14.3.Virtuoso Server Pages (VSP)

The Virtuoso Server Pages subsystem is an integral part of the Virtuoso server. A VSP page is a file system or DAV resident resource that contains Virtuoso/PL code intermixed with HTML or other static content.

Figure14.14.VSP Conceptual Diagram

VSP Conceptual Diagram

Virtuoso can serve Web pages to HTTP 1.0 and HTTP 1.1 clients. The HTTP document root is set by the ServerRoot parameter in the [HTTPServer] section of the Virtuoso INI file. By default this is set to vsp sub-directory of the installation. If this parameter is not set, then the default document root will be the server's working directory.

Executing VSP Pages. Directories under the ServerRoot directory, including the root itself are not automatically allowed to execute dynamic pages such as VSP. A virtual directory must first be created with an appropriate VSP user assigned to permit execution of dynamic pages. After this, Virtuoso will execute files with a ".vsp " extension as a VSP page.]

The VSP Stored Procedure. Each VSP page constitutes a Virtuoso stored procedure named after the URI of the page by taking the URI and pre-pending the WS.WS. qualifier and owner. Hence the page /test.vsp becomes the procedure "WS"."WS"."/test.vsp" . This is automatically performed when a page is first requested. Subsequent requests to the page will not reference the file. The VSP engine will check for changes in the .vsp source file before calling the procedure and re-compile as required. The ws_proc_def() SQL function can be used to explicitly update the procedure if the .vsp file is changed.

The VSP Transaction. Each URL is executed in its own transaction. All SQL statements in a page procedure, whether on local or remote data, make up one transaction, unless transaction control statements are explicitly used to divide the page into multiple transactions. If the page procedure returns through completing execution or through a return statement or as a result of a 'no data found' condition, the transaction is implicitly committed. If this commit fails, the output is discarded and the error message indicating the commit failure is sent to the HTTP client. If a VSP procedure returns as a result of an error the transaction is rolled back and the output discarded. The client gets the SQL error message as the HTML body of the reply. A VSP procedure can commit or roll back explicitly with the commit work or rollback work PL statements.

If the client closes the connection to the server while server-side processing is taking place this will be detected by the server and the transaction will be eventually interrupted and rolled back in the same way as if an ODBC client had disconnected. To control the server reacting or not on the HTTP client disconnecting the SET HTTP_IGNORE_DISCONNECT = ON/OFF should be used.

Each VSP page-procedure is called with three arguments:

path - the URI path of the VSP page itself.
params - the parameters from a previous POST to the server.
lines - the complete set of headers received from the calling client.

Each VSP procedure runs in a context that implicitly contains the stream to the HTTP client. The arguments of the VSP page procedure are represented as arrays of strings. For example, GETing the URL:


would cause the following arguments to be given to the page-procedure WS.WS./x/y.vsp :

path     ('x', 'y.vsp')
params   ('arg1', '1', 'arg2', '2')
lines    ('GET /x/y.vsp?arg1=1&arg2=2 HTTP/1.1', 'Host: www.test.net', ...)

Arrays are marked in parentheses with elements separated by commas. The page procedure is called as a result of either a GET or POST request in either HTTP/1.0 or HTTP/1.1. In the event of a POST request the params contains the post data.

Virtuoso can be configured to proxy certain requests to another web server. This allows using another web server for running cgi-bin's, Java servelets or other web server functions. Virtuoso, however, is capable of hosting many other dynamic engines also, such as PHP, JSP and ASP.Net.