This is a possible problem: the maximum id of an activity is 2^31-1 (which is about 2.147*10^9, about 2 billions).
Every time the user adds an activity, the next id is the size of maximum current id + 1.
If a user removes an activity, that id is removed.
Example: adds 1,2,3,4,5, removes 3 and 4 - next addition will be 6. So, we have 1,2,5,6.
My question is: may there appear problems because of limitation of max id to 2^31-1? Or should I not bother about that?