CorbaWeb: A Generic Object Navigator

WWW and Objects

• WWW is the "killer application" of the 1990s
  A user-friendly uniform interface to access any distributed resource

• WWW can be seen as an Object-Oriented Operating System:
  
  • A uniform naming with the Uniform Resource Locators
  • Persistent objects implemented via http daemons and their host file systems
  • An invocation meta-protocol through HTTP and a set of basic operations (GET, POST, ...)
  • Extensibility via the Common Gateway Interface protocol
• But
  • A same single "object interface" for all Web document resources
  • Other non standard Web resources are accessed by specific CGI programs
• Then
  • WWW needs a Uniform Object-Oriented Model to support new kinds of resources

CorbaWeb Overview

CORBA as a Uniform Object-Oriented Model

• Web resources are implemented by CORBA objects

CorbaWeb Environment

• Integration between WWW and CORBA worlds
• A generic gateway between WWW server side and CORBA objects
• Representation, navigation and invocation on any CORBA object
  even if they are not designed for WWW

Main features

• Object-Oriented Programming from CORBA
• Hypermedia navigation from the WWW

Common Object Request Broker Architecture

• A unified architecture for distributed application integration using object technologies (OMG)
  • Interoperability (hardware and software)
  • Component portability and reusability

• A client/server object-oriented middleware
• A component description language (IDL)
• ORB, an open support: multi-languages, several object adapters
• A dynamic environment:
  • Interface Repository to discover IDL definitions modelized by objects
  • Dynamic Invocation Interface to dynamically invoke object operations

OMG-IDL

• An object-oriented interface definition language
• A CORBA object is described by an interface
• An interface = a set of attributes and operations
• Parameters and attributes are strongly typed

• Example:
  

  interface computer {
       typedef unsigned long Positive;
       typedef sequence seqPositive;
       boolean is_prime_number (in Positive number);
       seqPositive prime_numbers (in Positive number);
       seqPositive decompose_into_prime_factors (in Positive number);
  };
  

• Allows multiple interface inheritance
• Mapping to several programming languages
  C, C++, SmallTalk, Java, Ada95, ...

Reuse of Current Technologies

Object-Oriented Programming from CORBA

• The Interface Definition Language to specify Web resources
• The Common Object Request Broker Architecture to support Web resources modelized by objects

Hypermedia Navigation from the WWW

• Web browsers as a single uniform user interface to navigate through objects
• The Uniform Resource Locators to access objects and to apply operations on them
• The HyperText Markup Language to describe the object state and generate Graphical User Interfaces on objects

Current Implementation

• CGI in Web servers as a gateway between the Web and objects

CorbaWeb Architecture

• Scripts are written in CorbaScript language
• CorbaWeb Repository contains meta information
• Meta scripts allow invocation, navigation and representation of any object

CorbaScript: A New CGI Language

• An interpreted scripting language
  • A script = a set of invocations on objects
  • An invocation = every service described with IDL
  • Any IDL description is accessible
  • Variables, exception management and control flow instructions
• Example

  $number := 18567                            #
  $computer := computer(...)                  # bind to object
  $v1 := $computer.is_prime_number ($number)  # check 'is_prime_number' with IR
                                              # invocation with DII
  if $v1 = true then
    echo $number , " is a prime number"
  end
  

• CGI parameters are accessible through CorbaScript variables
• Built upon the Dynamic Invocation Interface
• Control and type inference using the Interface Repository

Computer Example

Design a prime service for WWW

1. Design IDL Interface (computer example)
2. Implement the object server
3. Construct an HTML page with forms
4. Program the CGI to use the service

Step 3: Construct an HTML Page with Forms

Step 4: Program the CGI Script with CorbaScript

#! /usr1/bin/cwsh

echo "Content-Type: text/html\n\n<HTML>\n"

$computer := computer(my_computer:computerSvr:blanche)

if $QUERY_STRING = "is_prime" then ... invokes computer and displays result ... end

if $QUERY_STRING = "prime_numbers" then
  $result := $computer.prime_numbers($number)
  echo "The prime numbers less than the number " , $number , " are\n<P>\n"
  echo $result
end

if $QUERY_STRING = "decompose" then ... invokes computer and displays result ... end

echo "</HTML>\n"

From Scripts to Meta Scripts

• CorbaScript allows any operation invocation on any CORBA object

• The problem:
  • integration of a new service requires to program a new CGI script
  • integration of millions of object types requires a plethora of CGI scripts

• The solution: the meta scripts
  • Definition of generic actions on CORBA objects
  • These scripts take parameters like object references or scripts
  • They apply meta operations on these parameters such as an object browsing or a script execution
  • Scripts and meta scripts are written with CorbaScript language

Meta Scripts

Exec Meta Script

• Allows a user to invoke operations on any object from a Web browser

• Takes a script as parameter and executes it

Interface Meta Script

• Generates an HTML document representing object IDL interface

• Takes an object reference as parameter
  • Accesses the Interface Repository to retrieve all operations and attributes of the object interface
  • Automatically generates forms for each operation or attribute
    When an operation takes parameters, it generates HTML input text fields

• The action of each form refers to the Exec meta script

Interface Meta Script Example

Invocation of "Prime numbers less than ..."

Result:

HTML Representations of IDL Values

View Meta Script

• The meta script View represents the object state according to a user's profile
  

• Takes two parameters: an object reference and a user's profile (1)
  • It retrieves from the CorbaWeb Repository the appropriate view script according to the object type and the profile (2)
  • It executes the retrieved script (3)

• As a result (4), the retrieved view script:
  • calls IDL features to read the object state
  • generates an HTML document presenting the object state

A French Hospital Information System

• Only described by IDL interfaces
• Implemented with C++ or database

Browsing the Hospital Object

Browsing the Radiology Department

Browsing a Checkup Instance

Browsing an X-ray Instance

Conclusion

CorbaScript:

• An interpreted scripting language
• Built upon the Dynamic Invocation Interface and the Interface Repository
• Access every service described with IDL

CorbaWeb:

• Integration of WWW and Corba worlds (via a generic gateway)
• New WWW services designed with objects
• Meta scripts
  • Apply meta operations on objects (Interface, Exec, View meta scripts)
  • VRML documents could be generated by a VRML meta script to navigate in 3D objects
• WWW + CorbaWeb + Corba = A Generic Object Navigator

• Current implementation on Orbix 2.0