A system is a single instance of the database engine and the environment in which it runs. It consists of a system directory, zero or more databases, and a system-wide configuration. The system directory contains any persistent system-wide configuration parameters, or properties, specific to that system in a properties file called derby.properties. This file is not automatically created; you must create it yourself.

The system is not persistent; you must specify the location of the system directory at every startup.

However, the system and the system directory is an essential part of a running database or databases. Understanding the system is essential to successful development and deployment of applications. As the following figure shows, databases live in a system, which includes system-wide properties, an error log, and one or more databases.

This figure shows a Derby system that includes a database called Accounting and a database called Sales. The figure shows the derby.system.home system variable pointing to the databases and explains that this system variable tells Derby the name of your system directory. Additionally, this figure shows that the derby.properties file and the derby.log file are part of the Derby system.

The system directory can also contain an error log file called derby.log (see ).

Each database within that system is contained in a subdirectory, which has the same name as the database (see ).

In addition, if you connect to a database outside the current system, it automatically becomes part of the current system.