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Interoperable Heterogeneous Data
Access with CORBA and Ada 95

William W. Pritchett
Huet C. Landry

Table of Contents

1. Background

Today, DoD information systems are largely single-purpose systems that are difficult to change and expensive to maintain. These stovepipe systems are the result of a development paradigm which focuses on the system at hand, and not the enterprise as a whole. The DoD, however, is changing this approach by emphasizing "integration, interoperability, flexibility, and efficiency through the development of a common, multipurpose, standardsbased technical infrastructure"[1]. This infrastructure focuses on developing architectures to support both common facilities as well as mission-specific operations. Two early implementors of this strategy are Defense Intelligence Agency's (DIA) Modernized Integrated Database (MIDB) program and the Central Imagery Office's (CIO) Common Imagery Interoperability Facility (CIIF). Although a complete strategy has not been established, it clear that two emerging standards, the Common Object Request Broker Architecture (CORBA) and the new Ada standard (Ada 95) will play an important role.

1.1 Modernized Intelligence Database

MIDB is the core Military Intelligence Data System made up of several distributed databases and applications. One of the principal problems being examined by MIDB is how to integrate the various types of intelligence data (text, images, maps), in a manner that is transparent to the analyst. In current implementations, intelligence analysts are forced to use a variety disparate applications to get a complete picture of the situation at hand. Querying a relational database might reveal the existence of imagery data by specifying a file name. The analyst must then find the file and launch an application capable of viewing the image. This process is both time consuming and error prone as a result of intensive dependence on interactions that are software rather than mission related.

Ideally, analysts should be able to access all necessary information from the desktop with as little software specific interaction as possible. The notion of a single compound document as shown in Figure 1., containing text, images, etc., would be an example of a desired interface.

Figure 1. Example Intelligence Desktop

In a reference implementation co-sponsored by the Defense Information Systems Agency (DISA) Center for Standards, developers were able to demonstrate this seamless data access can be accomplished in an object-oriented manner using CORBA specifications and the Ada 95 standard. The DIA is actively pursing use of the CIIF as the primary image service for MIDB.

1.2 Common Imagery Interoperability Facility

CIO is also employing a standards based approach to tackle the problem of imagery interoperability by defining a CIIF. The CIIF is a specification that describes a group of related software interfaces that perform imagery-specific services such as image access and image processing. The goal of the CIIF is to enhance interoperability and portability of imagery applications by defining a standard set of application program interfaces (APIs). The CIO has chosen to define this API in the CORBA Interface Definition Language (IDL). The CIO believes this will lead to increased sharing of imagery data and services and will facilitate the insertion of commercial technology.

2. Standards-Based Architecture Approach

In recent years, the term architecture has come to mean many things to different people. At a minimum, a software architecture is the structure of components and how they are related to one another. Developing a sound architecture is the key to building good software. A well-formed architecture begins with clear definitions of requirements, services (facilities) to satisfy those requirements and interfaces for these services. This provides for a flexible implementation while preserving interoperability and mission support. The architecture also dictates how well the software will perform over time, as requirements change and technology evolves. Developing architectures, however, is difficult and as Mowbray and Zahavi note that "in order to design a successful, long-lived software architecture, software engineers must have an understanding of standards"[2]. They see the process of selecting and implementing a standards strategy as the first step in their systems integration approach.

The standards strategy is the roadmap for developing information systems. One of the most important portions of any standards strategy is the standards profile which contains the accepted and preferred system technology based on commonly accepted groups of requirements. For the DoD, the standards strategy and profile are contained in the Technical Architecture Framework for Information Management (TAFIM) [1]. The main objectives of the TAFIM are to 1) improve user productivity by promoting consistent user interfaces and data sharing, 2) improve development efficiency through common applications development, software reuse, and resource sharing, 3) improve portability and scalability, 4) improve interoperability by developing common infrastructures and promoting standards, 5) promoting vendor independence by developing interchangeable components and producing non-proprietary specifications, and 6) reduce life-cycle costs by reducing duplication of efforts and reducing software maintenance costs.

Additionally, the TAFIM provides guidance for designing client/server applications, the type of applications required by MIDB and CIO. In this environment, servers distributed throughout a network provide services to client applications. The guidelines suggested by the TAFIM include:

The Common Object Request Broker Architecture and the revision to the Ada programming language - Ada 95, can provide systems within these guidelines.

2.1 Common Object Request Broker Architecture

The Common Object Request Broker Architecture (CORBA) is a set of commercial specifications produced by the Object Management Group, a consortium of hardware and software interests representing industry, government and academia. All CORBA specifications are consistent with the current TAFIM standards framework. CORBA has been described as being a "distributed object bus" [3] for software components, much like a hardware bus is for electronic components. A bus being the medium through which data is transferred.

