The process modeled by transition t 1 is activated only after the process modeled by transition t 2 is activated. (d) Modeling exclusion; transitions t 1 and t 2 model writing and, respectively, reading with n processes to a shared memory. At any given time only one process may write, but any subset of the n processes may read at the same time, provided that no process writes. • t 4 fires and then the read operation is completed and the next state is represented by marking M 0 . • t 4 fires again and the marking becomes M 1 = ( n − 2 , n − 2 , 0 , 2) (13) This process can be repeated k < n times leading to the markings M k - 1 = ( n − k, n − k, 0 , k ) (14) Several possible firing sequences: • ( t 2 t 3 ) k = ⇒ k consecutive write operations. • ( t 1 t 4 ) k = ⇒ k consecutive read operations. • (( t 2 t 3 ) , ( t 1 t 4 ) k ) = ⇒ a write operation followed by k consecutive read operations. • (( t 2 t 3 ) n , ( t 1 t 4 ) k , ( t 2 t 3 )) = ⇒ n consecutive write operation followed by k consecutive read operations, followed by a write operation. The postsets of the transitions are: 14
• t 1 = ⇒ ( P 4 ) • t 2 = ⇒ ( P 3 ) • t 3 = ⇒ ( P 1 , P 2 ) • t 4 = ⇒ ( P 1 , P 2 ) The presets of the places are: • p 1 = ⇒ ( t 3 , t 4 ) • p 2 = ⇒ ( t 3 , t 4 ) • p 3 = ⇒ ( t 2 ) • p 4 = ⇒ ( t 3 ) It is not possible to construct a state-machine model of the system because a state-machine does not support concurrency and we have to model the concurrent read operations. Problem 6.Explain brieﬂy how the publish-subscribe paradigm works and discuss its appli-cation to services such as bulletin boards, mailing lists, and so on. Outline the design of anevent service based on this paradigm, see Figure 3(b). Can you identify a cloud service thatemulates an event service? Two sets of entities are involved in the publish-subscribe system, those who provide theinformation and those who consume it. The two sets can be disjoint, as is the case with a newsservice, or they can consist of the same entities, as is the case of bulletin boards and mailinglists. The actual operation of different publish-subscribe systems can be very different. Forexample in case of Facebook, an individual with a Facebook page chooses a set of individualswho have access to this information, the so called “friends,” and publishes items of intereston his/her page and decides who has access to this information, everybody with a Facebookaccount or only the friends who are made aware whenever a new item is added to the page.In case of a mailing list individuals subscribe to the list, there is a moderator who decideswhat items can be distributed to all those who have subscribed to the list.In case of an event service, the designer of the system defines a number of classes of eventsand then chooses the events to be included in each class. Once the event ontology is madepublic individuals subscribe to the events they are interested in. There is a queue of eventsin each class and once a new event is added to the queue for the particular event, a messageis sent to notify all the subscribers for that type of event.The SNS (Simple Notification Service) of AWS is an example of anIaaScloud servicebased on the publish-subscribe paradigm.According tothesteps required for using SNS are:•Create a topic.A topic is an “access point” identifying a specific subject or event typefor publishing messages and allowing clients to subscribe for notifications.•Set policies for the topic.Once a topic is created, the topic owner can set policies for it,such as limiting who can publish messages or subscribe to notifications, or specifyingwhich notification protocols will be supported (i.e. SQS, HTTP/HTTPS, email, SMS).A single topic can support notification deliveries over multiple transport protocols.
- Summer '17
- ALBERT DOMINIC
- The Land, cloud service, Cloud delivery models, Cloud Computing delivery