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CS551 Object Oriented Middleware (I) (Chap. 3 of EDO). Yugi Lee STB #555 (816) 235-5932 [email protected] www.cstp.umkc.edu/~yugi. Outline. Computer Networks TCP UDP Types of Middleware Transaction-Oriented Middleware Message-Oriented Middleware Remote Procedure Calls

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Cs551 object oriented middleware i chap 3 of edo

CS551 Object Oriented Middleware (I)(Chap. 3 of EDO)

Yugi Lee

STB #555

(816) 235-5932

[email protected]

www.cstp.umkc.edu/~yugi

CS551 - Lecture 11


Outline

  • Computer Networks

    • TCP

    • UDP

  • Types of Middleware

    • Transaction-Oriented Middleware

    • Message-Oriented Middleware

    • Remote Procedure Calls

  • Object-Oriented Middleware

  • Developing with Object-Oriented Middleware

CS551 - Lecture 11


Computer Networks

  • The communication between processes on different machines

  • At the high levels, a connection to another process on another machine to simply send each message as a high level logical unit.

    • logical messages each want to send to each other: simple strings or arrays of bytes (binary data).

  • At the underlying layers, responsible for sending the message may split it into smaller units for physical transfer called PDU's (protocol data units).

    • With each of these smaller packets will be a wrapper with additional header information: Address of sender/receiver, Error detection codes, Protocol control information

CS551 - Lecture 11


Transport Layer

  • Concerned with the transport of information through a network.

  • Two facets in UNIX/Windows networks:

    • TCP

    • UDP

ISO/OSI Reference Model

Application

Presentation

Session

Transport

Network

Data link

Physical

CS551 - Lecture 11


Protocols

  • An agreement between entities (programs) on the transmission of data.

  • Different layers of network software have different concerns.

    • At lower levels it allow for determining the amount of data to be sent, the size of packets and the speed of transmission.

      • Connection Control /Data transfer

      • Flow control/Error control

      • Synchronization/Addressing

    • At higher levels it may cover the syntax and sequencing of logical messages.

CS551 - Lecture 11


Transmission Control Protocol (TCP)

  • Provides bi-directional stream of bytes between two distributed components by establish a virtual connection

    • Steps: Connection establishment, Data transfer, Connection termination.

    • Reliable: send logical messages without having to explicitly deal with these other issues (sequencing and re-transmission of lost packets).

    • Slow: as more time is required to set up and terminate the link and buffering at both sides de-couples computation speeds.

    • UNIX rsh, rcp and rlogin are based on TCP.

CS551 - Lecture 11


TCP for Request Implementation

Client

Server

Application

Application

Presentation

Presentation

Session

Session

Requests

Transport

Input Stream

Transport

Output Stream

Results

CS551 - Lecture 11



User Datagram Protocol (UDP)

  • Enables a component to pass a message containing a sequence of bytes (packet) to another component:

    • A program may send information to another at an indeterminate time with no explicit prior co-ordination

    • the packet may get lost or packets may arrive in a different order to that in which they were sent.

    • reorganize the packets and make requests for new packets when problems occur.

    • dynamically routed; split into a number of PDUs that take different routes to their destination depending on network congestion.

  • Unreliable but very fast protocol: restricted message length, queuing at receiver, e.g. UNIX rwho command.

CS551 - Lecture 11


UDP for Request Implementation

Client

Server

Application

Application

Presentation

Presentation

Session

Session

Request Datagrams

Transport

Transport

Result Datagrams

CS551 - Lecture 11


Datagram Communication

CS551 - Lecture 11


Direct Use of Network Protocols implies

  • Manual mapping of complex request parameters to byte streams

  • Manual resolution of data heterogeneity

  • Manual identification of components

  • Manual implementation of component activation

  • No guarantees for type safety

  • Manual synchronization of interaction between distributed components

  • No quality of service guarantees

CS551 - Lecture 11


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.

CS551 - Lecture 11


Forms of Middleware

  • Transaction-Oriented

    • IBM CICS

    • BEA Tuxedo

    • Encina

  • Message-Oriented

    • IBM MQSeries

    • DEC Message Queue

    • NCR TopEnd

  • RPC Systems

    • ANSA

    • Sun ONC

    • OSF/DCE

  • Object-Oriented

    • OMG/CORBA

    • DCOM

    • Java/RMI

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

CS551 - Lecture 11


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

CS551 - Lecture 11


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;

CS551 - Lecture 11


ISO/OSI Presentation Layer

Resolution of data heterogeneity

Common data

representation

Transmission of

data declaration

Marshalling andUnmarshalling

static

dynamic

CS551 - Lecture 11


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);

};

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);

};

  • Marshalling: Disassemble data structures into transmittable form

  • Unmarshalling: Reassemble the complex data structure.

