CSC 4320/6320 Operating Systems Lecture 13 Distributed CoordinationChapter 18 Distributed CoordinationChapter ObjectivesEvent OrderingRelative Time for Three Concurrent ProcessesImplementation of Distributed Mutual Exclusion (DME)DME: Centralized ApproachDME: Fully Distributed ApproachDME: Fully Distributed Approach (Cont)Desirable Behavior of Fully Distributed ApproachThree Undesirable ConsequencesToken-Passing ApproachTimestampingGeneration of Unique TimestampsDeadlock PreventionTimestamped Deadlock-Prevention SchemeWait-Die SchemeWound-Wait SchemeDeadlock DetectionTwo Local Wait-For GraphsGlobal Wait-For GraphElection AlgorithmsBully AlgorithmBully Algorithm (Cont)Slide 26Ring AlgorithmRing Algorithm (Cont)General’s ParadoxTwo-Phase CommitTwo phase commit exampleDistributed Decision Making DiscussionByzantine General’s ProblemByzantine General’s Problem (con’t)End of Lecture 13CSC 4320/6320Operating SystemsLecture 13Distributed CoordinationSaurav KarmakarChapter 18 Distributed Coordination•Event Ordering•Mutual Exclusion •Atomicity•Concurrency Control•Deadlock Handling•Election Algorithms•Reaching AgreementChapter Objectives•To describe various methods for achieving mutual exclusion in a distributed system•To explain how atomic transactions can be implemented in a distributed system•To show how some of the concurrency-control schemes discussed in Chapter 6 can be modified for use in a distributed environment•To present schemes for handling deadlock prevention, deadlock avoidance, and deadlock detection in a distributed systemEvent Ordering•Happened-before relation (denoted by )–If A and B are events in the same process, and A was executed before B, then A  B–If A is the event of sending a message by one process and B is the event of receiving that message by another process, then A  B–If A  B and B  C then A  CRelative Time for Three Concurrent ProcessesImplementation of  •Associate a timestamp with each system event–Require that for every pair of events A and B, if A  B, then the timestamp of A is less than the timestamp of B•Within each process Pi a logical clock, LCi is associated–The logical clock can be implemented as a simple counter that is incremented between any two successive events executed within a process »Logical clock is monotonically increasing•A process advances its logical clock when it receives a message whose timestamp is greater than the current value of its logical clock•If the timestamps of two events A and B are the same, then the events are concurrent–We may use the process identity numbers to break ties and to create a total orderingDistributed Mutual Exclusion (DME) •Assumptions–The system consists of n processes; each process Pi resides at a different processor–Each process has a critical section that requires mutual exclusion•Requirement–If Pi is executing in its critical section, then no other process Pj is executing in its critical section•We present two algorithms to ensure the mutual exclusion execution of processes in their critical sectionsDME: Centralized Approach•One of the processes in the system is chosen to coordinate the entry to the critical section•A process that wants to enter its critical section sends a request message to the coordinator•The coordinator decides which process can enter the critical section next, and its sends that process a reply message•When the process receives a reply message from the coordinator, it enters its critical section•After exiting its critical section, the process sends a release message to the coordinator and proceeds with its execution •This scheme requires three messages per critical-section entry:–request –reply–releaseDME: Fully Distributed Approach•When process Pi wants to enter its critical section, it generates a new timestamp, TS, and sends the message request (Pi, TS) to all other processes in the system•When process Pj receives a request message, it may reply immediately or it may defer sending a reply back•When process Pi receives a reply message from all other processes in the system, it can enter its critical section•After exiting its critical section, the process sends reply messages to all its deferred requestsDME: Fully Distributed Approach (Cont)•The decision whether process Pj replies immediately to a request(Pi, TS) message or defers its reply is based on three factors:–If Pj is in its critical section, then it defers its reply to Pi–If Pj does not want to enter its critical section, then it sends a reply immediately to Pi–If Pj wants to enter its critical section but has not yet entered it, then it compares its own request timestamp with the timestamp TS»If its own request timestamp is greater than TS, then it sends a reply immediately to Pi (Pi asked first)»Otherwise, the reply is deferredDesirable Behavior of Fully Distributed Approach•Freedom from Deadlock is ensured•Freedom from starvation is ensured, since entry to the critical section is scheduled according to the timestamp ordering–The timestamp ordering ensures that processes are served in a first-come, first served order •The number of messages per critical-section entry is 2 x (n – 1)This is the minimum number of required messages per critical-section entry when processes act independently and concurrentlyThree Undesirable Consequences•The processes need to know the identity of all other processes in the system, which makes the dynamic addition and removal of processes more complex•If one of the processes fails, then the entire scheme collapses–This can be dealt with by continuously monitoring the state of all the processes in the system•Processes that have not entered their critical section must pause frequently to assure other processes that they intend to enter the critical section•This protocol is therefore suited for small, stable sets of cooperating processesToken-Passing Approach•Circulate a token among processes in system–Token is special type of message–Possession of token entitles holder to enter critical section•Processes logically organized in a ring structure•Unidirectional ring guarantees freedom from starvation•Two types of failures–Lost token – election must be called–Failed processes – new logical ring establishedTimestamping•Generate unique timestamps in distributed scheme:–Each site generates a unique local timestamp–The