SECONDO is an Extensible Database System developed by FernUniversität in Hagen, This is a just mirror for the open sources of the SECONDO system on GitHub for convienience.
You can access all source and original documents in the following site.
http://dna.fernuni-hagen.de/secondo/
Secondo is an extensible database system supporting especially non-standard applications. Designed at the FernUniversität in Hagen, Secondo is a platform for implementing and experimenting with various kinds of data models.
Secondo can be used by groups at universities to pursue and prototype their research ideas for database systems. It can be used in teaching to show students in detail the architecture and implementation of database systems. Additionally, Secondo may be interesting for users in need of a database system able to handle non-standard data types usually not supported by standard systems.
The Secondo system is extensible by algebra modules, using a well defined interface. New data models and data structures together with their operations are integrated into the system in this way. The source code is available and enables the user to access and modify every part of the database system. Secondo comes with a carefully written user manual, programmer's guide and installation instructions.