Distributed Objects and Remote Invocation Chapter 5 B

Distributed Objects and Remote Invocation Chapter 5 B. Ramamurthy 2/24/2021 B. Ramamurthy 1

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. 2/24/2021 B. Ramamurthy 2

The Object Model 1. Object references n Objects are first-class values, meaning that they may be assigned to variables, passed as arguments and returned as results of methods. 2. Interfaces n n Interfaces provide signatures of the set of methods Signature is the type of arguments, return value and exceptions) 2/24/2021 B. Ramamurthy 3

The Object model (contd. ) 3. Action: Action is initiated by an object invoking a method in another object. 4. Exceptions: These provide clean way to deal with error conditions without complicating the code. n n n 2/24/2021 Each method heading explicitly lists the exceptions or error conditions that may occur allowing users of the method to deal with them A block of code may be defined to throw an exception when an error occurs. The control passes to another block of code that catches the exception B. Ramamurthy 4

The Object Model 5. Garbage Collection: it is necessary to provide means for freeing the space occupied by objects when they are no longer needed. 2/24/2021 B. Ramamurthy 5

The Distributed Object Model The invoker and the invoked are separated and remote to each other. Foundations of distributed systems Distributed objects may adopt client-server architecture. It may assume other architectural model – objects may be replicated for fault-tolerance and performance. Distributed object model allows for real encapsulation, heterogeneous systems, distribution. The concept of remote interfaces. 2/24/2021 B. Ramamurthy 6

RMI-based Distributed System 4. 5. 3. XYZ Client 3. Stub uses 1. XYZ interface Client Host 2/24/2021 2. XYZ Implementation implements Server Host B. Ramamurthy 7

Steps in RMI-based Application 1. Design the interface for the service. 2. Implement the methods specified in the interface. 3. Generate the stub, skeletons, and dispatchers (for enabling distributed access to the service). 4. Register the service by name and location. 5. Use the service in an application. 2/24/2021 B. Ramamurthy 8

Compile and Register Commands Finds object by name 5. rmiregistry rmic 3. XYZ Client 3. Skeleton Stub uses 1. XYZ interface Client Host 2/24/2021 Stores object by name 2. XYZ Implementation implements Server Host B. Ramamurthy 9

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. 2/24/2021 B. Ramamurthy 10

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. 2/24/2021 B. Ramamurthy 11

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: http: //iwin. nws. noaa. gov/iwin/us/traveler. html 2/24/2021 B. Ramamurthy 12

Temperature Client/Server Distributed Application rmic 4 compiles 5. 3. Temp. Client Temp. Impl_Stub uses generates uses 1. Temp. Server Client Host 2/24/2021 3. 2. Temp. Impl_Stub Temp. Server. Impl implements Server Host B. Ramamurthy 13

Defining Remote Interface import java. rmi. *; // the interface extends Remote interface // any class implementing Remote can be accessed remotely security permitting public interface Temperature. Server extends Remote { // specify methods that can be called remotely // each method “throws Remote. Exception” } 2/24/2021 B. Ramamurthy 14

Remote. Exception 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. 2/24/2021 B. Ramamurthy 15

Implementing the Remote Interface import java. rmi. *; import java. rmi. server. *; import java. net. *; // others as needed Temperature. Server. Impl extends Unicast. Remote. Object implements Temperature. Server { 2/24/2021 B. Ramamurthy 16

Temperature. Server. Impl 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. 2/24/2021 B. Ramamurthy 17

Temperature. Server. Impl (contd. ) It implements the service method get. Weather. Info which simply returns the weather data gathered. The main method instantiates an object for the service, and registers it with rmiregistry. 2/24/2021 B. Ramamurthy 18

Streaming URLs Using the open. Stream 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. 2/24/2021 B. Ramamurthy 19

Server Object Name Syntax for the server object name is: //host: port/remote. Object. Name Default port number for rmiregistry is 1099 For local host the object name: //localhost/Temp. Server For a remote host //127. 0. 0. 1/Temp. Server 2/24/2021 B. Ramamurthy 20

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. 2/24/2021 B. Ramamurthy 21

