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This chapter is from the book

The facility that permits SQL Server to be queried via HTTP resides in SQLXML's ISAPI extension DLL, SQLISn.DLL, commonly referred to as SQLISAPI. Although the Configure IIS Support tool provides a default, you can configure the exact extension DLL uses when you set up a virtual directory for use by HTTP queries.

If you attach to IIS (the executable name is inetinfo.exe) with WinDbg prior to running any HTTP queries, you'll see ModLoad messages for SQLISn.DLL as well as one or two other DLLs. An ISAPI extension DLL is not loaded until the first time it's called.

Architecturally, here's what happens when you execute a basic URL query.

  1. You supply the query as a URL in a Web browser.

  2. It travels from your browser to the Web server as an HTTP GET request.

  3. The virtual directory specified in your query indicates which extension DLL should be called to process the URL. IIS loads the appropriate extension and passes your query to it.

  4. SQLISn.DLL, the SQLISAPI extension DLL, gathers the connection, authentication, and database information from the specified virtual directory entry, connects to the appropriate SQL Server and database, and runs the specified query. If the query was passed as a plain T-SQL query, it comes into the server as a language event. If it was passed as a template query, it comes in as an RPC event.

  5. The server gathers the requested data and returns it to SQLISn.DLL.

  6. The ISAPI extension returns the result data to the Web server, which then, in turn, sends it to the client browser that requested it. Thus, the original HTTP GET request is completed.

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