Synchronization CSCI 4780/6780

Mutual Exclusion Concurrency and collaboration are fundamental to distributed systems Simultaneous access to resources Concurrency control prevents resource corruption due to simultaneous access Correctness –Mutual exclusion –No deadlocks –No starvation

Mutual Exclusion: A Centralized Algorithm a)Process 1 asks the coordinator for permission to enter a critical region. Permission is granted b)Process 2 then asks permission to enter the same critical region. The coordinator does not reply. c)When process 1 exits the critical region, it tells the coordinator, when then replies to 2

A Distributed Algorithm a)Two processes want to enter the same critical region at the same moment. b)Process 0 has the lowest timestamp, so it wins. c)When process 0 is done, it sends an OK also, so 2 can now enter the critical region.

A Token Ring Algorithm a)An unordered group of processes on a network. b)A logical ring constructed in software.

Comparison A comparison of three mutual exclusion algorithms. Algorithm Messages per entry/exit Delay before entry (in message times) Problems Centralized32Coordinator crash Distributed2 ( n – 1 ) Crash of any process Token ring 1 to  0 to n – 1 Lost token, process crash

Leader Election in Distributed Systems Many distributed systems need one process to act as a coordinator/initiator –“First among equals” not “one above the rest” Does not matter which process takes up responsibility –But all process should agree about who would be the leader Assumption: Each process has a unique identifier Bully Algorithm Ring Algorithm

Bully Algorithm When a process P notices that current coordinator has failed, it sends an ELECTION message to all processes with higher IDs If no one responds, P becomes the leader If a higher-up receives P’s message, it will send an OK message to P and execute the algorithm Process with highest ID takes over as coordinator by sending COORDINATOR message If a process with higher ID comes back, it takes over leadership by sending COORDINATOR message

Bully Algorithm - Example The bully election algorithm Process 4 holds an election Process 5 and 6 respond, telling 4 to stop Now 5 and 6 each hold an election

Bully Algorithm - Example (2) d)Process 6 tells 5 to stop e)Process 6 wins and tells everyone

Ring Algorithm When a process P notices current coordinator’s failure, it builds an ELECTION message –Message contains the P’s ID P sends message to it successor –Skip if the successor is down Each process adds its ID to the ELECTION message When election message reaches the initiator, it converts message to COORDINATOR –The process ID with highest ID is declared as new coordinator

Ring Algorithm - Example

Elections in Wireless Environments