Communication (Part II) - Amir H. Payberah€¦ · Communication (Part II) Amir H. Payberah...

Communication (Part II)

Amir H. Payberahamir@sics.se

Amirkabir University of Technology(Tehran Polytechnic)

Amir H. Payberah (Tehran Polytechnic) Communication 1393/12/19 1 / 27

Remote Procedure Call(RPC)

Introduction

I Many distributed systems have been based on explicit message ex-change between processes.

I However, the send and receive methods do not hide communica-tion at all, which is important to achieve access transparency indistributed systems.

I Proposed solution: to allow programs to call procedures located onother machines: Remote Procedure Call (RPC).

Introduction

Local Procedure Call

I a: Parameter passing in a local procedure call: the stack before thecall to read(fd, buf, bytes).

I b: The stack while the called procedure is active.

Remote Procedure Call (RPC)

I Principle of RPC between a client and server program.

Call local procedureand return results

Call remoteprocedure

Returnfrom call

Client

Request Reply

ServerTime

Wait for result

Basics of RPCs

I RPC abstracts procedure calls between processes on networked sys-tems.

I Stubs: client-side proxy for the actual procedure on the server.

I The client-side stub• Locates the server• Marshalls the parameters

I The server-side stub• Receives the message from client-side stub• Unpacks the marshalled parameters• Performs the procedure on the server

Basics of RPCs

Steps of a RPC (1/2)

1 Client procedure calls client stub.

2 Stub builds message, and calls local OS.

3 OS sends message to remote OS.

4 Remote OS gives message to server stub.

5 Server stub unpacks parameters and calls server.

Implementationof add

Client OS Server OS

Client machine Server machine

Client stub

Client process Server process1. Client call to

procedure

2. Stub buildsmessage

5. Stub unpacksmessage

6. Stub makeslocal call to "add"

3. Message is sentacross the network

4. Server OShands messageto server stub

Server stubk = add(i,j) k = add(i,j)

proc: "add"int: val(i)int: val(j)

Steps of a RPC (2/2)

6 Server makes local call and returns result to stub.

7 Stub builds message, and calls OS.

8 OS sends message to client’s OS.

9 Client’s OS gives message to stub.

10 Client stub unpacks result and returns to the client.

Implementationof add

Client OS Server OS

Client machine Server machine

Client stub

Client process Server process1. Client call to

procedure

2. Stub buildsmessage

5. Stub unpacksmessage

6. Stub makeslocal call to "add"

3. Message is sentacross the network

4. Server OShands messageto server stub

Server stubk = add(i,j) k = add(i,j)

proc: "add"int: val(i)int: val(j)

Parameter Passing (1/2)

I Parameter marshaling: there is more than just wrapping parametersinto a message.

I Client and server machines may have different data representations(think of byte ordering).

I Wrapping a parameter means transforming a value into a sequenceof bytes.

I Client and server have to agree on the same encoding:• How are basic data values represented (integers, floats, characters)• How are complex data values represented (arrays, unions)

I Client and server need to properly interpret messages, transformingthem into machine-dependent representations.

I Some assumptions:• Copy in/copy out semantics: while procedure is executed, nothing

can be assumed about parameter values.• All data that is to be operated on is passed by parameters. Excludes

passing references to (global) data.

I Conclusion: full access transparency cannot be realized.

I Observation: a remote reference mechanism enhances access trans-parency:

• Remote reference offers unified access to remote data.• Remote references can be passed as parameter in RPCs.

Asynchronous RPCs

I Try to get rid of the strict request-reply behavior, but let the clientcontinue without waiting for an answer from the server.

Call local procedure

Returnfrom call

Request Accept request

Wait for acceptance

Call local procedureand return results

Returnfrom call

Client Client

Request Reply

Server ServerTime Time

Wait for result

(a) (b)

Remote Method Invocation(RMI)

Introduction

I Remote Method Invocation (RMI)

I RMI = RPC + object oriented

I RPC in C and RMI in Java

Steps of a RMI (1/3)

1 The server program controls the remote objects.

2 The server registers an interface with a naming service: makes theinterface accessible by clients.

3 The interface contains the signatures for those methods of the objectthat the server wishes to make publicly available.

