Mutual exclusion in distributed system

Mutual exclusion is a concurrency control property which is introduced to prevent race conditions. It is the requirement that a process can not enter its critical section while another concurrent process is currently present or executing in its critical section i.e only one process is allowed to execute the critical section at any given instance of time. 

Mutual exclusion in single computer system Vs. distributed system: In single computer system, memory and other resources are shared between different processes. The status of shared resources and the status of users is easily available in the shared memory so with the help of shared variable (For example: Semaphores) mutual exclusion problem can be easily solved. In Distributed systems, we neither have shared memory nor a common physical clock and therefore we can not solve mutual exclusion problem using shared variables. To eliminate the mutual exclusion problem in distributed system approach based on message passing is used. A site in distributed system do not have complete information of state of the system due to lack of shared memory and a common physical clock. 

Requirements of Mutual exclusion Algorithm:

• No Deadlock: Two or more site should not endlessly wait for any message that will never arrive.
• No Starvation: Every site who wants to execute critical section should get an opportunity to execute it in finite time. Any site should not wait indefinitely to execute critical section while other site are repeatedly executing critical section
• Fairness: Each site should get a fair chance to execute critical section. Any request to execute critical section must be executed in the order they are made i.e Critical section execution requests should be executed in the order of their arrival in the system.
• Fault Tolerance: In case of failure, it should be able to recognize it by itself in order to continue functioning without any disruption.

Some points are need to be taken in consideration to understand mutual exclusion fully :

1) It is an issue/problem which frequently arises when concurrent access to shared resources by several sites is involved. For example, directory management where updates and reads to a directory must be done atomically to ensure correctness.
2) It is a fundamental issue in the design of distributed systems.
3) Mutual exclusion for a single computer is not applicable for the shared resources since it involves resource distribution, transmission delays, and lack of global information.
 

Solution to distributed mutual exclusion: As we know shared variables or a local kernel can not be used to implement mutual exclusion in distributed systems. Message passing is a way to implement mutual exclusion. Below are the three approaches based on message passing to implement mutual exclusion in distributed systems:

1. Token Based Algorithm:

• A unique token is shared among all the sites.
• If a site possesses the unique token, it is allowed to enter its critical section
• This approach uses sequence number to order requests for the critical section.
• Each requests for critical section contains a sequence number. This sequence number is used to distinguish old and current requests.
• This approach insures Mutual exclusion as the token is unique

Example : Suzuki–Kasami Algorithm

2. Non-token based approach:

• A site communicates with other sites in order to determine which sites should execute critical section next. This requires exchange of two or more successive round of messages among sites.
• This approach use timestamps instead of sequence number to order requests for the critical section.
• When ever a site make request for critical section, it gets a timestamp. Timestamp is also used to resolve any conflict between critical section requests.
• All algorithm which follows non-token based approach maintains a logical clock. Logical clocks get updated according to Lamport's scheme

Example : Ricart–Agrawala Algorithm

3. Quorum based approach:

• Instead of requesting permission to execute the critical section from all other sites, Each site requests only a subset of sites which is called a quorum.
• Any two subsets of sites or Quorum contains a common site.
• This common site is responsible to ensure mutual exclusion

Example : Maekawa’s Algorithm

FAQs:

How does the token-based algorithm handle the failure of a site that possesses the token?
If a site that possesses the token fails, then the token is lost until the site recovers or another site generates a new token. In the meantime, no site can enter the critical section.

What is a quorum in the quorum-based approach, and how is it determined?
A quorum is a subset of sites that a site requests permission from to enter the critical section. The quorum is determined based on the size and number of overlapping subsets among the sites.

How does the non-token-based approach ensure fairness among the sites?
The non-token-based approach uses a logical clock to order requests for the critical section. Each site maintains its own logical clock, which gets updated with each message it sends or receives. This ensures that requests are executed in the order they arrive in the system, and that no site is unfairly prioritized.