Assignment

Add the following to complete and execute the model

1. Write the following invariants: (20 marks)

• 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: (8 marks)

• 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 (20 marks)

• addPost[n1, n2:DistributedSN,u:User,p:Post] add post p by user u.
• delPost[n1, n2:DistributedSN,u:User,p:Post] delete post p from user u.
• addFriend[n1, n2:DistributedSN,u,v:User] add v as friend of u.
• addServer[n1, n2:DistributedSN,s:Server] add new server s to network n1.
• delServer[n1, n2:DistributedSN,s:Server] delete server s from network n1.

4. Check the following algebraic properties of these operations: (8 marks)

• delPost undoes addPost
• delServer undoes addServer