Writing Custom Extensions

While I was updating our Inventory Server from 2.4 to 2.8 I realized we where utilizing quite a few custom plugins.

If I understood correctly, these need to be changed to be extensions to work with the 2.8 version of the Server. The documenation on developing custom plugins as well as the requirements for them has thus far eluded me.

Are there any resources I might consult for help?

Regards,

Yannic