Weather. Info class It is very traditional class for keeping the information about the temperature at a single location. It has data fields : city. Name, temperature, and description and get methods for these. An array of objects of this class is used in the server implementation. 2/24/2021 B. Ramamurthy 22

Temperature Client import java. rmi. *; // import other packages constructor calls a private method get. Remote. Temp which takes care of lookup of remote object and access. In this application it also displays the information. 2/24/2021 B. Ramamurthy 23

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. 2/24/2021 B. Ramamurthy 24

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. Weather. Item class used in the Graphical display of the weather information. 2/24/2021 B. Ramamurthy 25

Preparing the Application 1. Compile all the class using javac. 2. Generate the stub and the skeleton: rmic -v 1. 2 Temperature. Server. Impl 3. Then start the registry (this will be running as a daemon) rmiregistry & 2/24/2021 B. Ramamurthy 26

Preparing the Application 4. Run the server which will register with the RMI registry. Java Temperature. Server. Impl & 5. Run the client. Java Temperature. Client & or java Temperature. Client {IPAddress} java Temperature. Client 192. 168. 0. 150 2/24/2021 B. Ramamurthy 27

Inside RMI http: //java. sun. com/j 2 se/1. 5. 0/docs/index. html Basic RMI classes: /usr/java 1. 1/src/java/rmi n java. rmi. registry. * n java. rmi. Naming class (static/class methods) n java. rmi. Remote interface (marker interface) n java. rmi. server. * n Default RMI port 1099 n Both lookup from local and remote are acceptable. 2/24/2021 B. Ramamurthy 28

Implementation of RMI (5. 2. 5) Access. Exception. java Remote. Exception. java Already. Bound. Exception. java Connect. Exception. java Server. Exception. java Connect. IOException. java Server. Runtime. Exception. java Marshal. Exception. java Stub. Not. Found. Exception. java Unexpected. Exception. jav Server. Error. java Unknown. Host. Exception. java No. Such. Object. Exception. java Unmarshal. Exception. java Not. Bound. Exception. java RMISecurity. Manager. java 2/24/2021 Remote. java Marshalled. Object. java Naming. java activation dgc Registry server B. Ramamurthy 29

The role of proxy and skeleton in remote method invocation server client object A proxy for B Request skeleton & dispatcher for B’s class remote object B Reply Remote Communication reference module servant Communication. Remote reference module Object A invokes a remote object in Object B for which it holds a remote object reference. “System Model” 2/24/2021 B. Ramamurthy 30

RMI Internals: Communication Module Carries out request-reply protocol; On the client side {message type, message id, remote reference to object} are gathered and sent out. At most once invocation semantics; On the server side, it gets local reference for remote reference from remote reference module, invokes a dispatcher with this reference. See Unicast. Remote (implements Unicast. Remote) 2/24/2021 B. Ramamurthy 31

RMI Internals: Remote Reference module Responsible for translating between local and remote object references and for creating remote object references. A remote object table has a mapping between local and remote references. A table at server (entry for object ref for B) and a table at client (entry for object ref for proxy B). 2/24/2021 B. Ramamurthy 32

RMI Internals: Remote References Action of remote reference module: See Remote. Ref. java interface n n 2/24/2021 When a remote object is to be passed as argument or result for the first time, the remote ref is asked to create a remote ref object which is added to the table. When a remote object reference arrives in a request or reply, the remote ref module is asked for corresponding local object ref, which may either a proxy or remote object. If it is not in the table RMI runtime creates it and asks remote ref module to add it to the table. B. Ramamurthy 33

RMI Internals: RMI software Layer of software between application level objects and communication and remote reference modules: “Middleware” Proxy: provides remote access transparency. One proxy for every remote object in the client. Dispatcher: A server has one dispatcher and skeleton for each class representing a remote object. It receives request message from comm. Module n It used Message. Id to select appropriate method in skeleton. n Proxy and dispatcher use same Message. Id. Skeleton: A class of remote object has a skeleton that implements of the remote interface. All the access dependencies are hidden in this class. A remote object has a servant that directly implements the methods. Java 5 creates this n dynamically. Proxies, dispatcher and skeleton are automatically generated by interface compiler. Binder: binds textual names to remote object references. RMIRegistry is a binder; Naming class; see fig. 5. 13 Server Threads: one thread per invocation Distributed garbage collection: See Andrew Birell’s paper [1995]. 2/24/2021 B. Ramamurthy 34