Officially, CORBA is a set of specifications defining:

Figure 2. OMG Architecture

In CORBA-based applications, everything is defined in terms of objects which can alternate between client and server roles. An object is a client when it initiates interaction with another object. An object is a server when it receives and responds to requests from client objects. There are several benefits to basing applications on CORBA and include the fact that low-level communication protocols are hidden from the programmer as is the location of server objects. Additionally, CORBA is hardware, operating system and programming language independent; interoperable, and interchangeable. It is easy to see that CORBA fulfills the guidelines set forth by the TAFIM.

2.2 CORBA Interface Definition Language

Programming language independence in CORBA is achieved through the use of an Interface Definition Language (IDL). IDL is the notation strictly used for specifying the interfaces of CORBA objects. Object interfaces contain only the definitions for the attributes and operations for a particular object and carry no behavioral information, much like an Ada package specification. IDL also supports inheritance across interfaces, allowing existing interfaces to be extended. The syntax to IDL is very similar to that of popular object-oriented programming languages. Many commonly used constructs can be defined in IDL including:

Implementing an IDL interface requires the interface to be compiled into a programming language that has a mapping from IDL. Current mapping exist for Ada 95, C, C++ and Smalltalk with mappings in the process for Fortran, COBOL, CLOS and Java. IDL is currently in the Committee Draft stage as part of the fasttrack process for standardization by the International Standards Organization (ISO). Adoption by ISO will mean that all application program interfaces (APIs) can be defined by a single, implementation dependent format, regardless of whether a CORBA-based implementation is used.

The emergence of a universal language for specifying APIs has both near-term and long-term implications. In the near-term, developers should start defining APIs in IDL and implementing systems in a cost-effective, reliable manner. Since IDL is a commercial standard, it may be possible to purchase much of the required functionality based on the desired interface, and develop the rest as needed. IDL definitions either exist or have been proposed for many common application facilities such as user interface, information management, systems management and task management. This enables the focus to shift to developing vertical or domain specific facilities, such as imagery or intelligence. To restate, the key is to have a stable, refined interface defined in IDL. As long as the interface does not change, it is likely that commercial implementations will be developed, thus freeing the organization from maintaining their proprietary implementation.

2.3 IDL and Ada 95

One of the languages that has an IDL mapping is the recent revision to Ada, popularly distinguished as Ada 95 [5]. Among the features new in Ada 95 are full support for object-oriented programming including inheritance and polymorphism, improved support for programming in the large with the addition of a hierarchical library structure, an improved tasking model, and better support for interfacing to other programming languages and commercial off-the-shelf software.

The IDL mapping to Ada 95 is a fairly natural one as Ada package specifications already contain many characteristics of IDL. Table 1. summarizes the how IDL is mapped into Ada 95.

    IDL Construct 			Mapping to Ada 95            

	File 					Package 

	Module                 			Hierarchical unit (package)            

	Interface              			Package w/ tagged type                 

	Operation              			Primitive subprogram                   

	Attribute              			Set_attribute and attribute_Of  subprograms                          

	Inheritance            			Tagged type derivation                 

	Data type             			Ada types                              
             	arithmetic             		        arithmetic types                       
		enumerations           			enumerations types                     
		structs, unions        			records, variant records               
		sequence               			instantiation of sequence generic      
		arrays, strings        			array types, strings                   

	Exceptions             				Exceptions and record type

Table 1. IDL to Ada 95 Mapping

A more specific example of how IDL maps into an Ada package specification is shown below. Consider the IDL definition of a person interface that has a name and an age as attributes and operations to eat and sleep as shown below.

/* Class definition of a person */
interface Person
    /* attributes */ 
    attribute string name;
    attribute integer age;

    /* exception definition */
    exception Busy {string explanation;};

    /* operations with exception than can be raised*/
    void Eat(in short how_much) 
	raises {Busy};

    void Sleep(in short how_long)
	raises {Busy};

This maps into the following partial Ada 95 package specification:

package Person is
  -- the type for the object reference
  type Ref is new CORBA.Object.Ref with null record;

  -- generated for the attribute - Name
  procedure Set_Name(Self : in Ref; 
                     To   : in CORBA.String);
  function Name_Of(Self : Ref) return CORBA.String;

  procedure Eat(Self     : Ref; 
                How_Much : in CORBA.Short);
  Busy : exception;

end person;

3. Heterogeneous Database Access

As previously stated, one of the major problems facing the intelligence community is how to integrate information from multiple sources and platforms containing heterogeneous data. The intelligence community has begun to adopt a TAFIM based approach by adopting SQL and by specifying a common database schema for common entities in different databases (definition of a unit in database x is the same as that in database y). They still, however, are faced with the task of maintaining sets of exported SQL scripts (which are largely vendor specific) for manipulating common data in different formats, and relying on vendor-specific multimedia interfaces. CORBA technology promises to help alleviate these problems. The goals of the intelligence community are to minimize the impact of database changes on the developers who are creating systems to access these databases, and to protect these databases from inadvertent corruption.

