Wikipedia
OrientDB is an open source NoSQL database management system written in Java. It is a multi-model database, supporting graph, document, key/value, and object models, but the relationships are managed as in graph databases with direct connections between records. It supports schema-less, schema-full and schema-mixed modes. It has a strong security profiling system based on users and roles and supports querying with Gremlin along with SQL extended for graph traversal. OrientDB uses several indexing mechanisms based on B-tree and Extendible hashing , the last one is known as "hash index", there are plans to implement LSM-tree and Fractal tree index based indexes. Each record has Surrogate key which indicates position of record inside of Array list , links between records are stored either as single value of record's position stored inside of referrer or as B-tree of record positions (so-called record IDs or RIDs) which allows fast traversal (with O(1) complexity) of one-to-many relationships and fast addition/removal of new links. OrientDB is the second most popular graph database according to the DB-Engines graph database ranking
The development of OrientDB still relies on a big open source community lead by OrientDB LTD company created by its original author Luca Garulli. The project uses GitHub to manage the sources, contributors and versioning, Google Group and StackOverflow to provide free support to the worldwide users. OrientDB also offers a free udemy course for those hoping to learn the basics and get started with OrientDB.