While the core replication logic and utility libraries are well-crafted and modular, the project bundles an older version of the ASIO networking library and retains C++03 compatibility shims, contributing to moderate technical drift. The repository is actively maintained with regular commits and issue triage.