Slides for Chapter 11: Coordination and Agreement From Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edition 3, © Addison-Wesley.

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1 Slides for Chapter 11: Coordination and Agreement From Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edition 3, © Addison-Wesley 2001

2 Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 3 © Addison-Wesley Publishers 2000 Figure 11.1 A network partition

3 Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 3 © Addison-Wesley Publishers 2000 Figure 11.2 Server managing a mutual exclusion token for a set of processes

4 Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 3 © Addison-Wesley Publishers 2000 Figure 11.3 A ring of processes transferring a mutual exclusion token

5 Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 3 © Addison-Wesley Publishers 2000 Algoritmo de Ricart e Agrawala zRicart e Agrawala [1981] zImplementa exclusão mútua distribuída entre N processos. zusa “multicast” e clocks lógicos zIdéia básica: Processos que requerem entrar em uma seção crítica “multicast” uma mensagem de “request”, e pode entrar somente quando todos os outros processos têm respondido a esta mensagem de “request”.

6 Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 3 © Addison-Wesley Publishers 2000 Algoritmo de Ricart e Agrawala zAs condições sob as quais um processo responde a um “request” são projetadas para garantir que as condições ME1(segurança), ME2(vivacidade)-ME3(ordenação) são satisfeitas. zOs processos p1, p2,..., pN arcam com identificadores numéricos distintos. zÉ assumido existirem canais de comunicação entre os processos e cada processo guarda um clock lógico de Lamport, atualizado de acordo com as regras LC1-LC4 (Cap10 – Tempo em SDs).

7 Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 3 © Addison-Wesley Publishers 2000 Algoritmo de Ricart e Agrawala zMensagens de “request” são da forma, onde T é o rótulo de tempo do processo enviando o “request” e pi é o identificador do processo que envia o “request”. zCada processo registra seu estado em uma variável state: - de estar fora da seção crítica (RELEASED), - esperando entrar na seção crítica (WANTED), - ou estando na seção crítica (HELD)

8 Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 3 © Addison-Wesley Publishers 2000 Algoritmo de Ricart e Agrawala zSe um processo requisita entrar e o estado de todos os processos é RELEASED, então todos os processo responderão imediatamente ao “request” e o processo requerente obterá a entrada na seção crítica. zSe algum processo está em HELD, então aquele processo não responderá a “requests” até que ele tenha terminado sua seção crítica.

9 Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 3 © Addison-Wesley Publishers 2000 Algoritmo de Ricart e Agrawala zE assim, o processo requerente não pode ganhar a entrada enquanto isso. zSe dois ou mais processos requerem entrar ao mesmo tempo em suas seções críticas, então qualquer que seja o processo requerente, aquele que suporta o menor rótulo de tempo será o primeiro a coletar N-1 respostas (replies), concedendo a ele próxima entrada na seção crítica.

10 Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 3 © Addison-Wesley Publishers 2000 Algoritmo de Ricart e Agrawala zSe os “requests” têm rótulos de tempo iguais, os “requests” são ordenados pelos identificadores correspondendo aos processos. zNote que, quando um processo “requests” entrada, ele adia o processamento de “requests” de outros processos até que seu próprio “request” tenha sido enviado e ele tenha registrado o rótulo de tempo T do “request”. zÉ assim que processos tomam decisões consistentes quando processando “requests”.

11 Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 3 © Addison-Wesley Publishers 2000 Algoritmo de Ricart e Agrawala zO algoritmo alcança a propriedade ME1: zSe fosse possível para dois processos pi e pj (i diferente de j) entrarem nas suas seções críticas ao mesmo tempo, então ambos os processos teriam que ter respondido ao outro. zMas, visto que, os pares e são totalmente ordenados, isto se torna impossível. zO algoritmo também satisfaz a ME2 e ME3.

12 Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 3 © Addison-Wesley Publishers 2000 Figure 11.4 Ricart and Agrawala’s algorithm On initialization state := RELEASED; To enter the section state := WANTED; Multicast request to all processes;request processing deferred here T := request’s timestamp; Wait until (number of replies received = (N – 1)); state := HELD; On receipt of a request at p j (i ≠ j) if (state = HELD or (state = WANTED and (T, p j ) < (T i, p i ))) then queue request from p i without replying; else reply immediately to p i ; end if To exit the critical section state := RELEASED; reply to any queued requests;

13 Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 3 © Addison-Wesley Publishers 2000 Algoritmo de Ricart e Agrawala zPara ilustar o algoritmo, considere a Figura 11.5, seguinte.

14 Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 3 © Addison-Wesley Publishers 2000 Figure 11.5 Multicast synchronization p 3 34 Reply 34 41 34 p 1 p 2 Reply

15 Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 3 © Addison-Wesley Publishers 2000 Algoritmo de Ricart e Agrawala zAssuma que p3 não está interessado em entrar na sua seção crítica. zP1 e p2 “requests” entrar concorrentemente. zO rótulo de tempo de p1, T1, é 41. zO rótulo de tempo de p2, T2, é 34.

