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Michael Mraka edited this page Sep 18, 2017 · 3 revisions
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DEPRECATED, NO LONGER USED

PXT

Overview

PXT stands for Pseudo-XML Templates. Essentially, you embed XML tags within HTML that are replaced dynamically upon each request. This is similar to custom tag library in JSP.

PXT was developed internally in 2001 for three main reasons:

  1. Separation of presentation and logic: Unlike JSP, PXT does not allow you to embed actual code within the dynamic HTML documents. This forces the webpages to be legible, and therefore more maintainable. All the heavy lifting for dynamic code is relegated to Perl modules.

  2. It's lightweight. The early predecessors to the current Satellite code once had speed issues, largely due to the use of Apache::ASP and its overhead. A simplified XML parser allows for fast parsing, resulting in little overhead spent around a request.

  3. Reduced complexity of integrating XMLRPC, SOAP, and other XML calls. This allows a page that serves HTML to also service those types of calls. This means creating a command line interface for clients or other forms of automation will be much easier, as the code to handle both the HTML display and the XML response can be shared. This simplifies interfaces and increases code reuse.

Sample .pxt file

Here's a simple PXT file (foo.pxt):
    
	<pxt:use module="ColorSniglet" />	
	<html>
	  <my_tag>
            <body bgcolor={bgcolor}>
	      Hello world.<br>
	      The background color is {bgcolor}.
	      The posted color is 
                <pxt:formvar>{formvar:posted_color}</pxt:formvar>.
            </body>
	  </my_tag>
	</html>
        }}}

Valid PXT Directives

1. `<pxt:use module="Foo"/>` (synonymous with `<pxt:use class="Foo"/>`)

The above registers a perl module that will watch for tags to replace (e.g.
`<body_tag>`), execute callbacks, or handle XML-RPC/SOAP calls.  For
our example, let's say that the HTML output after foo.pxt processes
is:
{{{
#!text/html
	<html>
	  <body bgcolor=red>
	    Hello world.
	    The background color is red.
	    The posted color is blue.
	  </body>
	</html>

The perl module (known as a "Sniglet" by the Satellite Web Team) registered the <body_tag> tag in the following subroutine (in ColorSniglet.pm):

#!perl
	sub register_tags {
	  my $class = shift;
	  my $pxt = shift;

	  $pxt->register_tag("my_tag" => \&my_tag);
	}

The &my_tag is a reference to a function. The function name does not have to match the tag, though adopting that convention improves maintainability. Here is the source for the body_tag subroutine:

#!perl
	sub my_tag {
	  my $pxt = shift;
	  my %params = @_;
	  
	  # equivalent to $pxt->block, this returns everything between
	  # <my_tag> and </my_tag>, in this case:
	  #
	  #	Hello world.<br>
	  #	The background color is {bgcolor}.
	  #	The posted color is {formvar:posted_color}.
	  #	  
	  my $ret = $params{__block__};

          # subtag substitution is most easily done through a simple s///
          # reg-exp expression.  
	  my $ret =~ s/\{bgcolor}\}/red/gism;

	  return $ret;
	}

$ret is going to be the replacement value for everything between the <my_tag></my_tag> tags, inclusive. Note that as a point of style, no HTML exists in the perl module. Use subtags (things within {}'s) appropriately so that the separation between presentation and logic remains clear.

If two Perl modules register the same tag, the first one registered "wins" and determines what handles the tag.

  1. <pxt:include file="Foo.pxt"/> or <pxt:include file="Foo.html"/>

<pxt:include/> reads the contents of a file and dumps it into the current document. Any <pxt:XXXX/> tags within the included document will be parsed, regardless of its extension. Note that all files are relative to the document root of the current virtual host. Also note that parsing is re-entrant and recursive, meaning tags registered are not necessarily registered when the pxt:include tag is expanded.

  1. <pxt:formvar>{formvar:foobar}</pxt:formvar>

In the url http://www.foo.com/index.pxt?foobar=1 "<pxt:formvar>{formvar:foobar}</pxt:formvar>" is automatically replaced with "1" by PXT.

  1. pxt:comment yada yada </pxt:comment>

Your basic comment. Anything between the start and end tags will be utterly ignored by PXT, including valid PXT tags. This is different from an HTML comment which, while not affecting the rendering of a page, is still visible in the source of the page. pxt:comments are filtered out before serving.

Tag and Callback Handlers

Typical form interaction on the web usually falls into a simple pattern: display form, process results, display results. PXT accommodates this model through the use of tags and callbacks.

