Middleware Technology

1
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

2
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)

3
Message-Oriented

• Message Passing
• Requires a Message Protocol (a standard)
• Client packages request into message format
• Client transmits message
• Server received message
• Server un-packs request from message
• Server computes/retrieves formulates reply
• Server packages reply into a message format
• Server transmits message
• Client receives message
• Client un-packs reply from message
• Etc.

4
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)

5
Concurrent Processing

• Multiple concurrent processes
• A form of parallel processing
• Introduces synchronization and communication
  issues
• A useful model Petri Nets

Resource
Process State
Token
Transition
Transitions may require resources
Process State
6
Client/Server Petri Net

• Synchronized Client/Server

Request
Reply