HSQLDB is a relational database engine written in Java, with a JDBC driver, conforming to ANSI SQL:2023. A small, fast, multithreaded engine and server with memory and disk tables, LOBs, transaction isolation, multiversion concurrency and ACID.
See http://www.cubrid.org/cluster. CUBRID Cluster is an open source clustered DBMS for high scalability including global schema, distributed partition and load balancing features. It is a spin-off project from CUBRID project and supported by NHN.
A disk based production quality RTree based upon Guttmans quadratic split algorithm having following features:
Buffering with dirty write,
Index joins,
Tree Seeding,
Window and NN queries,
Runtime STR Packing capability,
Completely Thread safe.
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This is an implementation of the java DOM specification that keeps all
its "parsed-node" data in disk files instead of RAM. This means you
can create really big DOM trees an not use all your RAM.