Lamport’s Algorithm for Mutual Exclusion in Distributed System

Prerequisite: Mutual exclusion in distributed systems

Lamport’s Distributed Mutual Exclusion Algorithm is a permission based algorithm proposed by Lamport as an illustration of his synchronization scheme for distributed systems. In permission based timestamp is used to order critical section requests and to resolve any conflict between requests. In Lamport’s Algorithm critical section requests are executed in the increasing order of timestamps i.e a request with smaller timestamp will be given permission to execute critical section first than a request with larger timestamp. In this algorithm:

• Three type of messages ( REQUEST, REPLY and RELEASE) are used and communication channels are assumed to follow FIFO order.
• A site send a REQUEST message to all other site to get their permission to enter critical section.
• A site send a REPLY message to requesting site to give its permission to enter the critical section.
• A site send a RELEASE message to all other site upon exiting the critical section.
• Every site S_i, keeps a queue to store critical section requests ordered by their timestamps. request_queue_i denotes the queue of site S_i
• A timestamp is given to each critical section request using Lamport’s logical clock.
• Timestamp is used to determine priority of critical section requests. Smaller timestamp gets high priority over larger timestamp. The execution of critical section request is always in the order of their timestamp.

Algorithm:

• To enter Critical section:
  • When a site S_i wants to enter the critical section, it sends a request message Request(ts_i, i) to all other sites and places the request on request_queue_i. Here, Ts_i denotes the timestamp of Site S_i
  • When a site S_j receives the request message REQUEST(ts_i, i) from site S_i, it returns a timestamped REPLY message to site S_i and places the request of site S_i on request_queue_j
• To execute the critical section:
  • A site S_i can enter the critical section if it has received the message with timestamp larger than (ts_i, i) from all other sites and its own request is at the top of request_queue_i
• To release the critical section:
  • When a site S_i exits the critical section, it removes its own request from the top of its request queue and sends a timestamped RELEASE message to all other sites
  • When a site S_j receives the timestamped RELEASE message from site S_i, it removes the request of S_i from its request queue

Message Complexity: Lamport’s Algorithm requires invocation of 3(N – 1) messages per critical section execution. These 3(N – 1) messages involves

• (N – 1) request messages
• (N – 1) reply messages
• (N – 1) release messages

Drawbacks of Lamport’s Algorithm:

• Unreliable approach: failure of any one of the processes will halt the progress of entire system.
• High message complexity: Algorithm requires 3(N-1) messages per critical section invocation.

Performance:

• Synchronization delay is equal to maximum message transmission time
• It requires 3(N – 1) messages per CS execution.
• Algorithm can be optimized to 2(N – 1) messages by omitting the REPLY message in some situations.

Advantages of Lamport’s Algorithm for Mutual Exclusion in Distributed System:

1. Simplicity: Lamport’s algorithm is relatively easy to understand and implement compared to other algorithms for mutual exclusion in distributed systems.
2. Fairness: The algorithm guarantees fairness by providing a total order of events that is used to determine the next process that can enter the critical section.
3. Scalability: Lamport’s algorithm is scalable because it only requires each process to communicate with its neighbors, rather than all other processes in the system.
4. Compatibility: The algorithm is compatible with a wide range of distributed systems and can be adapted to different network topologies and communication protocols. 

Disadvantages of Lamport’s Algorithm for Mutual Exclusion in Distributed System:

1. Message Overhead: The algorithm requires a lot of message passing between processes to maintain the total order of events, which can lead to increased network traffic and latency.
2. Delayed Execution: The algorithm can lead to delays in the execution of critical sections because a process may have to wait for messages to arrive from other processes before entering the critical section.
3. Limited Performance: The algorithm may not perform well in systems with a high number of processes because of the increased message overhead and delayed execution.
4. No Fault Tolerance: The algorithm does not provide any fault tolerance mechanism, which means that if a process fails or crashes, it may cause the entire system to fail or become unstable.

FAQs:

How does Lamport’s algorithm ensure mutual exclusion in a distributed system?
Lamport’s algorithm uses a permission-based approach and timestamps to order critical section requests and resolve any conflicts between them. Each process sends a request message to all other processes to enter the critical section, and the request is placed in the request queue of each process in the order of its timestamp. A process can enter the critical section if it has received a reply message from all other processes with a timestamp greater than its own, and its own request is at the top of its request queue.

What is the message complexity of Lamport’s algorithm?
Lamport’s algorithm requires 3(N-1) messages per critical section execution, where N is the number of processes in the system. These messages involve (N-1) request messages, (N-1) reply messages, and (N-1) release messages.

Is Lamport’s algorithm fault-tolerant?
No, Lamport’s algorithm does not provide any fault tolerance mechanism. If a process fails or crashes, it may cause the entire system to fail or become unstable.