Mutual Exclusion - DMJCCLT - dmj.one

Mutual Exclusion in Distributed Systems

1. Introduction and Basics

Mutual exclusion is a fundamental concept in distributed systems, ensuring that multiple processes do not simultaneously access shared resources or critical sections (CS). This guarantees data consistency and correctness by avoiding race conditions.

1.1 Why Mutual Exclusion?

1.2 Requirements

2. Distributed Mutual Exclusion

Distributed systems, unlike single operating systems, cannot rely on shared memory or global variables. They implement mutual exclusion using message passing across interconnected processes.

2.1 Centralized Algorithm

2.2 Ring-Based Algorithm

3. Ricart-Agrawala's Algorithm

The Ricart-Agrawala algorithm is a token-less approach to mutual exclusion in distributed systems. It leverages the concepts of causality and Lamport timestamps to ensure correctness, safety, and fairness without relying on a centralized master or token passing.

3.1 Steps

3.2 Safety Proof

What: The algorithm ensures that at most one process is in the CS at a time.

Why: It avoids conflicting access to shared resources by leveraging Lamport timestamps and reply acknowledgments.

How:

3.3 Performance

4. Maekawa's Algorithm

Maekawa's algorithm refines Ricart-Agrawala by reducing the number of processes involved in granting access to the critical section (CS). Instead of requiring responses from all processes, it uses subsets called voting sets to optimize bandwidth and delay.

4.1 Key Concepts

4.2 Steps

4.3 Analysis

5. Wrap-Up

Mutual exclusion is a cornerstone of distributed system design, ensuring consistency and correctness in accessing shared resources. The choice of algorithm depends on the system’s requirements for safety, liveness, bandwidth, and delay.

In real-world distributed systems, more advanced protocols are used to handle failures and ensure robust mutual exclusion:

These systems demonstrate how consensus protocols like Paxos ensure safety and eventual liveness in complex distributed environments.