man Embperl::Intro (3): Introduction to Embperl

Content

What is Embperl?
Embed Perl Code in HTML Documents
Meta-Commands
Dynamic Tables
Form Processing
Maintaining persistent (session) data
Breaking up your code in components
Debugging
Database Access
Security
Escaping/Unescaping

What is Embperl?

Embperl has started as a Perl module for simply embedding Perl into HTML and has grown to a full featured system to build dynamic content (not only) under mod_perl. The version 1.x focus on HTML documents, also it could be used for any sort of ascii files, and brings a lot of features especially useful in a web-environment. This features includes handling of form data and dynamic HTML tables/lists, session management and context sensitv escaping and unescaping. More over you can break up your documents in small reusable components/objects and build a object-oriented website out of such objects, by using inheritence and specificly overriding parts of the page. Also Embperl can cope with pages that are screw up by high-level HTML editors, so your designer can still use there favorite tool.

Embperl 2.0, which is a complete rewrite of the Embperl core, is not even much faster then 1.x, but adds new possibilities. You can extent or define your own syntax, thus giving the chance to trigger actions on certain tags or inventing your own tags (creating a taglib). It is much more modularized, so specific steps could be replaced by custom processor and more then one processor can act on a document before it goes to the browser (just like a Unix pipe). To enhances performance 2.0 indrocuces caching of the output or intermediate steps.

Due to this modularization, it is now possible, to replace Embperl parser by an XML parser and to do XML processing, for example by pluging in an XSLT processer in the processing pipeline. Embperl 2.0 can utilize libxml2 and libxslt for XML and XSLT processing.

All versions of Embperl can be used offline (as a normal CGI script or as a module from other Perl code), but its real power comes when running under mod_perl and Apache. It's directly integrated with Apache and mod_perl to achieve the best performance by directly using Apache functions and precompiling your code to avoid a recompile on every request.

How does it compare to other templating solutions ?

Embperl is not the only processor for embedded Perl code. I guess every second webprogrammer builds his own templating engine. Most of them are very similar and easy, but do a good job in a certain environment. Often after a while these programmer discover, they need more features and that other people in the same situation has already build a soltuion for their problems. At the moment there are five widely used Perl modules for generating dynamic web content (Embperl, Apache::ASP, HTML::Mason, Template::Toolkit and AxKit). All of these have their special strength. So why to use Embperl? We believe that Embperl, especialy the version 2.0, covers all of the aspects of these modules and integrates them in one module with addtionaly benefits that are unique to Embperl. Addtionaly Embperl is the fastes of these solutions, because it's engine is totaly written in C and is optimized for delivering dynamic content online.

A another competitor for Embperl is PHP. PHP is developing a strong user base, because it is rumored to be easy to learn, and was designed specifically for HTML. Also PHP is probably one of the strongest open source alternatives to using Perl in your HTML, it's target is very webcentric and you may discover at a certain point that is has it's limitations when you try to realize great projects. Also it's not true, like some anecdotal stories on the Web might want to make you belive, that PHP is faster then Perl. Perl, and therfore Embperl also, scales and performs very well for high end solution.

Focus of this document

This tries to be an introduction to the basics of Embperl. ``perldoc IntroEmbperlObject'' gives you an tutorial about how to build an object-oriented website.

How to Embed Perl Code in HTML Documents

Perl code can be embedded in three ways:

1.) [- ... -] Execute code

    [- $a = 5 -]  [- $b = 6 if ($a == 5) -]

The code between the [- and the -] is executed. No output will be generated in the HTML. This is mainly for assignments, function calls, database queries, etc.

2.) [+ ... +] Output the result

    [+ $a +]  [+ $array[$b] +] [+ "A is $a" +]

The code between the [+ and the +] is executed and the return value (the value of the last expression evaluated) is output (sent to the browser in the HTML stream).

3.) [! ... !] Execute code once

    [! sub foo { my ($a, $b) = @_ ; $a * $b + 7 } !]

Same as [- ... -], but the code is only executed for the first request. This is mainly for function definitions and one-time initialization.

Meta-Commands

Embperl support some meta commands to control the ``program flow'' within the Embperl document. This can be compared to preprocessor commands in C. The meta commands take the following form:

    [$ <cmd> <arg> $]

if, elsif, else, endif

The if command is just the same as in Perl. It is used to conditionally output/process parts of the document. Example:

 [$ if $ENV{REQUEST_METHOD} eq 'GET' $]
    This is a GET request
 [$ elsif $ENV{REQUEST_METHOD} eq 'POST' $]
    This is a POST request
 [$ else $]
    This is not GET and not POST
 [$ endif $]

This will output one of the three lines depending on the setting of $ENV{REQUEST_METHOD}.

while, endwhile

The while command can be used to create a loop in the HTML document. For example:

 [$ while ($k, $v) = each (%ENV) $]
    [+ $k +] = [+ $v +] <BR>
 [$ endwhile $]

The above example will display all environment variables, each terminated with a line break.

do, until

The do until also create a loop, but with a condition at the end. For example:

 [- @arr = (3, 5, 7); $i = 0 -]
 [$ do $]
    [+ $arr[ $i++ ] +]
 [$ until $i > $#arr $]

foreach, endforeach

Create a loop iterating over every element of an array/list. Example:

 [$ foreach $v (1..10) $]
    [+ $v +]
 [$ endforeach $]

var <var1> <var2> ...

