Calendar

I am trying to make a calendar/meeting program for my office. It needs to allow anyone to see their/another user's calendar, and it needs to find the earliest time that a group of people can all have a meeting. And then send them a popup message confirming they can make it, and finally creating that event.   I am writing this in java because I am retarded and can't into haskell.

I have some questions on how I should setup the server.

1. Should I keep each persons calendar in a separate file? or keep all the events in one file and have a userid for each event?
2. How do you deal with updating when a program is already running?
3. How do you stop two people from writing to a file at the same time and knowing that you have the most recent file?

Thanks

Just install a CalDAV server and be done with it.