RMI Internals: Distributed Garbage Collection Based on reference counts. Local garbage collectors and a distributed support. Each server holds the list of processes that hold remote object references: for example, B. Holders When a client C first receives a remote reference to a particular remote object, say B, it makes a add. Ref(B) invocation to server of that remote object and then creates proxy; server adds C to B. Holders. When client C’s garbage collector finds that proxy is no longer reachable (ref count), it makes a remove. Ref(B) invocation to server and then deletes proxy; the server removes C from B. Holders. When B. Holders is empty, server’s local garbage collector will reclaim the space occupied B unless there any local holders. These extra calls for updates occur during proxy creation and deletion and do not affect normal opertion. Tolerates communication failures: add. Ref() and remove. Ref() are idempotent: effects of N > 0 identical requests is the same as for a single request. If add. Ref() fails with an exception, proxy is not created, remove. Ref() is transmitted; remove. Ref() failures are dealt with by “leases” (Jini kind). 2/24/2021 B. Ramamurthy 35

RMI Internals: Use of Reflection What is reflection? See Reflection package Reflection enables Java code to discover information about the fields, methods and constructors of loaded classes, and To use reflected fields, methods, and constructors to operate on their underlying counterparts on objects, within security restrictions. http: //java. sun. com/docs/books/tutorial/reflect/class/index. html Reflection feature allowed for dynamic creation of skeleton and proxy in Java 2 version onwards. Read more about reflection model of computing. 2/24/2021 B. Ramamurthy 36

A Little bit of Reflection Method class, invoke method Invoke method requires two parameters: first the object to receive invocation, second an array of Object parameters. Invoke executes the method on the object and returns result as Object. Method m; Object result = m. invoke(String Obj. Ref, 2/24/2021 B. Ramamurthy 37 Args);

Reflection Java Reflection is a mechanism for inspecting the variables and methods of an unknown class at runtime. Reflection allows for • –finding methods, variables, constructors • –analyzing their types, parameters, results, modifiers • –changing variables • –calling methods or constructors 2/24/2021 B. Ramamurthy 38

Reflection (contd. ) • Reflection uses methods of java. lang. Class(and java. lang. reflect. *) e. g. • • java. lang. Classfor. Name(Stringclass. Name) java. lang. reflect. Contructor[] get. Constructors() java. lang. reflect. Field[] get. Fields() java. lang. reflect. Method[] get. Methods() 2/24/2021 B. Ramamurthy 39

Reflection (contd. ) Java Reflection: Example. . . Class clazz = Class. for. Name("My. Class"); Object my. Object= clazz. new. Instance(); Method[] methods= clazz. get. Methods(); Method first. Method= methods[0]; Objectresult= null; // try to invoke a method on my. Object if("test. Method". equals(first. Method. get. Name())) result= first. Method. invoke(my. Object, null); //. . . obtain class of specified name; create a new instance; obtain all public methods; obtain a method's name; invoke the method with no parameters on the class instance J 2 EE implementations use immense amounts of reflection! 2/24/2021 B. Ramamurthy 40

Using Reflection in RMI Proxy (on the client side) marshals info. about a method and its arguments into a request message. For a method it marshals an object of class Method into the request. It then adds an array of objects for the method’s arguments. The dispatcher unmarshals the Method object and its arguments from request message. The remote object reference is obtained from remote ref. table. The dispatcher then calls the “invoke” method on the object reference and array of arguments values. After the method execution the dispatcher marshals the result or any exceptions into the reply message. 2/24/2021 B. Ramamurthy 41

Figure 5. 1 Middleware layers Applications RMI, RPC and events Request reply protocol External data representation Operating System Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 Middleware layers

Remote and local method invocations local remote invocation A B C E invocation local invocation D Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 remote invocation F

A remote object and its remote interface remoteobject remote interface Data { m 1 m 2 m 3 implementation of methods Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 m 4 m 5 m 6

