II. Middleware for Distributed Systems

Principles of Object-Oriented Middleware 1

II. Middleware for Distributed Systems

Outline Principles of Object-Oriented Middleware CORBA, COM and Java/RMI ORB Runtime Resolving Heterogeneity Dynamic Object Requests


Principles of Object-Oriented Middleware

Outline Computer Networks Types of Middleware Object-oriented Middleware Developing with Object-Oriented Middleware


Principles of Object-Oriented Middleware

Network Operating System Facilitate physical interchange of electrical or optical signals as packets of

information Detect and correct transmission errors Implement routing of packets between hosts Compose packets into messages

Application Requesting operations from remote objects

Middleware Shield lower level details from applications

Middleware

Network OS

Component-1 Component-n…

Host-1

Hardware


Computer Networks

ISO/OSI Reference Model (7-layer model)

Physical

Data Link

Network

Transport

Session

Presentation

Application


Computer Networks

ISO/OSI Reference Model (7-layer model)

Physical

Data Link

Network

Transport

Session

Presentation

Application

Middleware


Available Middleware Transaction-Oriented Middleware

Support transactions across different distributed db systems Two-phase commit protocol to implement distributed transaction Products: IBM’s CICS, etc.

Message-Oriented Middleware Supports communication between distributed system components by facilitating message

exchange Support asynchronous message delivery naturally Support multi-casting Fault tolerance (message queue on temporarily persistent storage) De-coupling of client and server Products: IBM’s MQSeries

Object Oriented Middleware Has transaction-oriented middleware capabilities Support synchronous communication with at-most-once semantics Trend: to be integrated with message-oriented middleware


Available Middleware

RPC Call across host boundaries Origin of OO middleware Interface Definition Language


Remote Procedure Calls

Presentation Layer Resolution of data heterogeneity

Common data representation Transmission of data declaration

Marshalling and Unmarshalling static Dynamic

Client and server stubs Static implementations of marshalling and unmarshalling

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);

};


Remote Procedure Calls

Session Layer Enable client to locate an RPC server

Static binding: simple, lost location transparency Dynamic binding: depending on a deamon

– Support location transparency

Print_person (char* host, Player * pers) {

CLIENT * clnt;

clnt = clnt_create(host, 105040, 0, “udp”);

if (clnt == (CLIENT*) NULL) exit (1);

if (print_0(pers, clnt) == NULL)

clnt_perror(clnt, “call failed”);

clnt_destroy (clnt);


General Pattern for Remote Invocation

Call: marshal arguments convert to network format locate server transmit data

Client CodeClient Code

Stub

Server CodeServer Code

Skeleton

InfrastructureInfrastructure

Serve: receive data convert & unmarshal invoke method marshal return value transmit data


Object-Oriented Middleware

IDL Object types as parameters; Failure handling; inheritanceInterface Player: Object {

typedef struct Date {

short day; short onth; short year; };

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 Similar to RPC

Support client and server stubs Perform marshalling and unmarshalling Resolve heterogeneity of data representation

Different from RPC Define representation of object references Marshalling/unmarshalling object references



Session Layer Map object references to hosts Implements object activation policies in the object adapter Object adapters need to be able to start up severs, which register in an

implementation repository or registry Implement operation dispatch Implement synchronization

Object Reference Hosts Processes Objects

Obj

ect R

eque

st B

roke

r


Developing with Object-Oriented Middleware

Design

Interface Definition

Server Stub Generation

Client Stub Generation

Client Coding

Server Coding

Server Registration


General Architecture for a DOC System

IDL

Com

pil

ers

Registration Service

Object Skeleton

Object Storage

Object Manager

Naming Service

Client Application

ServerImplementation

Client StubInterface

Object Interface

Specification


CORBA

OMG standard Enable interoperability between applications in heterogeneous distributed

environment

Common architecture framework Common framework across heterogeneous hardware platforms and operating

systems Common framework for inter-communication of application objects

Open distributed object computing infrastructure Automate many common network programming tasks, such as

object registration, location, and activation; Request demultiplexing Framing and error-handling Parameter marshalling and un-marshalling Operation dispatching


CORBA

CORBA Message-passing Object-oriented programming

ORB Automate network functions Sit on the host between the data and the application layer Handles request messages from clients to servers in transparent manner


CORBA Objects

A CORBA Object has an interface and an implementation Interface

Interface is not bound to a specific implementation PL, Interface Definition Language


Interface Definition

Purpose of Defining Interface Instantiate meta-models Provide details to class diagrams Govern interaction between client and server Provide basis for distributing type information Provide basis for automatically generate client and server stub

Reading Material Brose’s book chapter 2


Stub Generation

Difference between Method Calls and Object Requests

Called

Caller

Stub

Called

Stub

Caller

Transport Layer (e.g. TCP or UDP)


IDL Compiler

For each .idl file, idl compiler generates 4 files

Test.idl

IDL-Compiler

Testsv.hh

Testsv.cc

Testcl.hh

Testcl.cc


Implementation of Client Objects

A static object request is made by a client calling a local method of a client stub

Stubs are typed Stubs can achieve access transparency Middleware may shortcut a stub if server and client reside on

the same host


Implementation of Server Objects

The generated server stub has to call the server implementation that an application builder designed.

Interfaces and inheritance make server object implementations type safe.

Player_Dispatch Player_Imp

Player_Server

<<uses>>Player_Dispatch <<Interface>>

Player_Imp

Player_Server

<<uses>>

<<Implements>>

Inheritance Interface


Server Registration

Server object must be registered Registry Implementation repository

System administrator maintain the repository

Middleware provide tools for maintain the repository Register new server objects Startup server objects Stop/delete existing server objects


Summary

OO middleware built on top of the transport layer OO middleware implements the session and presentation layers Other than OO middleware, there are transactional, message-

oriented, remote procedure call middleware Session layer implements an object adaptor

activating/deactivating objects, synchronize client and server objects

Presentation layer resolve data heterogeneity Development process for distributed objects

Idl, stub generation, coding, registration Type safety Implementation repository

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II. Middleware for Distributed Systems