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COM+ - the backbone of Windows DNA

Author: Mahesh Bhide
Date Submitted: June 1, 2000
Level of Difficulty: Enterprise
Languages Covered: COM+
Pre-required Reading: Windows DNA, COM, MTS

Introduction:

This article assumes that the reader is familiar with n-tier application development methodologies. The article explains the importance of use of COM+ in applications designed around Windows Distributed interNet Architecture (DNA).

Before finding out how COM+ is a backbone of Windows DNA, lets briefly consolidate our understanding about Windows DNA and COM+.

What is Windows DNA?

Microsoft Windows Distributed interNet Applications (Windows DNA) is an architecture that enables developers to integrate Web-based and client/server applications in a single, unified architecture.

Windows DNA integrates - and offers solution for - every aspect of Internet application development, including:

Directory Services, Component Services, Web Services, Data Access, Message Queuing, Online Business, XML Interoperability and Workflow, Host Interoperability, Database, Development Tools, Messaging and Collaboration.

The goal of Windows DNA is to help developer concentrate more on business logic than on systems integration.

What is COM+?

COM+ is the evolution of Component Object Model (COM) and Microsoft® Transaction Server (MTS). It is a run-time core that enables an application execution environment that allows developers choose the services for their applications (COM components) - the services such as transactions, security, queuing etc. So it actually provides a high performance application-hosting environment that's very easy to manage.

To understand how COM+ forms a backbone of Windows DNA lets build a scenario.

Lets say we have decided to develop a simple Web application running on Windows platform that interacts with database. We draw a multi-tier logical diagram for our application showing the application that will run at each tier.

User Services	Business Services	Data Services
Client using browser	Business Logic Components Data Access Components	Multiple Databases

Since we have decided to use Windows platform Windows DNA architecture helps us to choose the solution for the applications:

Browser Internet Explorer
Business Logic Components and Data Access Components will be developed as COM components using development tools like Visual Basic/Visual C++
Database Microsoft SQL Server 7.0
Web Server Microsoft Internet Information Server

The web application should be developed according to following performance guidelines:

The functionality should be made available to the user depending on his role
The application should support transactions to update databases reliably. In case of failure to update the original state of the database should restored.
The application should be scalable in the sense that the perceived performance shall not degrade when the number of users increases.

The above guidelines demand following actions for us, developers

The business logic components we develop should authenticate the user based on his/her role
We should develop the components so that they support transactions and transactions span component boundaries
To make components scalable we have to take care of resource management such as thread allocation and object pooling

COM+ comes to the rescue of developers

If we host our site on Windows 2000 which now includes Internet Information Server as a service, we can make use of COM+ that comes to the rescue of developers and takes care of all the actions we have listed above.

With COM+ now the developers do not have to worry about the thread management for their COM components. They can write components in any language that supports COM, including C, C++, Java, Visual Basic, Delphi, and even some implementations of COmmon Business Oriented Language (COBOL). Once the COM component is hosted in COM+ Application, COM+ will take care of thread management and object pooling.
Developers have to do minimal coding in their components to support transactions (inheriting transaction and using methods such as SetComplete and SetAbort). All they to do is host their component in COM+ Application and then choose and select the transaction properties out of - Requires a new transaction, requires a transaction, supports transaction and does not support transaction.
For authenticating the user COM+ offers a role-based security. Once the component is hosted in COM+ application you add roles (could be Windows NT groups) that can access these components. The same security could be extended to authenticate users to use SQL server.

If you are using Windows NT then the same can be achieved with the help of Microsoft Transaction Server. In fact COM+ is combination of COM, DCOM and MTS. COM+ name is given to indicate that the new offering is essentially COM plus more.

This shows that COM+ in Windows 2000 is very important part of Windows DNA since it helps to integrate all tiers (User, Business, and Data) in a unified interface.

Here we have just covered the importance of COM+ in Windows DNA. As you start exploring COM+ you will realize how easy it is to use COM+ and take full advantage of the services it offers.

Summary:

In this article we saw how Windows DNA helps us to choose solution for every aspect of Web application and importance of COM+ in integrating these solutions easily. COM+ is a combination of COM, DCOM and MTS along with some new feature such as Queued Components, Loosely Coupled Events etc.