ISAPI

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The Internet Server Application Programming Interface (ISAPI) is an N-tier API of Internet Information Services (IIS), Microsoft's collection of Windows-based web server services. The most prominent application of IIS and ISAPI is Microsoft's web server.

ISAPI Filters dialog in Microsoft IIS

The ISAPI has also been implemented by Apache's mod_isapi module so that server-side web applications written for Microsoft's IIS can be used with Apache.

1 Big Picture
2 Performance and compromise
3 ISAPI applications
4 ISAPI Development
5 See also
6 Footnotes
7 References
8 External links

[edit] Big Picture

The word "server" can refer to a computer ( "box" ) or a piece of software ( for example, SQL Server ). Microsoft's web server application is called Internet Information Services, which is made up of a number of "sub-applications" and is very configurable. ASP.NET is one such slice of IIS, allowing a programmer to write web applications in his or her choice of programming language ( Visual Basic, C#, J# ) that's supported by the Microsoft .NET CLR. ISAPI is a much lower-level programming system, giving much better performance, at the expense of complexity.

[edit] Performance and compromise

In contrast to CGI applications, an ISAPI application is loaded into the same process space as the IIS web server. This removes the need for inter-process calls allowing improved performance of ISAPI applications over CGI applications. However, if an ISAPI application crashes, it can cause the whole of the web server to crash along with it, if it is not configured properly (recommended mode: "High (isolated)").

Compared to PHP or ASP, which both are server-side scripting technologies, ISAPI is a real executable part of the system - thus: having the highest performance possible. To issue a statement: A Microsoft webserver executes ASP and PHP by running an ISAPI Server Extension. Comparing PHP/ASP with ISAPI is like comparing Microsoft Office with Microsoft Windows: Office can never run faster than Windows - its operating system - itself.

Before switching to ASP.Net Microsoft's MSDN-Library (msdn.microsoft.com) was fully developed with ISAPI. Ebay still uses the "eBayISAPI.dll", although they declare that they are using Java-Technology powered by Sun.^[1]

The greatest disadvantage of developing web applications at ISAPI level is the required development time, which is at least twice as high as using ASP or PHP. Another penalty is the lack of native support for many standard web application features such as Session handling, which are common features in technologies such as ASP and PHP.

[edit] ISAPI applications

ISAPI consists of two components: Extensions and Filters. These are the only two types of application that can be developed using ISAPI. Both Filters and Extensions must be written in C++ and compiled into DLL files which are then registered with IIS to be run on the web server.

ISAPI applications can be written using just C++ but there are a couple of C++ libraries available which help to ease the development of ISAPI applications. MFC includes classes for developing ISAPI applications. Additionally, there is the ATL Server technology which includes a C++ library dedicated to developing ISAPI applications.

[edit] Extensions

ISAPI Extensions are true applications that run on IIS. They have access to all of the functionality provided by IIS. ISAPI extensions are implemented as DLLs that are loaded into a process that is controlled by IIS. Clients can access ISAPI extensions in the same way they access a static HTML page.

[edit] Filters

ISAPI filters are used to modify or enhance the functionality provided by IIS. They always run on an IIS server and filter every request until they find one they need to process. Filters can be programmed to examine and modify both incoming and outgoing streams of data.

Filters are implemented as DLL files and can be registered on an IIS server on a site level or a global level (i.e., the apply to all sites on an IIS server). Filters are initialised when the worker process is started and listens to all requests to the site on which it is installed.

Common tasks performed by ISAPI filters include:

Changing request data (URLs or headers) sent by the client
Controlling which physical file gets mapped to the URL
Controlling the user name and password used with anonymous or basic authentication
Modifying or analysing a request after authentication is complete
Modifying a response going back to the client
Running custom processing on "access denied" responses
Running processing when a request is complete
Run processing when a connection with the client is closed
Performing special logging or traffic analysis.
Performing custom authentication.
Handling encryption and compression.

[edit] Common ISAPI applications

This is a list of common ISAPI applications implemented as ISAPI extensions:

Active Server Pages (ASP), installed as standard
ASP.NET, installed as standard on IIS 6.0 onwards
Perl ISAPI (aka PerlIIS), available for free to install
PHP, available for free to install

[edit] ISAPI Development

ISAPI applications can be developed using any development tool that can generate a Win32 DLL. Wizards for generating ISAPI framework applications have been available in Microsoft development tools since Visual C++ 4.0.

[edit] See also

[edit] Footnotes

^ eBay was initially developed using Microsoft technology for both the front end and back and processing (re: The Perfect Store by Adam Cohen). They later adopted a Java application server for the request processing but the web client software still uses an ISAPI application.