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Intro to Remote Invocation & Marshalling in Distributed Systems (CS 5523 Lecture 8), Study notes of Operating Systems

University of Texas - San Antonio Operating Systems

An introduction to remote invocation and marshalling in distributed systems. It covers the concepts of remote method invocation (rmi), remote procedure call (rpc), and event notification. The document also explains the process of marshalling and unmarshalling data for transmission between processes. The instructor's guide for coulouris, dollimore and kindberg's distributed systems: concepts and design is referenced for further reading.

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Uploaded on 07/30/2009

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CS 5523 Lecture 8:

Introduction to Remote invocation

]Questions on Laboratory 1

]Models of programming in distributed systems

]Objects and remote objects

]Remote invocation and remote object references

]A simple “Hello World” application in CORBA

]Marshalling

Programming models for distributed applications:

]Remote method invocation (RMI) – an object in one process can

invoke methods of objects in another process

]Remote procedure call (RPC) – client calls the procedure in a

server program that is running in a different process

]Event notification – objects receive notification of events at other

objects for which they have registered

These mechanism must be location-transparent. The first two are

traditional client-server (pull), while event notification is a push

strategy

Discover Study notes of Operating Systems University of Texas - San Antonio

Partial preview of the text

Download Intro to Remote Invocation & Marshalling in Distributed Systems (CS 5523 Lecture 8) and more Study notes Operating Systems in PDF only on Docsity!

CS 5523 Lecture 8:

Introduction to Remote invocation

] Questions on Laboratory 1

] Models of programming in distributed systems

] Objects and remote objects

] Remote invocation and remote object references

] A simple “Hello World” application in CORBA

] Marshalling

Programming models for distributed applications:

] Remote method invocation (RMI) – an object in one process can

invoke methods of objects in another process

] Remote procedure call (RPC) – client calls the procedure in a

server program that is running in a different process

] Event notification – objects receive notification of events at other

objects for which they have registered

These mechanism must be location-transparent. The first two are

traditional client-server (pull), while event notification is a push

strategy

Basic steps for client-server (pull strategies):

] Client or its proxy marshalls the information that would be used

for local access (do operation, call, or invocation) into a message

and sends to the remote server.

] The server or its proxy unmarshalls the message and performs

the request as though it were made locally.

] The server or its proxy then marshalls the result into a message

and sends it to the remote client.

] The client or its proxy unmarshalls the message and treats the

result as though it were obtained locally.

What is a proxy and why might it be useful?

Marshalling:

] marshalling – process of transforming a collection of data items

into a form suitable for transmission as a message

] unmarshalling – process of disassembling a message into its

pre-marshalled equivalent.

The process requires a predefined format. Examples:

\ XDR standardized external data representation (RPC)

\ CORBA common data representation (CDR)

\ Java object serialization (Java RMI)

\ Convert to ASCII (HTTP)

\ Microsoft’s format

Review of objects:

] An object encapsulates both data and methods

] Objects are accessed via object references

] Interfaces – provide definitions of signature of a set of methods

] Actions are performed in OO by having objects invoke methods

of other objects, the invoker is called a “client” of the object

] Invocation can cause:

\ the state of the receiver to be changed (modifier methods)

\ additional invocations of methods on other objects

] Exceptions are thrown when an error occurs. If object doesn’t

“catch” the exception, the exception is delivered to the caller

(similar to signals, but at the programming language level)

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

Figure 5.

Remote and local method invocations

invocation invocation

remote

invocation

remote

local

invocation

A

B

C

D

E

F

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

Figure 5.

A remote object and its remote interface

interface

remote

m

Data

implementation

remoteobject

{ of methods

Remote object reference:

] An object must have the remote object reference of an object in

order to do remote invocation of an object

] Remote object references may be passed as input arguments or

returned as output arguments.

] Parameters of a method in Java are input parameters

] Returned result of a method in Java is the output parameter

] Objects are serialized to be passed as parameters

] When a remote object reference is returned, it can be used to

invoke remote methods

] Non-remote serializable objects are copied by value

Remote interface (RMI):

] The remote interface specifies the methods of an object that are

available for remote invocation

] Input and output parameters are specified. The parameters may

be objects

] Use:

\ When the remote method is invoked, the actual arguments corresponding

to the input parameters are marshalled into a packet and sent to the server.

\ The server demarshals the packet, performs the procedure, remarshals the

output arguments, and sends the return packet to the caller.

\ Client demarshals the return packet

\ Need a common format definition for how to pass objects (e.g., CORBA

IDL or Java RMI)

Interfaces:

] Specify procedures (methods) and variables that can be

accessed in a module

] No information other than that specified by the interface can be

communicated.

] Do not specify an implementation

] Types of interfaces:

\ Service interface (RPC)

\ Remote interface (RMI)

Remote interface:

] CORBA – uses IDL to specify remote interfaces

] JAVA – uses ordinary interfaces that are extended by the

keyword remote.