Instantiation of remote objects Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005

Invocation semantics Fault tolerance measures Retransmit request Duplicate message filtering Invocation semantics Re-execute procedure or retransmit reply Maybe No Not applicable Yes No Re-execute procedure. At-least-once Yes Retransmit reply Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 At-most-once

Role of client and server stub procedures in RPC in the context of a procedural language client process server process Request client program Reply client stub procedure Communication module server stub procedure Communication dispatcher module Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 service procedure

Dealing room system External source Dealer’s computer Notification Information provider Notification Dealer’s computer Notification Dealer Information provider Notification Dealer External Instructor’s Guide for Coulouris, source Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005

Architecture for distributed event notification Event service subscriber object of interest 1. notification object of interest 2. object of interest observer notification subscriber notification observer 3. subscriber notification Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005

Java Remote interfaces Shape and Shape. List import java. rmi. *; import java. util. Vector; public interface Shape extends Remote { int get. Version() throws Remote. Exception; Graphical. Object get. All. State() throws Remote. Exception; 1 } public interface Shape. List extends Remote { Shape new. Shape(Graphical. Object g) throws Remote. Except Vector all. Shapes() throws Remote. Exception; int get. Version() throws Remote. Exception; } Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005

The Naming class of Java RMIregistry void rebind (String name, Remote obj) This method is used by a server to register the identifier of a remote object by name, as shown in Figure 15. 13, line 3. void bind (String name, Remote obj) This method can alternatively be used by a server to register a remote object by name, but if the name is already bound to a remote object reference an exception is thrown. void unbind (String name, Remote obj) This method removes a binding. Remote lookup(String name) This method is used by clients to look up a remote object by name, as shown in Figure 15. 15 line 1. A remote object reference is returned. String [] list() Instructor’s Guide for Coulouris, This method returns an. Dollimore array and of Kindberg Strings containing the names Distributed Systems: Concepts bound in the registry. and Design Edn. 4 © Pearson Education 2005

Java class Shape. List. Server with main method import java. rmi. *; public class Shape. List. Server{ public static void main(String args[]){ System. set. Security. Manager(new RMISecurity. Manager()); try{ Shape. List a. Shape. List = new Shape. List. Servant(); Naming. rebind("Shape List", a. Shape. List ); 2 System. out. println("Shape. List server ready"); }catch(Exception e) { System. out. println("Shape. List server main " + e. get. Message()); } } } Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005

Java class Shape. List. Servant implements interface Shape. List import java. rmi. *; import java. rmi. server. Unicast. Remote. Object; import java. util. Vector; public class Shape. List. Servant extends Unicast. Remote. Object implements Sha private Vector the. List; // contains the list of Shapes private int version; public Shape. List. Servant()throws Remote. Exception{. . . } public Shape new. Shape(Graphical. Object g) throws Remote. Exception { version++; Shape s = new Shape. Servant( g, version); the. List. add. Element(s); return s; } public Vector all. Shapes()throws Instructor’s Guide Remote. Exception{. . . } for Coulouris, Dollimore Remote. Exception and Kindberg public int get. Version()Distributed throws {. . . } Systems: Concepts } and Design Edn. 4 © Pearson Education 2005

Java client of Shape. List import java. rmi. *; import java. rmi. server. *; import java. util. Vector; public class Shape. List. Client{ public static void main(String args[]){ System. set. Security. Manager(new RMISecurity. Manager()); Shape. List a. Shape. List = null; try{ a. Shape. List = (Shape. List) Naming. lookup("//bruno. Shape. List") Vector s. List = a. Shape. List. all. Shapes(); } catch(Remote. Exception e) {System. out. println(e. get. Message()); }catch(Exception e) {System. out. println("Client: " + e. get. Message()); } } Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005

Classes supporting Java RMI Remote. Object Remote. Server Activatable Unicast. Remote. Object <servant class> Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005

Summary We discussed designing a distributed system using RMI We also looked at RMI internal We also learned about marker interface, distributed garbage collection, object marshalling, registry, server-port binding, Naming class of RMI, … 2/24/2021 B. Ramamurthy 56