Revisiting Logical Clocks: Mutual Exclusion Problem statement: Given a set of n processes, and a shared resource, it is required that: –Mutual exclusion At any time, at most one process is accessing the resource –Liveness If a process requests for the resource, it can eventually access the resource

Solution to mutual exclusion How could we do this if all processes shared a common clock –Each process timestamps its request –The process with lowest timestamp is allowed to access critical section What are the properties of clocks that enable us to solve this problem? Is there another type of clock that could help is to solve this problem

Solution to mutual exclusion, based on logical clocks Messages are timestamped with logical clocks Each process maintains a queue of pending requests When process j wants to access the resource, it adds its timestamp to the queue, and sends a request message containing its timestamp to all other processes When process k receives a request message from j, it sends a reply message to j

Solution to mutual exclusion, based on logical clocks (continued) Process j accesses the resource (enters critical section) iff –it has received a reply from every other process –its queue does not contain a timestamp that is smaller than its own request After a process is done accessing its critical section, it sends a release message to all processes and removes its own request from the pending queue When a process k receives the release message from j,it removes the entry of j from its pending queue

Solution to mutual exclusion, based on logical clocks (continued) This is called Lamport’s mutual exclusion algorithm What is the number of messages sent for every access to critical section?

Optimizations Should a process wait for a reply message from every other process? Should a process send a reply message immediately? Answer these questions to obtain a protocol where only 2 (n-1) messages are used for each critical section

Classification of Mutual Exclusion Algorithms Quorum Based –Each node is associated with a quorum Q j –When j wants to enter critical section, it asks for permission from all nodes in this quorum –What property should be met by the quorums of different processes? Token Based –A token is circulated among nodes; the node that has the token can access critical section –We will look at these later

Which category would Lamport’s protocol fit in? What is the quorum of a process in this algorithm? What are the possibilities of different quorums

Taking Quorum Based Algorithms to Extreme Centralized mutual exclusion –A single process `coordinator' is responsible for ensuring –mutual exclusion. –Each process requests the coordinator whenever it wishes to access the resource. –The coordinator permits only one process to access the resource at a time. –After a process accesses the resource, it sends a reply to the coordinator. Quorum for all processes is {c} where c is the coordinator

Centralized mutual exclusion Problem : What if the coordinator fails? Solution : Elect a new one