Developing Web Applications

Out-of-Process Components

Out-of-process components are COM components implemented as executables; these start as a separate process on the same computer as the client application. Out-of-process components, also called “local servers,” are different from out-of-process applications (known, once again, as isolated processes). When you start an out-of-process component on the Web server, IIS 5.0 becomes the client application.

COM objects are implemented inside a server. Most of the time, the COM server is implemented as a DLL that executes in the same process as the client application. Sometimes the COM server is implemented as an executable that runs in a separate process from the client. For instance, when you create an instance of an Active Document server such as Microsoft® Word, you actually start a copy of the server application. When you instantiate such an object from ASP, you create a new process on the Web server. Because the Active Document server is an executable rather than a DLL, it cannot be loaded into the IIS 5.0 process.

When you use Server.CreateObject in an ASP page to start an out-of-process component, and IIS 5.0 has not been configured to allow out-of-process components, it will return the following error:

Server object error 'ASP 0196'
Cannot launch out of process component
/myvroot/launch_exe.asp, line 16

This is the result of an ASP safety mechanism that prevents executables (but not DLLs) from being started directly from ASP.

There are several reasons for this safeguard. Not all executables are safe to use on the server, and some may pose security risks. Also, because in-process component DLLs are faster, more secure, and can be hosted by Component Services, they are much better suited for server-side use.

Furthermore, out-of-process components often create individual server processes for each object instance, reducing their performance to that of CGI applications. They simply do not scale as well as component DLLs that run in process with IIS 5.0 or Component Services. If performance and scalability are priorities for your site, using out-of-process components is strongly discouraged. On the other hand, intranet sites that receive moderate to low traffic might be able to use an out-of-process component without adversely affecting the site’s overall performance.