Question

I have to write a distributed system with four processes running on four different nodes. The distributed system is supposed to work in the following way: a random number generator generates a random number at each process. The objective is to even out these values in all processes by message passing between processes. Such that process A is the server who gets the numbers from all proceses and then orders them to send a portion of their number to one or more other processes in order to even out all numbers the processes hold. For example A’s count is 30, B’s count is 65, C’s count is 35 and D’s count is 70. A computes 30+65+35+70 = 200 divided by 4 = 50. Now process A, the server, knows who has less than average and who has more than average. Now the question is how does A decide who sends what number to who? to even out the values of all processes. please note that A can’t directly instruct a process to decrement or increment its count e.g. it can’t send a message to B and tell it to decrement by 15 and then send another message to C and tell it to increment by 15. A must send a message to B that will tell B to decrement by 15 and then send a message to C and tell it to increment by 15 or in other words it tell B to send 15 of your count to C. Thanks in advance. Zaki.

Answer 1

The problem that you describe is known as distributed aggregation. There are a number of solutions appropriate for different assumptions on the network (what nodes are connected? can messages be lost?), the function to compute (average? sum?), and so on. A good overview, with references to algorithms that you can use, can be found at http://arxiv.org/abs/1110.0725.