Asp programming

Microsoft IIS, COM+, and ASP

Microsoft doesn't actually sell an application server, so what the heck is it doing in this roundup? Well, a great many sites are using Microsoft technologies to do the very things that application servers do. One advantage to this approach is that if you're using Microsoft Windows 2000 Advanced Server, you've already got the engine for application server functions.

Other pieces of Microsoft's solution are Visual Studio, Internet Security and Acceleration Server (ISA), and Application Center. The Microsoft equivalent to J2EE's EJB components is COM+ objects, and instead of JSP (Java Server Pages), we have ASP (Active Server Pages). One advantage of COM+ objects is that they can be built in several languages, and multilanguage programming is common in COM+ systems.

ISA can improve the performance of a Web site by caching frequently accessed static content. It handles requests directly, without having to contact an upstream Web server. ISA servers can also be configured in arrays, for fault tolerance and load balancing. Application Center helps administrators work with clusters, letting them, for example, change configuration settings on all servers or perform rolling upgrades of applications.

At the heart of COM+ is a simple premise: Write the code and declare what services it requires. Developers can write code using RAD (rapid applications development) tools such as Visual Basic (VB) or Delphi and package it into COM+ components, or they can get more control over components by coding in C++.

One of the key innovations in COM+ is the concept of attributes. An attribute is data that declares what services a component needs at runtime. In practice, different people decide on a component's attributes. In one case, a developer declares that one component must have a transaction. In another case, an administrator declares which groups of users are allowed to access a given component.

Attributes are the key to extending COM+; developers can add future services by defining new attributes. Alas, only Microsoft can add new attributes today. This limitation is removed in the .NET Framework, where developers can define their own attributes. Visual Studio.NET is currently in beta, and Microsoft says it will ship this year.

Another important service that COM+ provides is its thread and database connection pools. New requests are executed on the next available thread from the pool. Code that uses the Active Data Objects (ADO) API to talk to databases gets connections from the pool.

COM+ also provides a simple, transparent security model. A component can request that COM+ perform access checks prior to having its methods invoked. Developers define the roles that are allowed access to the component. Administrators define which Windows user groups (or users) belong to which roles.

IIS can broker incoming HTTP requests and hand them off to COM+ components for processing. COM+ applications bypass an extra network round-trip to a separate application server, resulting in increased throughput. This is why logical three-tier, physical two-tier designs are popular in COM+ systems.

ASP exposes an object model to developers working in scripting languages such as JScript, PerlScript, or VBScript. This object model lets applications interact with three key sub-systems: the current HTTP request, the current COM+ transaction (if any), and external COM+ components. Each ASP first receives and pre-processes an incoming HTTP request. Next, it instantiates a COM+ component in the business tier and invokes methods using DCOM (Distributed Component Object Model). Finally, it generates the HTTP response and releases its reference to the COM+ component.

We set up our test bookstore site using the same machine configuration as the J2EE products. The two front-end Web servers were load-balanced using a simple round-robin DNS scheme. They delegated the real work to the three application servers running our business logic.

Our testing of Microsoft, though not directly comparable, because it used a different code base than that used for testing the J2EE products, showed COM+ performing on a par with the fastest J2EE products when using Oracle and even faster with Microsoft SQL Server.