MW TechIS 8040 Data Comm and NetworkingDr. Hoganson

Middleware Technology

Communication Mechanisms

• Synchronous – process on client side must stop and wait until server reply is received.

– Classic programming model

• Asynchronous – process on client side may proceed to do other work while waiting for server reply

– Modern multi-threaded languages

– Java, VB, Delphi

– Adds another “wrinkle” – communication is not just between machines, but between specific processes on specific machines – TCP/IP protocol accommodates this concept – Port #

MW TechIS 8040 Data Comm and NetworkingDr. Hoganson

Remote Procedure Call

• Procedural languages – Cobol, Pascal, C, etc

• The client “calls” a subroutine that happens to reside on the server rather than the local machine

• Synchronous – stop and wait for reply

• DCE “open” standard (Distributed Computing Environment)

MW TechIS 8040 Data Comm and NetworkingDr. Hoganson

Message-Oriented

• Message Passing

• Requires a Message Protocol (a standard)

1. Client packages request into message format

2. Client transmits message

3. Server received message

4. Server un-packs request from message

5. Server computes/retrieves formulates reply

6. Server packages reply into a message format

7. Server transmits message

8. Client receives message

9. Client un-packs reply from message

10. Etc.

MW TechIS 8040 Data Comm and NetworkingDr. Hoganson

Object-Oriented

• Object to Object, rather than procedure to procedure

• Object sends a message (a method call) to another object

• Objects can be multithreaded, so supports both synchronous and asynchronous

• CORBA: Common Object Request Broker Architecture

• DCOM: Distributed Common Object Model (Microsoft proprietary)

MW TechIS 8040 Data Comm and NetworkingDr. Hoganson

Concurrent Processing

• Multiple concurrent processes

• A form of parallel processing

• Introduces synchronization and communication issues

• A useful model: Petri Nets

Process State

Transition

Process State

Process State

Transition

Process State

Resource

Transitions may require resources

Token

MW TechIS 8040 Data Comm and NetworkingDr. Hoganson

Client/Server Petri Net

• Synchronized Client/Server

Computing

Idle

Waiting

ProcessReply

Waiting

ProcessRequest

Request

Reply