Lab 8 - Distributed protocols

Due: week 13.

Goal

The goal of this lab is to deal with the consistency issues of the distributed programming.

Requirement

Implement a simple distributed shared memory (DSM) mechanism. The lab shall have a main program and a DSM library. There shall be a predefined number of communicating processes. The DSM mechanism shall provide a predefined number of integer variables residing on each of the processes. The DSM shall provide the following operations:

• write a value to a variable (local or residing in another process);
• a callback informing the main program that a variable managed by the DSM has changed. All processes shall receive the same sequence of data change callbacks; it is not allowed that process P sees first a change on a variable A and then a change on a variable B, while another process Q sees the change on B first and the change on A second.
• a "compare and exchange" operation, that compares a variable with a given value and, if equal, it sets the variable to another given value. Be careful at the interaction with the previous requirement.

Notes:

• Only nodes that subscribe to a variable will receive notifications about changes of that variable, and only those nodes are allowed to change (set) that variable;
• The subscriptions are static and each node knows, for each variable it is subscribed to, which are the other subscribers for that variable.
• We assume that most variables are accessed locally (within a small group of a few computers); we don't want a centralized server that hold all variables, because it would be a central bottleneck of the system. Therefore, as a result of changing a variable, all the messages should be exchanged only between the subscribers of that variable.
• The computers are not faulty.