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Increasing Availability under Mutual Exclusion Constraints with Dynamic Vote Reassignment

Report ID:
October 1986
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Voting is used commonly to enforce mutual exclusion in distributed systems. Each node is assigned a number of votes and only the group with a majority of votes is allowed to perform a restricted operation. This paper describes techniques for dynamically reassigning votes upon node or link failure, in an attempt to make the system more resilient to future failures. We focus on autonomous methods for achieving this, i.e., methods that allow the nodes to make independent choices about changing their votes and picking new vote values, rather than group consensus techniques that require tight coordination among the remaining nodes. Protocols are given which allow nodes to install new vote values while still maintaining mutual exclusion requirements. The lemmas and theorems to validate the protocols are presented. A simple example shows how to apply the method to a database object-locking scheme; the protocols, however, are versatile and can be used for any application requiring mutual
exclusion. In addition, policies are presented that allow nodes to autonomously select their new vote values. Simulation results are presented comparing the autonomous methods to static vote assignments and to group consensus strategies. These results demonstrate that under high failure rates, dynamic
vote reassignment shows great improvement over a static assignment of votes in terms of availability. In addition, many autonomous methods for determining a new vote assignment yield almost as much availability as a group consensus method and at the same time are faster and more flexible.

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