presentation 4: principles of object-oriented middleware

Presentation 4:Principles of

Object-Oriented Middleware

Types of Middleware- and why to use …

Ingeniørhøjskolen i ÅrhusSlide 3 af 23

Rolling your own distributed object

• We may build directly on the TCP or UDP protocols to build a distributed system

• In Java: quite easy – thanks to the java.net package – which contains a complete network API incl. socket support for TCP/UDP

• BUT: if we want to ensure OO – more complex

• Let us take a look at how this may be done


The Basic Class to OO Class

public class HelloWorldBasic { public static void main(String[] args) { System.out.println("HelloWorld!"); }}

HelloWorld – canit be more simple?

public class HelloWorldOO { public HelloWorldOO() { } public void sayHelloWorld() { return “HelloWorld"; } public static void main(String[] args) { HelloWorldOO helloWorldOO = new HelloWorldOO(); System.out.println(helloWorldOO.sayHelloWorld()); }}

To OO:


First Issue: Making the Object Distributed- Method Call vs. Object Request

CalledCalledCalledCalled

StubStub

StubStubStubStub

CallerCaller

CalledCalledCalledCalled

CallerCallerCallerCaller

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


Repetition Group Work

• You already know how to do this from Dist• 10 Minutes group work – followed by discussion

– On paper – using pseudo code – describe what to code in order to make HelloWorldOO distributed depending solely on the datagram API (UDP) technology

– Optimize the code for access transparency– Consider introducing location transparency– Consider introducing scalability transparency– Change the interface to allow for returning a HelloWorldData

object with:• Name• Greetings Text• java.util.Data object

– Consider how to allow a C++ client to access the code– Consider the problems of your design – what might fail?


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


Types of Middleware

• Transaction-Oriented– Suited for Database Applications– IBM CICS– BEA Tuxedo– Encina

• Message-Oriented– Suited for Asynchronous (EDI,Batch)– IBM MQSeries– DEC Message Queue– NCR TopEnd– (SOAP)

• RPC Systems– Suited for Access transparency etc– ANSA– Sun ONC– OSF/DCE– (SOAP)

• Object-Oriented– OMG/CORBA– DCOM– Java/RMI/EJB– .NET Remoting– (SOAP)

• First look at RPCs to understand origin of object-oriented middleware


Remote Procedure Calls

• Enable procedure calls across host boundaries• Call interfaces are defined using an Interface

Definition Language (IDL)• RPC compiler generates presentation and session

layer implementation from IDL• RPC is the “legacy” of oo middleware


Type Safety and Heterogenity

• How can we make sure that – servers are able to perform

operations requested by clients?– actual parameters provided by

clients match the expected parameters of the server?

– results provided by the server match the expectations of client?

• Middleware acts as mediator between client and server to ensure type safety.

• Achieved by interface definition in a mutually agreed upon language

• Supports Heterogenity


IDL Example (Unix RPCs)

const NL=64;

struct Player {

struct DoB {int day; int month; int year;}

string name<NL>;

};

program PLAYERPROG {

version PLAYERVERSION {

void PRINT(Player)=0;

int STORE(Player)=1;

Player LOAD(int)=2;

}= 0;

} = 105040;


RPC Marshalling and Unmarshalling

• Marshalling: Disassemble data structures into transmittable form

• Unmarshalling: Reassemble the complex data structure

char * marshal() { char * msg; msg=new char[4*(sizeof(int)+1) + strlen(name)+1]; sprintf(msg,"%d %d %d %d %s", dob.day,dob.month,dob.year, strlen(name),name); return(msg);};void unmarshal(char * msg) { int name_len; sscanf(msg,"%d %d %d %d ", &dob.day,&dob.month, &dob.year,&name_len); name = new char[name_len+1]; sscanf(msg,"%d %d %d %d %s", &dob.day,&dob.month, &dob.year,&name_len,name);};

char * marshal() { char * msg; msg=new char[4*(sizeof(int)+1) + strlen(name)+1]; sprintf(msg,"%d %d %d %d %s", dob.day,dob.month,dob.year, strlen(name),name); return(msg);};void unmarshal(char * msg) { int name_len; sscanf(msg,"%d %d %d %d ", &dob.day,&dob.month, &dob.year,&name_len); name = new char[name_len+1]; sscanf(msg,"%d %d %d %d %s", &dob.day,&dob.month, &dob.year,&name_len,name);};


RPC Stubs

• Creating code for marshalling and unmarshalling is tedious and error-prone.

• Code can be generated fully automatically from interface definition.

• Code is embedded (hidden away – de-coupled) in stubs for client and server.

• Client stub represents server for client, Server stub represents client for server.

• Stubs achieve type safety.• Stubs also perform synchronization.• NOT object-oriented, no support for exceptions or

advanced error handling

Object-Oriented Middleware


Interface Definition Language

• Every object-oriented middleware has an interface definition language (IDL)

• Beyond RPCs, object-oriented IDLs support object types as parameters, failure handling (exceptions) and inheritance

• 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);};


Presentation Layer Implementation

• Ensuring compatible encoding across platforms• In addition to RPC presentation layer

implementation, object-oriented middleware needs to– define a transport representation for object references– deal with exceptions– need to marshal inherited attributes


ObjectReferences Hosts Processes Objects

Session Layer Implementation- Mapping of Object references to hosts-Activation & deactivion of objects-Invocation of requested operation-Synchronization of client & server (state)

- Mapping of Object references to hosts-Activation & deactivion of objects-Invocation of requested operation-Synchronization of client & server (state)

Developing with Object-Oriented Middleware


InterfaceDefinition

Design

Server StubGeneration

Client StubGeneration

ServerCoding

ClientCoding

ServerRegistration

Development Steps


Team.idlTeam.idl

included ingeneratesreads

IDL-Compiler

Teamcl.hh

Teamcl.cc Teamsv.cc

Teamsv.hh

Stub Generation

Team.wsdlTeam.wsdlTeam.javaTeam.java

Team.midlTeam.midl


C++ Compiler, Linker

Server

Client.ccClient.cc Server.ccServer.cc

C++ Compiler, LinkerC++ Compiler, Linker

Client

Team.idlTeam.idl

included ingeneratesreads

IDL-Compiler

Teamcl.hh

Teamcl.cc Teamsv.cc

Teamsv.hh

Client and Server Implementation

presentation 4: principles of object-oriented middleware

Documents

helloworldoo helloworldoo

public helloworldoo

public class helloworldoo

new helloworldoo

distributed systemsgets

distributed objectwe

public void sayhelloworld

distributed systemin