4 Clients use the naming service to obtain a reference to this interface:called a stub.

5 The stub is a local surrogate for the remote object.

6 On the server system, there is another surrogate called a skeleton.

7 When the client program invokes a method of the remote object, itappears to the client as though the method is being invoked directlyon the object.

8 What is actually happening, however, is that an equivalent methodis being called in the stub.

5 The stub is a local surrogate for the remote object.

6 On the server system, there is another surrogate called a skeleton.

7 When the client program invokes a method of the remote object, itappears to the client as though the method is being invoked directlyon the object.

8 What is actually happening, however, is that an equivalent methodis being called in the stub.

9 The stub forwards the call and any parameters to the skeleton onthe remote machine.

10 Only primitive types and those reference types that implement theSerializable interface may be used as parameters.

11 Upon receipt of the byte stream, the skeleton converts this streaminto the original method call and associated parameters.

12 The skeleton calls the implementation of the method on the server.

RMI Implementation

I Setting up a RMI connection four steps:1 Create the interface.2 Define a class that implements this interface.3 Create the server process.4 Create the client process.

Setting up a RMI Connection (1/4)

I Create the interface.

I This interface should extend interface Remote.

import java.rmi.*;

public interface Hello extends Remote {

public String getGreeting() throws RemoteException;

I Define a class that implements this interface.

I The implementation class must extend class RemoteObject or oneof RemoteObject’s subclasses.

• E.g., UnicastRemoteObject that supports TCP point-to-pointcommunication.

I We must provide a constructor for our implementation object.

import java.rmi.*;

import java.rmi.server.*;

public class HelloImpl extends UnicastRemoteObject implements Hello {

public HelloImpl() throws RemoteException { ... }

public String getGreeting() throws RemoteException {

return ("Hello there!");

Setting up a RMI Connection (3/4) - Part 1

I Create the server process.

I The server creates object(s) of the above implementation class andregisters them with a naming service called the registry.

I Establishes a connection between the object’s name and its refer-ence, by using method rebind that takes two arguments:

1 a string that holds the name of the remote object as a URL withprotocol rmi.

2 a reference to the remote object.

I Clients will then be able to use the remote object’s name to retrievea reference to that object via the registry.

Setting up a RMI Connection (3/4) - Part 2

import java.rmi.*;

public class HelloServer {

private static fi nal String HOST = "localhost";

public static void main(String[] args) throws Exception {

//Create a reference to an implementation object...

HelloImpl temp = new HelloImpl();

//Create the string URL holding the object’s name...

String rmiObjectName = "rmi://" + HOST + "/Hello";

//’Bind’ the object reference to the name...

Naming.rebind(rmiObjectName, temp);

System.out.println("Binding complete...\n");

I Create the client process.

I The client obtains a reference to the remote object from the registry,by calling method lookup.

import java.rmi.*;

public class HelloClient {

private static fi nal String HOST = "localhost";

public static void main(String[] args) {

//Obtain a reference to the object from the registry

Hello greeting = (Hello)Naming.lookup("rmi://" + HOST + "/Hello");

//Use the above reference to invoke the remote object’s method...

System.out.println("Message received: " + greeting.getGreeting());

} catch(Exception ex) { ... }

RMI Implementation

I Compiling and running a RMI application consists of four steps:1 Compile all files with javac.2 Start the RMI registry: rmiregistry.3 Run the server.4 Run the client.

Summary

I Send and receive methods do not hide provide access transparency.

I Remote Procedure Call (RPC) - in C

I Client stub and server stub (skeleton)

I Parameter passing: marshaling

I Remote Method Invocation (RMI) - in Java

Reading

I Chapter 4 of the Distributed Systems: Principles and Paradigms.

I Chapter 5 of An Introduction to Network Programming with Java.

Questions?

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