As discussed earlier, tags can create arbitrary HTML output. This obviously includes form elements, such as and so forth. So the first stage, displaying a form, is typically handled in this way. Whether your PXT tag generates all of the necessary form elements, or whether most are in the HTML itself is up to the designer. Typically, though, it is best to let an HTML designer make the form to their satisfaction, and then create a tag that renders the form itself and inserts appropriate values. For instance:

#!text/html
  <FORM METHOD="POST">
    Username: <INPUT TYPE="TEXT" NAME="USERNAME" VALUE=""><BR>
    Password: <INPUT TYPE="PASSWORD" NAME="PASSWORD" VALUE=""><BR>
  </FORM>

would become:

#!text/html
  <pxt:use class="Sniglets::Users">
  <rhn:login_form METHOD="POST">
    {login:error_message}
    Username: <INPUT TYPE="TEXT" NAME="USERNAME" VALUE="{formvar:USERNAME}"><BR>
    Password: <INPUT TYPE="PASSWORD" NAME="PASSWORD" VALUE=""><BR>
    {login:login_hidden_formvars}
  </rhn:login_form>

The code to render the above would essentially return the block it is handed wrapped inside of a true

tag. Note the use of {formvar:USERNAME} to insert the value of a form variable named USERNAME, should it exist.

Also note the use of {login:error_message} -- this is where you might display something to the effect of "Invalid username or password" so that the user has visual feedback as to why they didn't log in. Such a situation is where the formvar:USERNAME substitution would make sense as well, so that the user need not retype their username. (Note: there is actually core functionality for generally handling displaying messages to the user like the above using a system of message queues. This is necessary when you wish to display a message outside of a block inside of the tag handler that might generate the tag).

The next phase is called a trap or callback, which corresponds to the "process results" stage. This phase would handle events such as: verify username and password; update a database record; load more database records to display. In other words, this is the place you place your code that handles various input form variables and decides what to do with them. In the above example, you would use the USERNAME and PASSWORD form variables to either log a user in or present them with an error.

A callback is similar to a tag, in that the first parameter it receives is the ubiquitous $pxt object. However, it receives no other parameters, because it isn't declared as an XML tag (in other words, where a tag handler would receive tag attributes, the callback handler receives nothing).

Typically, a callback will verify data (though often it isn't capable of doing so meaningfully without tying unnecessary business rules at this level). It will then either shove it in a database, update a given record, or perhaps register a user as logged in. It could also perform some kind of search, or any number of things. Once it is done, the callback can either simply return, which then tells PXT to go ahead and render the page loaded, or it can issue a redirect, so that the user is sent to a different page (for instance, the page that shows that you have successfully logged in, or the page that will display search results). Use of redirection depends largely on the ACTION= attribute of the form itself. Typically, leaving ACTION= blank and issuing a redirect to wherever a request should go is what seems to work well, but there is no clear best practice in this case.

PXT itself decides whether or not to execute a callback based on a simple test. If there is a form variable named pxt:trap, and if a .pxt file has registered a callback whose name is the contents of that form variable, then it executes it. Otherwise it completely ignores it. Its very important to have a form's action handler point to a pxt that registers the callback you wish to use. Otherwise, it will silently ignore you. This is typically why I choose to have ACTION= point back to the same page. Also note that having one Sniglet point to a callback in another is bad practice, in that it creates possibly tangled dependencies. It should be avoided when possible, and documented clearly when it is necessary.

Tag handler can also have a priority. Lower numbers execute 1st and negative #'s are legal.

#!perl
  # order of 2 means it happens later than something of order < 2...  if you 
  # don't specify order, it defaults to 0.
  # negative orders are legal.
  $pxt->register_tag('some_tag' => \&some_tag_handler, 2);

PXT::Request object

Every tag, callback handler, XMLRPC handler, and SOAP handler receives a PXT::Request object (in Satellite code, it's usually the $pxt variable). This $pxt object is the first parameter passed to the handling function.

The $pxt object is the only way a given tag handler, callback handler, xmlrpc handler, or any other kind of handler can interact with the web server or user. The $pxt object is where you get information about the user who is logged in, any session data, form variables, cookies, pnotes, etc. It is absolutely discouraged to directly speak with the mod_perl Apache->request module in a Sniglet. All interaction MUST go through the $pxt object.

Since the $pxt object is the chief method of interacting with the server or user, it has a large number of methods. However, they can typically be grouped together into functional sets. For instance, there are the methods typically used when a PXT registration begins (register_handler, register_callback, etc). Others typically are used to ask Apache about the user's request: what IP address are they from, what headers did they send, what cookies did they send, what formvars they provided. Others alter the current state: redirect, for instance, pnotes, push_message. Some are shortcuts: prefill_form_values, message_tag_handler. However, all revolve around the same axiom: if you need to know something about the HTTP request, the user, or the server, use the $pxt object. It is your connection to everything that involves HTTP and the web server.

