1.2.4. Common Gateway Interface

Definition

"Common Gateway Interface (CGI) is an important World Wide Web technology that enables a client to request data from a program executed on the Web Server. CGI specifies a standard for passing data between the client and the program." (WIKIPEDIA)

Connection of Client, Server and CGI Program

As you have seen in the unit URL we are able to execute queries by entering the query-strings (e.g. ?q=cartouche&q=e-learning) directly in the URL. These parameters are sent to the server. Since the Web Server cannot handle these query parameters a Common Gateway Interface is necessary to process the data.
In the unit URL we used the query-string "?q=cartouche&q=e-learning" with the parameters "q=cartouche" and "q=e-learning" in a searching machine to look for documents (web pages, pdf etc.) that contain these terms. But what happens with these terms? The following graphic gives you the solution.

CGI in connection of a google query

Important is, that the server calls the CGI to process the data. The CGI response is mostly an HTML-code, but it can also be in the form of XML, SVG, JPEG, etc.

Forms

Often, you do not have to enter the query-strings in the URL because the Web Page offers a form where you have to enter some terms. This can be your name, your credit card number, the terms in a searching machine, etc. After having entered all data, you typically have to click on a symbolic submit button to initiate the sending of the entered information back to the server machine.
This process is the same as it is described above. The input of the completed form is automatically transformed into the URL notation with the query-string. Therefore the information is sent to the server which calls the CGI to process the received data.

You find a typical form when using a search machine on the Web. You always have to enter the term you are looking for.

Exercise

Enter some searching terms in the search bar of an arbitrary search machine. After having pressed the "Start-Search-Button" have a look at the URL that will be created automatically. You don't have to understand the meaning of every single parameter but normally the meanings can be guessed.

You can discuss your results on the discussion board "Internet Techniques", if you want.