Developing Web Applications
Developing Web Applications |
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An ISAPI extension is a run-time dynamic-link library (DLL) that is usually loaded in the same memory address space occupied by IIS 5.0. Since it is a DLL, only one instance of the ISAPI extension needs to be loaded at a time. Of course, the ISAPI extension must be thread-safe, so that multiple client requests can be received simultaneously.
Although ISAPI extensions are more complex than CGI applications, ISAPI uses a relatively simple Application Programming Interface (API). For each client request, the Web server invokes the HttpExtensionProc ISAPI call and passes a pointer to an ISAPI Extension Control Block (ECB), which contains information about the request. The ISAPI DLL can use server callback functions to access information such as server variables. The ISAPI ECB also provides the developer with access to some general-purpose support functions, such as URL redirection, session management, and response headers, which are not available to CGI applications.
Despite the obvious benefits over CGI, ISAPI extensions present some maintenance problems. For instance, if you want to make even a minor change to the HTML returned by an ISAPI extension, you have to recompile and link it. Also, an ISAPI DLL can cause the Web server to crash, if it hasn’t been thoroughly tested and verified before being deployed and run in the Web server process.
You can select which ISAPI extensions are loaded in process with IIS 5.0 and which extensions should be loaded in a separate process. ISAPI extensions in a separate process can be stopped and restarted independently of the server process, and can be restarted automatically after a crash. Although out-of-process extensions are slower than in-process ones, being able to isolate and reload applications that are under development offers advantages in service reliability. For more information about out-of-process extensions, see Data Access and Transactions in this book.
ISAPI can also be used to create ISAPI filters. Filters are a fairly new concept in Web server extensibility—there is no CGI counterpart. ISAPI filters can intercept specific server events before the server itself handles them. The calling convention for filters is very similar to that of extensions. When a filter is loaded (usually as the Web service starts), it indicates what sort of event notifications it will handle. If these events occur, the filter has the option of processing the events, passing them on to other filters, or sending them to the server. In this way, you can use ISAPI filters to provide custom authentication techniques, or to automatically redirect requests based on HTTP headers sent by the client, such as Accept-Language.
However, filters can degrade performance, if they are not written carefully. With IIS 5.0, ISAPI filters can be loaded for the Web server as a whole or for specific Web sites. However, they cannot be run out of process.