Presentation: Special Repetition Recap on Distributed Principles.

Presentation:Special Repetition

Recap on Distributed Principles

Ingeniørhøjskolen i ÅrhusSlide 2 af 17

Outline

• Some troubles with the understanding detected at last weeks exercises– Mapping principles with technology– We will recap on some principles (apx. 10 min.)– You will form groups of 3-4 too discuss the problems at

hand – and make the mappings yourselves (apx. 15 min.)

– We will discuss your conclusions (10-15 min.)• After today – I expect you to understand all

principles – if not -> read and discuss in your study groups!


Method Call vs. Object Request

CalledCalled

StubStubStubStub

CallerCaller

CalledCalled

CallerCallerCallerCaller

Transport Layer (e.g. TCP or UDP)Transport Layer (e.g. TCP or UDP)


Server Class – The Skeleton (Stub)…public class HelloWorldServerThread extends Thread { protected DatagramSocket socket = null; public HelloWorldServerThread() throws IOException { super(); socket = new DatagramSocket(4450); }

public void run() { while (true) { …

if (request.startsWith("sayHelloWorld")) { new String(packet.getData()); // figure out response String response = null; //Call the HelloWorldOO method & place it in response HelloWorldOO helloWorldOO = new HelloWorldOO(); response = helloWorldOO.sayHelloWorld(); buf = response.getBytes(); … socket.send(packet);…

CalledCalled

StubStub


The Client Stub - HelloWorldProxypublic class HelloWorldProxy { public HelloWorldProxy() { } public String sayHelloWorld() {… // get a datagram socket socket = new DatagramSocket(); // send the request byte[] buf = new byte[256]; buf = methodCalled.getBytes(); InetAddress address = InetAddress.getByName("localhost"); DatagramPacket packet = new DatagramPacket(buf, buf.length, address, 4450); socket.send(packet); // get a response packet = new DatagramPacket(buf, buf.length); socket.receive(packet); response = new String(packet.getData()); } catch (Exception e) { e.printStackTrace(); } socket.close(); return response;…

StubStub

CallerCaller


Achieving Access Transparency

package chapter1;public class HelloWorldClient { public static void main(String[] args) { System.out.println("Recieved from Server: " + new HelloWorldProxy().sayHelloWorld()); }}

… and thus we have achieved access transparencyusing rather simple constucts (the Proxy pattern).

We could have achieved an even higher degree of transparency by using an interface definition – and thus a contract between Client & Server.

StubStub

CallerCaller


Direct Use of Network Protocols implies

• Manual mapping of complex request parameters to byte streams or datagrams – is complex & error prone

• Manual resolution of data heterogeneity– encoding differs on NT & UNIX (what is the format of an Integer?)

• Manual identification of components– References must be established manually (host add., port nr. etc.)

• Manual implementation of component activation• No guarantees for type safety• Manual synchronization of interaction between distributed

components– Developers need to handle TCP/UDP output stream/responses

• No quality of service guarantees– Transactions, Security, Concurrency, Scalability, Reliability


= Reason for Using Middleware

• Layered between Application and OS/Network• Makes distribution transparent• Resolves heterogeneity of

– Hardware– Operating Systems– Networks– Programming Languages

• Provides development and run-time environment for distributed systems

• “Gets you of the hook” – concerning the nasty stuff in network programming


Flashback to HelloWorld Example

• How to map complex types?– e.g. returning a struct (in C, C++) or an object (in C++,

Java, C#, etc.)?– How would you do it?


InterfaceDefinition

Facilitating Type Safety

ServerClient

Request

Reply

Client & Server has a contract


Interface Definition Language

• Every object-oriented middleware has an interface definition language (IDL)– In Web services = WSDL, CORBA = IDL, DCOM =

MIDL, Java RMI = Java Interfaces

• Object-oriented middleware provide IDL compilers that create client and server stubs to implement session and presentation layer


IDL Example

interface Player : Object { typedef struct _Date { short day; short month; short year; } Date; attribute string name; readonly attribute Date DoB;};interface PlayerStore : Object { exception IDNotFound{}; short save (in Player p); Player load(in short id) raises(IDNotFound); void print(in Player p);};


HelloWorld.jws?wsdl


InterfaceDefinition

Design

Server StubGeneration

Client StubGeneration

ServerCoding

ClientCoding

ServerRegistration

Development Steps

SOAP: WSDLJava2WSDL

WSDL2JAVA

AXISSOAP


Team.idl

included ingeneratesreads

IDL-Compiler

Teamcl.hhTeamcl.cc Teamsv.cc

Teamsv.hh

Stub Generation

Team.wsdlTeam.java

Team.midl


C++ Compiler, Linker

Server

Client.cc Server.cc

C++ Compiler, Linker

Client

Team.idl

included ingeneratesreads

IDL-Compiler

Teamcl.hhTeamcl.cc Teamsv.cc

Teamsv.hh

Client and Server Implementation


Assignment

• Find the excerises you have made so far– Ultra simple UDP solotion– AXIS Web service

• Discuss the following in student groups of 3-4:1. What technologies are used for building Web serivces?2. When are the individual technologies used in Web services (every

time a request is made or …)?3. How do the specific technologies from above (from 2.) map to the

general principles discuessed earlier on? (IDL, stub, proxy, skeleton, design, client, server,etc)

4. Make a figure (in your own words and style) describing the development steps needed – and map these to the Web service technologies

Presentation: Special Repetition Recap on Distributed Principles.

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