CORBA solutions not only work with new systems, but can also be used when integrating legacy systems. This is accomplished by creating object wrappers. Object wrapper techniques include:

Of these techniques, wrappers for mediators and brokers are of the most interest when integrating heterogeneous data sources. This is a "smart wrapper" that can, among other things, perform data conversion as well as actively search out data from other sources, all transparent to the client. The client makes a simple request and the broker or mediator does the rest. It is this form of wrapper that was used in the reference implementation sponsored by DISA.

3.1 Reference Implementation

The purpose of the reference implementation was to demonstrate the applicability of CORBA and Ada 95 to systems developed for the Intelligence community. The main goal was to demonstrate that CORBA and Ada 95 technology could be used to seamlessly integrate different forms of data, while hiding the details from client applications [6]. For our purposes, we chose to integrate a traditional relational database with images stored on a Unix file system. The tools used to create the implementations are shown in the Table 2 below.

ORBOrbix (Iona)
CompilerGNAT (NYU/ACT)
GUI BuilderScreenMachine (OIS)
CASE ToolParadigm Plus

Table 2. Reference Implementation Tool Suite

The application itself maintained and manipulated data on villages and people living in any region of the world. Users could update and retrieve information on villages and people to include both textual information and photographs. The focus was not on the application, but on demonstrating that the supporting technology not only existed, but could be used in this application domain. The details of the design and implementation follow.

3.2 Design and Implementation Overview

A multi-tiered approach was used for the overall system consisting of a client used only for the presentation layer (user interface), a server which contained the business logic and responded to requests from the client through an object request broker (ORB), and a relational database management system (Oracle) as shown in Figure 3. The server communicated to the database via an Open Database Connectivity (ODBC) interface which is used for portable, distributed, SQL compliant database access.

Figure 3. Reference Implementation General Architecture

For the design, we decided to proceeded as if the database schema already existed (which would be the case in a legacy application). With this assumption, the first step was to take the database schema and wrap it in IDL. Wrapping means to develop an IDL specification mapping the database tables into IDL interfaces, with the column names being the IDL interface attributes. For this reference implementation, the mapping was one-to-one, however, this will more than likely not be the case in a real application. As an example, consider the following table structure shown in Figure 4.

Figure 4. Reference Implementation Entity-Relationship Diagram

This mapped into the following IDL interfaces:

interface Village
  attribute string Name;
  attribute string Location;
  attribute Photo Current_Photo;
  /* Plus operations to add and  
     remove people */

interface Person
  attribute string Date_Of_Birth;
  attribute string Gender; 
  attribute string Name;
  attribute string Lives_In;
  attribute Photo  Current_Photo;
  attribute string Comments;


interface Civilian : Person { attribute string Occupation; }; interface Warrior : Person { attribute string Rank; attribute string Unit; }; interface Photo { attribute string filename; attribute string caption; };

Additional container classes were used to store collections of people and villages as well as factory classes to create civilians and warriors. For brevity, these were not shown here. Additionally, the data types of many of the attributes are shown as strings in this example when in the real implementation they were mostly user-defined types. A complete listing of the IDL specification can be found in the CORBA/Ada 95 Reference Implementation Server Interface Definition Report [7] which can viewed in the standards document library at http://www.itsi.disa.mil. Of particular interest, however, is how the image data was handled in the design. In Figure 4. notice the relationship established by the current_photo field in the person and village tables. If an image is present, this field contained the full file name of the image associated with the object. In IDL this mapped into the attribute, Current_Photo, found in the person and village interfaces. Using a combination of either the person or village interface with the photo interface, the client software can retrieve and display the associated photograph without additional user action. Clients only have knowledge of the interface presented and do not have to know the underlying data representation.

Once the IDL interface definitions were complete, the next step was to generate Ada code by running the specifications through an IDL compiler. This produces at a minimum, two Ada 95 packages (specification and body of each), a client stub and the interface implementation. Client applications make calls against the operations defined in the client stub. The functionality of the objects are added to the package representing the interface implementation.

