6/13/2015B.Ramamurthy1 System Models Bina Ramamurthy (Based on Slides from CDK text)

6/13/2015B.Ramamurthy2 Fundamental Issues There is no global time. All communications are by means of messages. Message communication may be affected by network delays and can suffer from a variety of failures and security attacks. How does one express a solution/process for handling an issue? One of the ways is to establish a model.

6/13/2015B.Ramamurthy3 System Models Interaction model deals with performance and setting time limits in a distributed system, say, for message delivery. Failure model gives specification of faults and defines reliable communication and correct processes. Security model specifies possible threats and defines the concept of secure channels. Architectural model defines the way in which the components of the system interact with one another and the way in which they are mapped onto the underlying network of computers.

6/13/2015B.Ramamurthy4 Architectural Model Abstracts the functions of the individual components. Defines patterns for distribution of data and workload. Defines patterns of communication among the components. Example: Definition of server process, client process and peer process and protocols for communication among processes; definition client/server model and its variations.

6/13/2015B.Ramamurthy5 Software and hardware service layers in distributed systems

6/13/2015B.Ramamurthy6 Middleware Layer of software whose purpose is to mask the heterogeneity and to provide a convenient programming model for application programmers. Middleware supports such abstractions as remote method invocation, group communications, event notification, replication of shared data, real-time data streaming. Examples: CORBA spec by OMG, Java RMI, MS’s DCOM. We will look at RMI in this discussion.

6/13/2015B.Ramamurthy7 Clients invoke individual servers EX: browser, web client EX: Web server EX: 1. File server, 2. Web crawler

6/13/2015B.Ramamurthy8 A service provided by multiple servers EX: akamai, altavista, Sun’s NIS (data replication)

6/13/2015B.Ramamurthy9 Web proxy server and caches Proxy servers + cache are used to provide increased Availability and performance. They also play a major role Firewall based security.

6/13/2015B.Ramamurthy10 A distributed application based on peer processes Ex: distributed Whiteboard Application; EJB-based?

6/13/2015B.Ramamurthy11 Web applets EX: Look at Object by value in CORBA

6/13/2015B.Ramamurthy12 Spontaneous networking in a hotel Internet gateway PDA service Music service Discovery Alarm Camera Guests devices Laptop TV/PC Hotel wireless network EX: Jini, blue tooth protocol, UDDI

6/13/2015B.Ramamurthy13 Remote Method Invocation Remote Method Invocation (RMI) is Java’s implementation of object-to-object communication among Java objects to realize a distributed computing model. RMI allows us to distribute our objects on various machines, and invoke methods on the objects located on remote sites. Source code for the demo is a modified version of code in Chapter 20 of Deitel & Deitel’s text Java : How to Program.

6/13/2015B.Ramamurthy14 RMI-based Distributed System XYZ Implementation 4. Client Host Server Host XYZ Client Stub Skeleton XYZ interface uses implements

6/13/2015B.Ramamurthy15 Steps in RMI-based Application 1. Design the interface for the service. 2. Implement the methods specified in the interface. 3. Generate the stub and the skeleton. 4. Register the service by name and location. 5. Use the service in an application.

6/13/2015B.Ramamurthy16 Compile and Register Commands Client Host Server Host XYZ Client Stub Skeleton XYZ interface uses implements XYZ Implementation rmic rmiregistry Stores object by nameFinds object by name

6/13/2015B.Ramamurthy17 More Details Once the object (or service) is registered, a client can look up that service. A client (application) receives a reference that allows the client to use the service (call the method). Syntax of calling is identical to a call to a method of another object in the same program.

6/13/2015B.Ramamurthy18 Parameter Marshalling Transfer of parameters (or marshalling) is done by the RMI. Complex objects are streamed using Serialization. RMI model of networking for distributed system involves only Java. No need to learn IDL or any other language.

6/13/2015B.Ramamurthy19 Case Study : Temperature Service Lets create a distributed system using RMI model for networking (remote access). Basically this program will download the weather (temperature) information from the site:

6/13/2015B.Ramamurthy20 Defining Remote Interface import java.rmi.*; // the interface extends Remote interface // any class implementing Remote can be accessed remotely security permitting public interface TemperatureServer extends Remote { // specify methods that can be called remotely // each method “throws RemoteException” }

6/13/2015B.Ramamurthy21 RemoteException Any time you depend on outside entities there is a potential for problems in communication, networking, server crash etc. Any exception due to these should be handled by the services. This feature imparts robustness to the application. Java mandates this feature for any RMI service.

6/13/2015B.Ramamurthy22 Implementing the Remote Interface import java.rmi.*; import java.rmi.server.*; import java.net.*; // others as needed TemperatureServerImpl extends UnicastRemoteObject implements TemperatureServer {

6/13/2015B.Ramamurthy23 TemperatureServerImpl This class’s constructor calls a private method which in turn: 1. Connects to the url specified 2. Streams into a buffer the page referenced. 3. Parses the buffer to get the required data. 4. Creates an array of weather information.

6/13/2015B.Ramamurthy24 TemperatureServerImpl (contd.) It implements the service method getWeatherInfo which simply returns the weather data gathered. The main method instantiates an object for the service, and registers it with rmiregistry.

6/13/2015B.Ramamurthy25 Streaming URLs Using the openStream of java.net.URL class you can stream in the file spefied by an universal resource locator(url). It can be streamed into a buffer where it can be analyzed for information. Any number of urls can be streamed in. Unicast Communication : When you are interested in a particular remote site you will direct your net connection to that particular site using unicast.

6/13/2015B.Ramamurthy26 Server Object Name Syntax for the server object name is: //host:port/remoteObjectName Default port number for rmiregistry is 1099 For local host the object name: //localhost/TempServer For a remote host // /TempServer

6/13/2015B.Ramamurthy27 Name Binding rebind method binds a server’s object name to the object’s name as it is in the registry. Clients use the name in the registry. There is also a bind() method. But rebind is better since it binds the most recently registered object.

6/13/2015B.Ramamurthy28 WeatherInfo class It is very traditional class for keeping the information about the temperature at a single location. It has data fields : cityName, temperature, and description and get methods for these. An array of objects of this class is used in the server implementation.

6/13/2015B.Ramamurthy29 Temperature Client import java.rmi.*; // import other packages constructor calls a private method getRemoteTemp which takes care of lookup of remote object and access. In this application it also displays the information.

6/13/2015B.Ramamurthy30 Temperature Client (contd.) The main method in this client can get the IP address of the remote host as a command line argument. Command line argument is an array of String of items in the command line after the name of the application.

6/13/2015B.Ramamurthy31 Client Details The name of the server object along with the IP of the remote location is used in Naming class’s lookup method to get an object reference. This object reference is then used for remote method calls. Observe that there is no difference between the local and remote call.

6/13/2015B.Ramamurthy32 WeatherItem class Is used in the Graphical display of the weather information.

6/13/2015B.Ramamurthy33 Preparing the Application 1. Compile all the class using javac. 2. Generate the stub and the skeleton: rmic -v1.2 TemperatureServerImpl 3. Then start the registry (this will be running as a daemon) rmiregistry &

6/13/2015B.Ramamurthy34 Preparing the Application 4. Run the server which will register with the RMI registry. Java TemperatureServerImpl & 5. Run the client. Java TemperatureClient & or java TemperatureClient {IPAddress} java TemperatureClient

6/13/2015B.Ramamurthy35 Summary We discussed the various models of distributes systems. Java RMI was used to illustrate the distributed system concepts. Temperature examples shown illustrates some of the distributed system model discussed and all the important RMI features.