First a little history...
Derby started out long ago as Cloudscape. Cloudscape was bought by Informix. Informix was bought by IBM. IBM didn't understand Cloudscape and decided to open source the project under APL. Hence Derby was born.
Derby is an excellent lightweight 100% java database. So when you have a Java framework, using Derby makes a lot of sense. Derby is used to persist some environment information and I believe its used in part of some of the unit testing.
Where Derby has been replaced by MySQL is when someone wanted a multi-user database and they were more comfortable with MySQL than they were with Derby. (Hint: Derby can be started as an embedded single user database, or as a multi-user database by changing its invocation at startup. ;-)
So I would guess the initial reason to go with Derby was that its released under APL and there were no licensing issues. ;-)
> Date: Tue, 9 Aug 2011 15:17:35 +0530
> Subject: Derby with Hadoop --Why?
> From: [EMAIL PROTECTED]
> To: [EMAIL PROTECTED]
> What is the significance of Derby in Hadoop Project.
> Why people are using Derby along with Hadoop
> Saravana Kumar.J