Adding pxt Directives

When the httpd/mod_perl gets a request for a .pxt file, it's sent to the PXT::ApacheHandler->handler function. All utility data objects, registers tag handlers and callbacks, does any redirect voodoo necessary, and also initiates the actual parsing/replacement of the tags. Adding any pxt:XXX/ directives will require full comprehension of the flow of execution from PXT::ApacheHandler->handler down.

PXT <--> Satellite

Overview

Creating a website to talk to Satellite using PXT involves a number of points of understanding. You need to first understand how PXT interact with sniglets, and then understand how to leverage the RHN::XXXX and RHN::DB::XXXX classes from within your sniglets. Then add to the list session management, and authentication issues.

TODO: make this pretty

Visually:

                           +-------------+
                           |  index.pxt  |
                           +-------------+
                           |   Sniglet   |
                           +-------------+
                           |    RHN::    |
                           +-------------+
                           |  RHN::DB::  |
                           +-------------+
                           |     DBI     |
                           +-------------+

Arranged in that way, it looks like a cake. No layer of the cake can touch (or even knows the existence of) any layer it doesn't touch. Specifically, a Sniglet handler doesn't talk to DBI; an RHN:: object doesn't talk to DBI; RHN:: doesn't speak HTML.

Sniglets and PXT

Sniglets are what the Satellite team calls the perl modules that register tag handlers and callback handlers. Building a web application primarily will involve writing pxt files containing html and PXT tags, as well as writing the sniglets that handle said PXT tags.

For Satellite, we've created a group of classes that contain the business logic, as well as the necessary knowledge about the database. The RHN::XXXX modules are the hooks to build websites upon. Nearly every RHN::XXXX module has a corresponding RHN::DB::XXXX module that communicates with the DB.

If you're going to be making a website that ties into Satellite, your sniglets will be making use of the RHN::XXXX modules. Occasionally, the RHN::XXXX functions will return an RHN::DB::XXXX object that you can use.

PXT Sessions

Included as a part of PXT is session handling, and is made available in the following manner:

#!perl
$session = $pxt->session();

The $session object itself is defined in RHN::Session, not under PXT::* The session object in PXT is created at the initialization phase of the request.

A session is a place to store permanent data A session is not a place to store transient data

RHN::Session utilizes a database, and there are better places than a db to store transient data.

Session data for a visitor is stored in key/value fashion, with the exception of the visitors user id. To set a new session variable during a request:

#!perl
	# some request
	$session = $pxt->session();
	$session->set('visits',1);

In order to access session information during a request:

#!perl
	# another seperate request
	$session = $pxt->session();
	$visits = $session->get('visits'); 
	print "You have visited $visits pages for this session";

Notice that you do not need to check for the existence of a session, because every user is given a session at the initialization phase. Also, note that a users uid is retrieved directly from the following session method and not from $session->get:

#!perl
	$session = $pxt->session();
  	$userid = $session->uid;

RHN::User Object in PXT

As a convenience you can access RHN::User via an accessor method in PXT. Note that unlike sessions, an RHN::User object is not created during initialization nor at any other time. A user of PXT retrieves the user object as follows:

#!perl
$user = $pxt->user();			#object returned from RHN::User
print "no such user exists" if(!$user);

In addition, you can clear the user from pxt, if you so wish:

#!perl
$pxt->clear_user();

XML-RPC and PXT

in mod_perl the httpd.conf controls what directories are "XML-RPC listeners".

These listeners correspond to an XML-RPC handler (which is nothing more than a wrapper around Frontier::RPC2) and a server file. There can only be one server file per directory. Listeners can branch out and obtain routines from other files, but there must be a single point of entry, making a somewhat rigid framework to work with... (Theoretically you could create a filetype as a handler for Frontier to handle directly, but it would be rather non-intuitive, and not exactly the "norm"..)

PXT works a little differently. Instead of registering directories or special filetypes, PXT uses the exact filenames that you would be talking to via a web browser.

For example,

http://localhost/foo/product_list.pxt and via a web browser could, say bring up a list of products on a nice html page... (maybe some buttons to some links to start messing with the data)...

http://localhost/foo/product_list.pxt called by an XML-RPC client would have the following methods available to them.

product.list - returns an arrayRef of objects product.modify - takes an arugment, returns the returned objects affected ..

The PXT would inherit a few functions from PXT itself... All describing methods about the functions that have been registered...

methods.list - returns a list of documentation on the above two functions