16 Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 3 © Addison-Wesley Publishers 2000 Algoritmo de Ricart e Agrawala zQuando p3 recebe seus “requests”, ele responde imediatamente, a p1 e a p2. zQuando p2 recebe o “request” de p1, ele descobre que seu próprio “request” tem o rótilo de tempo menor e assim, não “reply”, retendo p1 a esperar. zContudo, p1 descobre que o “request” de p2 ^tem o rótulo de tempo menor do que o seu próprio “request” e assim responde imediatamente.

17 Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 3 © Addison-Wesley Publishers 2000 Algoritmo de Ricart e Agrawala zNo recebimento do segundo “reply”, p2 pode então entar na sua seção crítica. zQuando p2 sai da sua seção crítica, ele responderá ao “request” de p1 e assim, concede a ele a entrada. zPara obter a entrada, o algoritmo proporciona 2(N-1) mensagens, seguidas por (N-1) respostas. zA vantagem do algoritmo é que seu atraso de sincronização é “round-trip syncronization”, ou seja, somente sobre o tempo de ida-e-volta para transmissão das mensagens “requests” e “replies”.

18 Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 3 © Addison-Wesley Publishers 2000 Figure 11.6 Maekawa’s algorithm – part 1 On initialization state := RELEASED; voted := FALSE; For p i to enter the critical section state := WANTED; Multicast request to all processes in V i – {p i }; Wait until (number of replies received = (K – 1)); state := HELD; On receipt of a request from p i at p j (i ≠ j) if (state = HELD or voted = TRUE) then queue request from p i without replying; else send reply to p i ; voted := TRUE; end if Continues on next slide

19 Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 3 © Addison-Wesley Publishers 2000 Figure 11.6 Maekawa’s algorithm – part 2 For p i to exit the critical section state := RELEASED; Multicast release to all processes in V i – {p i }; On receipt of a release from p i at p j (i ≠ j) if (queue of requests is non-empty) then remove head of queue – from p k, say; send reply to p k ; voted := TRUE; else voted := FALSE; end if

20 Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 3 © Addison-Wesley Publishers 2000 Figure 11.7 A ring-based election in progress Note: The election was started by process 17. The highest process identifier encountered so far is 24. Participant processes are shown darkened

21 Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 3 © Addison-Wesley Publishers 2000 Figure 11.8 The bully algorithm The election of coordinator p 2, after the failure of p 4 and then p 3

22 Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 3 © Addison-Wesley Publishers 2000 Figure 11.9 Open and closed groups

23 Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 3 © Addison-Wesley Publishers 2000 Figure 11.10 Reliable multicast algorithm

24 Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 3 © Addison-Wesley Publishers 2000 Figure 11.11 The hold-back queue for arriving multicast messages

25 Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 3 © Addison-Wesley Publishers 2000 Figure 11.12 Total, FIFO and causal ordering of multicast messages Notice the consistent ordering of totally ordered messages T 1 and T 2, the FIFO-related messages F 1 and F 2 and the causally related messages C 1 and C 3 – and the otherwise arbitrary delivery ordering of messages.

26 Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 3 © Addison-Wesley Publishers 2000 Figure 11.13 Display from bulletin board program Bulletin board: os.interesting Item FromSubject 23A.HanlonMach 24G.JosephMicrokernels 25A.HanlonRe: Microkernels 26T.L’HeureuxRPC performance 27M.WalkerRe: Mach end

27 Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 3 © Addison-Wesley Publishers 2000 Figure 11.14 Total ordering using a sequencer

28 Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 3 © Addison-Wesley Publishers 2000 Figure 11.15 The ISIS algorithm for total ordering 2 1 1 2 2 1 Message 2 Proposed Seq P 2 P 3 P 1 P 4 3 Agreed Seq 3 3

29 Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 3 © Addison-Wesley Publishers 2000 Figure 11.16 Causal ordering using vector timestamps

30 Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 3 © Addison-Wesley Publishers 2000 Figure 11.17 Consensus for three processes

31 Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 3 © Addison-Wesley Publishers 2000 Figure 11.18 Consensus in a synchronous system

32 Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 3 © Addison-Wesley Publishers 2000 Figure 11.19 Three byzantine generals p 1 (Commander) p 2 p 3 1:v 2:1:v 3:1:u p 1 (Commander) p 2 p 3 1:x1:w 2:1:w 3:1:x Faulty processes are shown shaded

33 Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 3 © Addison-Wesley Publishers 2000 Figure 11.20 Four byzantine generals p 1 (Commander) p 2 p 3 1:v 2:1:v 3:1:u Faulty processes are shown shaded p 4 1:v 4:1:v 2:1:v3:1:w 4:1:v p 1 (Commander) p 2 p 3 1:w1:u 2:1:u 3:1:w p 4 1:v 4:1:v 2:1:u3:1:w 4:1:v