Social Network

Consider the following model of distributed social network.

sig User {}
sig Post {}
sig DistributedSN {
servers : set Server,
    friends : User -> User
}
sig Server {
     posts : User -> Post, 
     capacity : Int
}

1. Write the following invariants:
   • Each post is owned by at most one user
   • A user cannot be his or her own friend
   • Friendship is a symmetric relation
   • Server capacity is positive
   • Servers cannot exceed their capacity
2. Explore different (alternative) replication strategies for posts:
   • Each post is stored in at most one server
   • Each post is stored in at least two servers
3. Specify the following operations ensuring they preserve the above invariants
   • addPost