Example of CORBA IDL

module HelloApp

interface Hello

string sayHello();

oneway void shutdown();

idlj -fall Hello.idl

This generates the following files in the HelloApp subdirectory:

Hello.java, HelloHelper.java, HelloHolder.java,

HelloOperations.java, HelloPOA.java and _HelloStub.java.

Map into Java by running the IDL-to-java:

HelloServer.java (continued)

public class HelloServer {

public static void main(String args[]) { try{ ORB orb = ORB.init(args, null); // create and initialize the ORB // get reference to rootpoa & activate the POAManager POA rootpoa = POAHelper.narrow(orb.resolve_initial_references("RootPOA")); rootpoa.the_POAManager().activate(); HelloImpl helloImpl = new HelloImpl(); // create servant and register it with the ORB helloImpl.setORB(orb); // get object reference from the servant org.omg.CORBA.Object ref = rootpoa.servant_to_reference(helloImpl); Hello href = HelloHelper.narrow(ref); // get the root naming context org.omg.CORBA.Object objRef = orb.resolve_initial_references("NameService"); // Use NamingContextExt which is part of the Interoperable Naming Service (INS) spec NamingContextExt ncRef = NamingContextExtHelper.narrow(objRef);

String name = "Hello"; // bind the Object Reference in Naming NameComponent path[] = ncRef.to_name( name ); ncRef.rebind(path, href); System.out.println("HelloServer ready and waiting ..."); orb.run(); // wait for invocations from clients } catch (Exception e) { System.err.println("ERROR: " + e); e.printStackTrace(System.out); } System.out.println("HelloServer Exiting ..."); } }

Running CORBA (server on pandora, port 20000):

] Compile the client and server:

javac HelloClient.java HelloApp/*.java

javac HelloServer.java HelloApp/*.java

] Start the Java Object Request Broker Daemon on server host:

] Start the HelloServer on server host:

orbd –ORBInitialPort 20000 –ORBInitialHost pandora.cs.utsa.edu &

] Start the client on another machine, say ten23:

java HelloServer –ORBInitialPort 20000

java HelloClient –ORBInitialHost pandora.cs.utsa.edu –ORBInitialPort 20000

] Be sure to kill your orbd when finished….

Why is this easier than just doing sockets?

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

Intro to Remote Invocation & Marshalling in Distributed Systems (CS 5523 Lecture 8), Study notes of Operating Systems

Related documents

Partial preview of the text

Download Intro to Remote Invocation & Marshalling in Distributed Systems (CS 5523 Lecture 8) and more Study notes Operating Systems in PDF only on Docsity!

CS 5523 Lecture 8:

Introduction to Remote invocation

] Questions on Laboratory 1

] Models of programming in distributed systems

] Objects and remote objects

] Remote invocation and remote object references

] A simple “Hello World” application in CORBA

] Marshalling

Programming models for distributed applications:

] Remote method invocation (RMI) – an object in one process can

invoke methods of objects in another process

] Remote procedure call (RPC) – client calls the procedure in a

server program that is running in a different process

] Event notification – objects receive notification of events at other

objects for which they have registered

These mechanism must be location-transparent. The first two are

traditional client-server (pull), while event notification is a push

strategy

Basic steps for client-server (pull strategies):

] Client or its proxy marshalls the information that would be used

for local access (do operation, call, or invocation) into a message

and sends to the remote server.

] The server or its proxy unmarshalls the message and performs

the request as though it were made locally.

] The server or its proxy then marshalls the result into a message

and sends it to the remote client.

] The client or its proxy unmarshalls the message and treats the

result as though it were obtained locally.

What is a proxy and why might it be useful?

Marshalling:

] marshalling – process of transforming a collection of data items

into a form suitable for transmission as a message

] unmarshalling – process of disassembling a message into its

pre-marshalled equivalent.

The process requires a predefined format. Examples:

\ XDR standardized external data representation (RPC)

\ CORBA common data representation (CDR)

\ Java object serialization (Java RMI)

\ Convert to ASCII (HTTP)

\ Microsoft’s format

Review of objects:

] An object encapsulates both data and methods

] Objects are accessed via object references

] Interfaces – provide definitions of signature of a set of methods

] Actions are performed in OO by having objects invoke methods

of other objects, the invoker is called a “client” of the object

] Invocation can cause:

\ the state of the receiver to be changed (modifier methods)

\ additional invocations of methods on other objects

] Exceptions are thrown when an error occurs. If object doesn’t

“catch” the exception, the exception is delivered to the caller

(similar to signals, but at the programming language level)

Figure 5.

Remote and local method invocations

invocation invocation

remote

invocation

remote

local

local

local

invocation

invocation

A

B

C

D

E

F

Figure 5.

A remote object and its remote interface

interface

remote

m

m

m