Active Client Pages at the Server

We said above that if you are using Ajax, the first thing that will happen in the server is that a Perl file will be executed. This executing Perl file may have to search for the string in a file (text), in a directory at the server that is different from the root directory. I show you how to achieve that in this section.

Well, I will give you a very basic explanation. A full explanation would call for a whole article. I will just show you how you can read the content of a whole file in a directory at the server and send it to the browser.

Let us refer to the directory at the server that has the executable Perl file as the current directory. This may be the cgi-bin directory. In this section, I show you how you can read the content of a whole file from this directory or from a sub directory of this directory. What I show you here will work with a text file or an HTML file; just replace the text file in the example below with an HTML file.

A filehandle

A filehandle is a special type of Perl variable that is associated with an output destination or input source. It is used to tell your program where you want output to go or input to come from. In our case, we want input to come from a file in the current directory or a sub directory to the current directory.

The following code, which is in the executable Perl file, will read the content of the  input.txt file in the same directory as the executable Perl file, and send to the Ajax function in the browser at the client (see explanation below).

open(INFO, "< input.txt") || die("can't open datafile: $!");

while ( <INFO> )

{

print $_;

}

close INFO;

The first statement creates and opens a filehandle called INFO. This file handle will represent the input.txt file. The first occurrence of INFO is in the first argument of the open() function. This function can be used to create and open filehandles.

There are two arguments in the function. The first one is the INFO filehandle. The second argument for the open() function is a string. There are two parts to this string. First you have the "<’"character. This character indicates that you are opening a file for reading. Next in the string, you can have a space of any length. After that, in the string, you have the name of the file. When there is no path associated with the file, as in the above code, it means that the file is in the current directory -- the same directory as the executable Perl file.

There are two functions in the first line of the code above, the open() function and the die() function (see explanation below). In between these functions you have the logical OR operator, ||. If the open() function fails, the die function sends an error message to the standard location where errors are sent; I will not say more than this about the die() function in this article.

If the opening of the filehandle is successful, the statements in the code below the first line will be executed. After the first line, you have a while-loop. In the while-condition you have the following expression:

<INFO>

The angle brackets and the filehandle inside the angle brackets copy the next line (terminated by the new-line character) in the file for each iteration, beginning of course from the first line. During each iteration, this next line is copied into the default variable, namely $_. For each iteration, the print statement sends the line of text to the Ajax function at the server. The Ajax function receives all the lines in one call.

After opening a filehandle, the filehandle has to be closed. You close the filehandle after using it. To close a filehandle, you use the “close” function as shown in the code above.

So the above code has to be in the executable Perl file called by Ajax from the client.

Now if the file is in a sub directory of the current directory, then you have to precede the file name in the open() function with the sub directories, separated by forward slashes. Let us say you have an immediate sub directory to the current directory called "subdir" and the file is inside this sub directory. The first line in the code would be:

open(INFO, "< suddir/input.txt") || die("can't open datafile: $!");

Getting content from the database

You can use Ajax at the client browser to get a string from a table cell of a database at the server. You need to know what to do at the server to get the string from the database. The explanation of what you have to do needs more than one article. I will not do that here. You can consult books or search for tutorials on the Internet.

With Active Client Pages, the analysis, logic and production of web pages for the web session are done at the client. You need just what I have explained in this article for the server. Well, as I said above, when it comes to databases, you still need to learn how to get data from the database tables; however, the bulk of the programming is at the client.

I hope you appreciated this article.