Migrating a Web Server to IIS 5.0

Performance vs. Development Work

Regarding the first consideration, ISAPI and the ASP scripting environment yield applications that are faster and more scalable than CGI applications. However, the work involved to migrate a particular CGI application to ISAPI or ASP can vary a great deal, depending on the structure of the application. In the case of a simple, clearly structured CGI application, you may be able to simply preserve the business logic and implement IIS 5.0 built-in input and output techniques. For a discussion of this approach, see Migrating from CGI to ASP. However, if the application is complex and the business logic is not easily separated from the rest of the application, rewriting it can be time-consuming. At this point, you need to carefully weigh the expected performance gains against the required effort.

Performance differences between CGI, ISAPI, and ASP are due to differences in server process overhead. CGI applications do not use server resources efficiently because a new, separate process is created for every client request, even if a single client submits more than one request. At any given time, the server must support a separate process for each ongoing request.

Unlike CGI applications, ISAPI extensions and ASP applications can be configured to either run in the same process as the Web service, or run in a pooled process. The server does not create a new process for each request, which greatly reduces the drain on resources. As a result, some benchmarks show that CGI applications run anywhere from two to three times slower than ASP applications and from three to five times slower than ISAPI extensions.

Besides avoiding server process overhead, ASP provides other performance-enhancing features, such as ODBC connection pooling and Microsoft® OLE DB session pooling, as described in Data Access and Transactions in this book.

Besides using more resources, CGI applications scale poorly on IIS 5.0. Adding additional processing power or RAM typically does not allow the application to support correspondingly more concurrent users. Therefore, if a CGI application is heavily used, you have compelling reasons to move to a solution that uses ISAPI or ASP, even when the application is complex and will be time-intensive to convert.

In deciding between ISAPI and ASP, bear in mind that ISAPI extensions offer some performance advantages over ASP applications, but ASP applications can often be developed more quickly.