CS551 - Lecture 11


Called

Caller

Caller

Called

Stub

Method Call vs. Object Request

Caller

Stub

Transport Layer (e.g. TCP or UDP)

CS551 - Lecture 11


Stubs

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

  • Code can be generated fully automatically from interface definition.

  • Code is embedded 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.

CS551 - Lecture 11


Synchronization

  • Goal: achieve similar synchronization to local method invocation

  • Achieved by stubs:

    • Client stub sends request and waits until server finishes

    • Server stub waits for requests and calls server when request arrives

CS551 - Lecture 11


Type Safety

  • 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 an agreed language.

CS551 - Lecture 11


Facilitating Type Safety

Interface

Definition

Server

Request

Client

Reply

CS551 - Lecture 11


Session Layer

  • Implements

    • identification of RPC servers

    • activation of RPC servers

    • dispatch of operations

Application

Presentation

Session

Transport

Network

Data link

Physical

CS551 - Lecture 11


Example: RPC Server Identification

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);

}

CS551 - Lecture 11


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 and inheritance

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

CS551 - Lecture 11


Why do we use an IDL?

PL

PL

PL

PL

1

2

1

2

PL

PL

PL

IDL

PL

6

3

6

3

PL

PL

PL

PL

5

4

5

4

CS551 - Lecture 11


Smalltalk

C++

Ada-95

IDL

Common

Object

Model

Java

C

Cobol

CORBA Programming Language Bindings

CS551 - Lecture 11


IDL Example (OO Middleware)

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);

};

CS551 - Lecture 11


Presentation Layer Implementation

  • 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

CS551 - Lecture 11


Session Layer Implementation

Object

References

Hosts

Processes

Objects

CS551 - Lecture 11


Server Stub

Generation

Client Stub

Generation

Server

Coding

Client

Coding

Development Steps

Design

Interface

Definition

Server

Registration

CS551 - Lecture 11


Facilitating Access Transparency

  • Client stubs have the same operations as server objects

  • Hence, clients can

    • make local call to client stub

    • or local call to server object

      without changing the call.

  • Middleware can accelerate communication if objects are local by not using the stub.

CS551 - Lecture 11


Facilitating Location Transparency

  • Object identity

  • Object references

  • Client requests operation from server object identified by object reference

  • No information about physical location of server necessary

  • How to obtain object references?

CS551 - Lecture 11


Stub Generation

The CORBA IDL is translated

into stubs and skeletons (server stubs):

IDL-compiler generates four files

(two for client stubs and two for

server stubs)

included in

generates

reads

Team.idl

IDL-Compiler

Teamcl.hh

Teamsv.hh

Teamcl.cc

Teamsv.cc

CS551 - Lecture 11


Team.idl

Client.cc

Server.cc

IDL-Compiler

Teamcl.hh

Teamsv.hh

Teamcl.cc

Teamsv.cc

C++ Compiler, Linker

C++ Compiler, Linker

included in

generates

reads

Server

Client

Client and Server Implementation

The client implementation (Client.cc) includes Teamcl.hh so as to enable the compiler to check consistency between the declaration and use of client stub implementation. When translated to an executable, Client.cc is linked with the object code of the client stub (Teamcl.cc). Similar to Server.

CS551 - Lecture 11


Type Safe Server Object Implement.

Inheritance is used in classes

Play_Dispatch and Player_Impl,

which are contained in the server stub

for object type Player.

An Interface-based implementation

with the relationships of interfaces

and classes involved on the server side

<<uses>>

<<uses>>

Player_Impl

<<interface>>

Player_Dispatch

Player_Dispatch

Player_Impl

<<implements>>

Player_Server

Player_Server

CS551 - Lecture 11


Server Registration

  • Object adapters need to be able to locate and start servers

  • Server objects are registered in some form of implementation repository

  • Registration processes is middleware and product-specific

  • Object adapter performs implementation repository lookup prior to activation

CS551 - Lecture 11


Key Points

  • Middleware builds on the transport layer

  • There are several forms of middleware

  • Object-oriented middleware provides IDLs

  • Object-oriented middleware implements session and presentation layer

  • Presentation layer implementation in client/server stubs is derived from IDL

  • Session layer is implemented in object adapters

CS551 - Lecture 11


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