Objects, Components, and the WWW

Introduction

• Want to use Web as an interface to libraries of objects
• Want interoperable objects, since Web runs on most platforms
• Want object-based (or compound) docs (vs. Web docs)
• Web=data only
• Object-based docs = data + access methods
• Object technologies:
• CORBA (common object request broker architecture)
• DCOM (MS's distributed component object model)
• Java
• Compound doc technologies:
• OpenDoc (by Object Management Group)
• OLE (Object Linkng and Embedding, by Microsoft)
• Components:
• Java Beans
• Active-X (for OLE)

Trends

• Web browser becoming universal interface to docs
• Web is more like a file system than a distributed computer
• Want to include one applet or application inside another one

Objects provide a new paradigm.

Distributed Objects

• Have normal object properties (encapsulation, inheritance, ...)
• Also want
• clients to be unaware where server object is
• local or remote
• if remote, which machine
• client and server objects in different languages, OSs to interoperate
• provide services such as security, transactions, state management, licensing

Component

Examples:
• Demos in Java Bean Development Kit (BDK) 1.0 are components.
• Microsoft Internet Explorer is an Active-X component.

• A stand-alone object
• Not bound to any program
• The "Integrated Circuit" of software
• May be "visibile" (e.g., has an interface) or "invisible"
• Could work stand-alone on one machine, or use a distributed object architecture to run across a network.
Example:
• Java Beans uses CORBA when used across machines.

What makes component-based software?

1. Components
2. Containers
3. Scripting

Services provided by Component-based software:

1. Interface publishing and discover
2. Event Handling
3. Persistence
4. Layout Control
5. Application Builder Support

Compound Document Framework

• Allows different desktop apps to access, modify data objects created by peer apps regardless of data content and format.
• Act as containers into which heterogeneous components can be placed to build custom apps.
• Act as front-end to apps using distributed objects.
Example:
• Plug-ins in a Web browser:  loading a spreadsheet in a Windows-based Web browser might invoke an Active-X component that is Excel.

Summary: Key Technologies

• Distributed Object Architecture
• Components and Containers
• Compund Document Frameworks

Paradigms for Distributed & Mobile Code

• Object Bus: a messaging mechanism
• Static (compile-time defined) and dynamic method invocations
• Object repositories
• Distribution services (names, creates, destroys objects)

CORBA

1. Object request broker (ORB) as object bus: middleware that mediates interactions between client apps and server objects
2. Interface Definition Language (IDL)
3. Interface Repository: database of object defns.
4. Object adaptor: provides run-time environment for server app to handle incoming client calls
5. CORBA services: defines persistence, transactions, concurrency, database querrie, licensing, ...

Compound Doc Architectures

• Acts as a container in which a continuous stream of various kinds of data can be placed.
• Each formof content has associated controls that modify the doc in-place, with uniform user interfaces.

OpenDoc

• IBM’s CORBA-compliant System Object Model is Object Bus.
• Bento defines a platform neutral container format used in files, net streams, ….
• Uniform Data Transfer allows drag, drop, link, paste, etc. in same or different docs.
• Open scripting extends Apple Events to allow objects to react to creation, edit, ...

OLE:

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Last modified on 6 May 1997.
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