Jade,
You can remove most of your design if you utilize acs-service contract:
"We add to calendar a table which lists other packages which are registered as calendar sources." These would be the acs service contract implementations.
"the the associated Tcl proc to call in order to get the information from those calendar sources" acs service contract implementation alias is the name of the tcl procedure.
So you should not need any additional tables.
You just need to define what operations are needed including the input and output parameters for them.
Take a look at the existing service contacts for ideas and post here for feedback.
