OrientDB
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 third most popular graph database according to the DB-Engines graph database ranking, as of September 2017. 

The development of OrientDB still relies on an open source community led 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 Stack Overflow 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.
Q82231
NoSQL
MISC
Q8513
database management system
Concept
Q251
Java
Concept
Q22907102
Multi-model database
Concept
Q595971
graph
MEDIA
Q1235236
document
Concept
Q20706915
key/value
Concept
Q118171
object
Concept
Q5607337
Gremlin
MEDIA
Q47607
SQL
MEDIA
Q677051
B-tree
MEDIA
Q5421911
Extendible hashing
Concept
Q6666764
LSM-tree
Concept
Q17014592
Fractal tree index
Concept
Q871776
Surrogate key
Concept
Q128555
Array list
Concept
Q677051
B-tree
MEDIA
None
O(1) complexity
UNK
Q49000680
DB-Engines
UNK
Q364
GitHub
ORG
Q840820
Google Group
MEDIA
Q549037
Stack Overflow
MEDIA
Q7877165
Udemy
MEDIA
Q808716
OrientDB
ORG
Q808716
OrientDB
ORG
Q808716
OrientDB
ORG
Q808716
OrientDB
ORG
Q808716
OrientDB
ORG
Q808716
OrientDB
ORG
Q808716
OrientDB
ORG
2017-09-01T00:00:00Z^^http://www.w3.org/2001/XMLSchema#dateTime
September 2017
DATE
+1^^http://www.w3.org/2001/XMLSchema#decimal
1
NUMBER
instance of
programming language
instance of
instance of