 Hello everyone. Myself Mahesh Mohanth, I am working in the CSC department as an assistant professor in the Valchin Institute of Technology, in Sulapur. Today we are going to discuss the topic models of distributed systems. Learning outcomes are at the end of the session student will be able to explain different models of distributed systems. Also compare between network and distributed operating systems. There are five models of distributed computing systems. The first one is mini computer model. The second model is workstation model. Third model is workstation server model. Fourth model is processor pool model. And fifth model is hybrid model. First we are going to discuss what is the meaning of distributed system. Distributed system is nothing but the interconnection of different computers or systems by using communication network for resource sharing. The resources are maybe hardware resources or software resources. Now we are going to discuss each and every model one by one. The first model is mini computer model. In this model mini computers, first mini computer, second mini computer and third mini computer. And these three mini computers are connected by using the ARPANET so that they can share the information among them. These are the different diskless workstations are interconnected to each mini computer. These total nine diskless workstations are connected to each and every mini computers. So every mini computer having three diskless workstations. So there is interconnection between diskless workstations and mini computer by using this communication network ARPANET. Now if any diskless workstations want to log into this communication network, first it will be logged into this mini computer. Then it will become a remote machine and share the resources in this network. So this is the extension of centralized time sharing system. First user must log on his or her mini computer. Thereafter he or she can log on a remote machine by Telnet. The main purpose of this model is resource sharing. Just I told you that sharing of the hardware resources or software resources, just an information databases or high-performance devices. The example of this kind of model is ARPANET. The second model is workstation model. In this model different workstations are interconnected with each other by using communication network so that each workstation can share the hardware and software resources among this network. Now each workstation is a single user PC. Here we can migrate any process. Means what? Process migrations can be take place. If anyone node is overloaded or it will not be having sufficient processing power. At that time that work should be migrated or that execution should be migrated to another workstation which is having the higher processing capability and it will be in idle state. So here we can process migration is the main advantage of this workstation model. So in process migration user first log on his or her personal workstation. If there is an idle remote workstation a heavy job may migrate to one of them. So actually there are major problems in the workstation model. First it is very difficult to find the idle workstation. Then second one is it is very difficult to migrate a job on to the other machine. And because on the other machine the remote machine that was busy executing some other process in another workstation. So there are three approaches they are considered. Suppose if the another machine is executing some process. Sharing of resources between the remote process and local process. Either this machine will be share some resources in the network system or kill the remote process. Just third one is migrate the remote process back to its workstations. This is called a primitive process migration. The best example of this model are the Sprite system, X-Punt system developed at Xerox Spark. These are the some distributed system examples of this kind of model. The third model is workstation server model. In the workstation server model different workstations and some servers are interconnected by using some communication network. As shown in the figure there are 100 gbps LAN should be connected to these workstations and the servers. So the workstations are executing the some workstations are this full workstation some workstations are diskless workstation. This full workstation means what they'll be having some hard disk and diskless workstation means what they will not contains any hard disk. They are executing their procedure by sending some parameters to the server and at the server side the process will be executing and the result will be redirected to the workstation. So in here three workstations are shown in the figure and four servers are kept. So the first server is file server, second server is HTTP server, third server is cycle server and fourth database server. So these servers provide different kind of services to the different workstation as per their requirement. So we can increase any number of servers here in the server side and we can add any number of workstations in the workstation side means what we can add number of workstations. So here more scalability in workstation server model. The client workstation most are diskless and few are diskful. Graphics, interactive application processed in local these are the all file print, HTTP and event cycle competition request are sent to the server. So desired operations can be performed at the server side and the response is sent to the desired client. So server mini-computer so each mini-computer dedicated to one or more different types of services. So every server provide different different kind of services. For example data DBMS server provide all the database related services, file server provide all the file related services. So this kind of HTTP server provides all the web services. So these kind of servers are kept in the server side for providing services to the client or workstations. Actually there is a communication between the client and the server can be takes place in the following by using following methods. So RPC remote procedure call here the one procedure from the one workstation is called from the another machine. So the process will be written at one location and that will be executed on the another location so by calling its name. So this is called RPC. Second method remote method invocation. Here we can change the runtime parameters. So RMI is the most important method of this client server communication model. A client process calls the server process function. So client process always called the servers process for the execution. No process migration is invoked. Here there will not be any process migration mechanism. For example network file system is the best example of this kind of model. Now just I'm asking one question to all the students and you have to pause your video and write down the answer of this question. What are the advantages of workstation server model advantages of workstation server model? This is the cheaper model. This this diskless workstation are more preferred than diskful workstations in the maintenance point of view. Installation of new software by using the file server flexibility to log into the any workstation and request the services request response protocol is used to access the services. It does not need process migration facility. A user has guaranteed guaranteed response and workstation are not used for executing the remote processes. The V system is the best example of this model. The next model is processor pool model. Here the diskless workstations are connected with the number of processors by using some communication network. So client they log in any one of the terminal. All services are dispatched to the server servers necessary number of processors are allocated to each user from the pool, not suitable for a high interact application run server allocate desired servers to the client. So here the run server would decided which processor will be allocated to the which workstation or which client. So in processor pool model advantage utilization of more processing power greater flexibility that explained without need to install anyone computer disadvantage it unable to process high speed interactive applications. Amoeba and and came Amoeba and Cambridge are the example of this kind of network. Last model is hybrid model. The workstation server model hybrid is most widely used for building distributed application. It performs simple interactive tasks such as editing, job sending, males and executing small program. Processor pool model is suitable for jobs in group of user who need massive computations. Here hybrid model combination of workstations server model and processor pool model. It is based on workstation server model but the addition of processor pool of processors. The processor are in the pool are kept dynamically for large computations. This also give the guaranteed response to interactive job by processing them on local workstations. Now there is difference between network and distributor operating system. Three features are considered for this different system image and autonomy and fault tolerance. In network operating systems, the multiple system image will be there means what in different operating systems are kept on different workstation in distributed system only single system image will be existing. Autonomy is high in the network operating system. So local operating system at each computer no job global job coordination that will be autonomy is low in distributed operating system. That was single system wide operating system global job coordination. Fault tolerance is very little in the network operating system. That's why unavailability grows as the faulty machine increases. But unavailability remains little in the faulty machine increases in distributed operating system. So references. Thank you.