A simplified example of how client applications can get and display a photo of a person in Ada 95 is as follows:

procedure Demo is
  The_Person  : Person.Ref;
  His_Age     : Person.Age_Type;
  The_Village : Village.Ref;
  The_Photo   : Photo.Ref;
  -- Given a village, give me the person with the name
  The_Person := Village.Person_Of(The_Village, “Mr. CORBA”);

  -- Given a person, get his age (text stored in RDBMS)
  His_Age := Person.Age_Of(The_Person);

  -- Given a person, get the photograph (an image)
  The_Photo  := Person.Current_Photo_Of(The_Person);

  -- Display the photograph (server takes care of display)
end Demo;

As shown, the client application simply requests the photo of the person from the particular person object. Then the client application asks the photo to display itself on the client display. Although the actual interface is simple, at this level the example demonstrates that CORBA technology can be used to achieve seamless (to client applications) heterogeneous database access.

4. Summary and Conclusions

In conclusion, we have demonstrated that CORBA fits well within a standards based architecture and can be used with existing technology to provide heterogeneous database access. CORBA-based systems promise a stable, lasting foundation for interoperable, portable and maintainable systems. The proof of this claim will be seen as vendors begin to implement the CORBA 2.0 interoperability specification. Building upon this reference implementation, the next step for the MIDB program is to develop a migration strategy to incorporate this technology into their long-range plans and to investigate scaling up the reference implementation into a more complete system. The Common Imagery Working Group is already using IDL to define the CIIF and defining additional common facilities within their own migration architecture. The expectation is that IDL, Ada 95 and CORBA will provide a key means of integrating these two architectures.


[1] Defense Information Systems Agency, Technical Architecture Framework for Information Management, Vol. 1, Version 2.0, 30 June 1994.

[2] Mowbray, Thomas J. and Zahavi, Ron, The Essential CORBA, Wiley Press, New York, New York, 1995.

[3] Orfali, Robert, Harkey, Dan and Edwards, Jeri, The Essential Distributed Objects Survival Guide, Wiley Press, New York, New York, 1996.

[4] Object Management Group, Object Management Architecture Guide, Revision 3.0, Wiley Press, New York, New York, 1995.

[5] International Standards Organization, Reference Manual for the Ada Programming Language, ANSI/ISO/IEC-8652:1995, February 1995.

[6] Defense Information Systems Agency, CORBA/Ada 95 Reference Implementation System Identification Report, 9 June 1995.

[7] Defense Information Systems Agency, CORBA/Ada95 Reference Implementation Interface Definition Report, 28 July 1995.

William W. Pritchett IV

Mr. William Pritchett is a Senior Computer Systems Analyst with the Systems & Technology Division of CACI, Inc.-FEDERAL in Fairfax, Virginia where he provides technical services on a variety of software engineering topics to Government clients. Before joining CACI, Mr. Pritchett was an electronics engineer with the Indian Head Division of the Naval Surface Warfare Center in Indian Head, Maryland where he designed simulators for surface launched guided missiles.

Mr. Pritchett received a B.S. degree in Electrical Engineering from Virginia Polytechnic Institute and State University and was later awarded an M.S. degree in Computer Science from The George Washington University. He is currently pursuing his Ph.D. in Information Technology from George Mason University.

William Pritchett
3930 Pender Dr.
3rd Floor
Fairfax, VA 22030
Voice : (703) 277-6765
Fax: (703) 277-1025z
Internet: bpritchett@std.caci.com

Huet C. Landry

Huet Landry is currently a Standards Development Analyst in the DISA Center For Standards, where he is in charge of Software Engineering Environment Standards - including Object Technology and Languages. Mr. Landry also serves as the Chairman of the Object Management Group's (OMG) CORBAUsers SIG, which is responsible for defining requirements for CORBA technology.

Within the DOD, Mr. Landry is a member of the Intelligence Systems Board Standards Panel, where he is working to develop a migration strategy for greater use of Object and Internet Technologies. He is also supporting similar work for the DIA's Multiplatform Integrated Database (MIDB) project.

Prior to his current position, Mr. Landry served 12 years as an Air Force officer, completing his service as the first Executive Officer for the Center for Standards. Other previous assignments include tactical C3I systems testing, technical trainer (Systems Analysis and Design) and Air Force Command level systems acquisition and development planning.

Huet C. Landry
10701 Parkridge Blvd.
Reston, VA 22091-4398
Voice: (703) 735-3565
Fax: (703) 735-3257
Internet: landryh@ncr.disa.mil