# Recommended practices

When developing Derby applications, create a single directory to hold your database or databases.

Give this directory a unique name, to help you remember that:

- All databases exist within a system.
- System-wide properties affect the entire system, and persistent system-wide properties live in the system directory.
- You can boot all the databases in the system, and the boot-up times of all databases affect the performance of the system.
- You can preboot databases only if they are within the system. (Databases do not necessarily have to live inside the system directory, but keeping your databases there is the recommended practice.)
- Once you connect to a database, it is part of the current system and thus inherits all system-wide properties.
- Only one instance of Derby can run in a JVM at a single time.
- The error log is located inside the system directory.

Parent topic: Derby system
Related concepts
Booting databases
The error log
derby.properties
Double-booting system behavior
Connecting to databases outside the system directory
Embedded systems and properties