Chapter 3: Objects, Components, and the Web Textbook IT Architectures and Middleware, Second Edition Chris Britton and Peter Bye AIT 600 Jeff Schmitt September 30, 2008

History of Middleware ● Chapter 2 was history of middleware into 1980's ● Chapter 3 continues at end of 1980s with resurgence in OO programming and a new kind of OO middleware  requestor calls a remote object  like RPC, replaces RPC/transactional  does not replace the two other kinds of middleware: message queue and remote DB  notable example: CORBA

CORBA ● Common Object Request Broker ● a standard, not a product ● developed by Object Management Group (OMG) a consortium of software vendors ● influental but seldom seen in implementations, similar situation with ISO 7-layer network model ● Problem: its complexity, interoperativity ● Rise of component technology made it less important

Component Technology ● Key characteristics  code file – can be interpreted or executed  run-time code has its own private data and provides an interface  can be deployed many times and on many different machines (reused)  component object creates one or more objects and makes the interface of these available

Microsoft Components ● Microsoft COM (Component Object Model), by end of 1990s  used extensively in windows  called from any language ( C++, Visual Basic)  don't need to know file name – find it in system registry ● Middleware Distributed COM (DCOM)  call a COM object on another Windows machine

Java Components ● Java emerged as an important language by 1995 ● Java component model is called JavaBean ● Java runs in Java Virtual Machine (JVM) – macnine independent ● Middleware called Remote Method Invocation (RMI)

Transactional Component Middleware ● solves some problems of using object component ● Microsoft Transaction Server evolved into COM+ ● Enterprise JavaBeans (EJB) ● issue is management of sessions, discussed later

Using Object Middleware ● program must have a reference pointing to the object on the other machine ● works like a pointer (to ram) but details are hidden ● Two steps  getting pointer to the object  repeatedly calling an operation(s) on the object  give up the reference

Object Interfaces ● Interface Definition Language  interfaces more explicit than in OOP  gives flexibility and encapsulation  generates a stub that converts messages into operation calls  marshalling, converting object reference to binary string  interface is not the same as a class – one interface can be used by many classes

OO Middleware compared to RPC ● OO middleware is a natural for OOP languages ● OO middleware is more flexible  interfaces delinked from the server program – this simplifies deployment by hiding details  changes to an interface can be handled by adding a new interface to an object, rather than creating a new object – eventually old interface can be removed allowing time for rewriting dependent code

Transactional Component Middleware ● a term coined by the textbook authors ● fills same niche in object middleware as transaction monitors in RPCs ● makes transaction processing easier to implement and more scalable ● components can be deployed with different settings to behave in different ways ● examples: COM+ and EJB

COM+

EJB

More about TCM ● compare COM+ and EJB – which is better solution? Disciples cannot agree ● Go with the one your skill level and experience favors ● EJB is operating system independent ● COM+ is language independent ● COM+ is now called Enterprise Services, a part of.NET ● New development: SOA and Service Orientation (next chapter)

Internet Applications

Summary ● TCM is the dominant technology for