By default, you do not need to declare any variables you use within an Embperl page. Embperl takes care of deleting them at the end of each request. Sometimes, though, you want to declare them explicitly. You can do this by using var:

 [$ var $a @b %c $]

Has the same effect as the Perl code:

 use strict ;use vars qw {$a @b %c} ;

hidden

hidden is used for creating hidden form fields and is described in the form field section below.

Dynamic Tables

A very powerful feature of Embperl is the processing of dynamic tables. This feature was designed mainly to display Perl arrays (one or two dimensional, regular and irregular), but can also be used in other ways.

Display a Perl Array

 [- @a = ( 'A', 'B', 'C') ; -]
 <TABLE BORDER=1>
   <TR>
        <TD> [+ $a[$row] +] </TD>
   </TR>
 </TABLE>

The above example simply displays a table with three rows containing A, B and C.

The trick is done by using the magical variable $row which contains the row count and is incremented for every row. The table ends if the expression which contains $row returns <undef>. The same can be done with $col for columns and $cnt can be used if you need a table which wraps after a certain number of elements.

This works with table/select/menu/ol/dl/dir

Simple DBI Example

Here is a simple DBI example that displays the result of a query as a two dimension table, with field names as headings in the first row:

 [-
 # connect to database
  $dbh = DBI->connect($DSN) ;
 # prepare the sql select
 $sth = $dbh -> prepare ("SELECT * from $table") ;
 # excute the query
 $sth -> execute ;
 # get the fieldnames for the heading in $head
 $head = $sth -> {NAME} ;
 #continues on the next page...
 # get the result in $dat 
 $dat = $sth -> fetchall_arrayref ;
 -]
 <table>
    <tr><th>[+ $head->[$col] +]</th></tr>
    <tr><td>[+ $dat -> [$row][$col] +]</td></tr>
 </table>

Form fields

Posted form data available in %fdat/@ffld

The hash %fdat contains all values of form fields. The array @ffld contains the names in the order in which they were submitted.

Input/Textarea/Select tags take values from %fdat

If you do not specify a default value for an input tag and a value for that input tag is available in %fdat, Embperl will automatically insert this value and send it to the browser. This is similar to the behavior of CGI.pm. This means that if you post a form to itself, the browser will display the values you just entered.

[$ hidden $]

[$ hidden $] creates hidden form fields for all fields not in another input field. This can be used to transport data through confirmation forms. (For example, a wizard.)

A simple Text input / Confirmation form

The following example shows many of the possibilities of Embperl. It's a simple form where you can enter your name, your email address and a message. If you hit the send button, you see the data you just entered and can confirm the information by hitting the ``send via mail'' button, or you can go back to the input form to change the data. If you confirm your input, the data will be sent to a predefined e-mail address. The example also shows how you can implement error checking---if you miss your name or your e- mail address, you will get a corresponding error message and the input form is shown again.

The first part is the error checking; the second part the confirmation form; the third part sends the mail if the input was ok and is confirmed; the last part is the input form itself.

Depending on the values of $fdat{check}, $fdat{send} and if $fdat{name} and $fdat{email} contains data, the document decides which part to show.

 [-  $MailTo = 'richter\@ecos.de' ;
  @errors = () ;
  if (defined($fdat{check}) || defined($fdat{send}))
    {
    push @errors, "**Please enter your name" if (!$fdat{name}) ;
    push @errors, "**Please enter your e-mail address" if (!$fdat{email}) ;
    }
 -]
 [$if (defined($fdat{check}) and $#errors == -1)$]
 [-
  delete $fdat{input} ;
  delete $fdat{check} ;
  delete $fdat{send}
 -]
 <hr><h3> You have entered the following data:</h3>
 <table>
  <tr><td><b>Name</b></td><td>[+$fdat{name}+]</td></tr>
  <tr><td><b>E-Mail</b></td><td>[+$fdat{email}+]</td></tr>
  <tr><td><b>Message</b></td><td>[+$fdat{msg}+]</td></tr>
  <tr><td align="center" colspan="2">
     <form action="input.htm" method="GET">
       <input type="submit" name="send"
              value="Send to [+ $MailTo +]">
       <input type="submit" name="input" value="Change your data">
       [$hidden$]
    </form>
    </td></tr>
 </table>
 [$elsif defined($fdat{send}) and $#errors == -1$]
 [- MailFormTo ($MailTo,'Formdata','email') -]
 <hr><h3>Your input has been sent</h3>
 [$else$]
 <hr><h3>Please enter your data</h3>
 <form action="input.htm" method="GET">
  <table>
    [$if $#errors != -1 $]
      <tr><td colspan="2">
      <table>
    <tr><td>[+$errors[$row]+]</td></tr>
      </table>
      </td></tr>
    [$endif$]
    <tr><td><b>Name</b></td> <td><input type="text"
                                        name="name"></td></tr>
    <tr><td><b>E-Mail</b></td> <td><input type="text"
                                          name="email"></td></tr>
    <tr><td><b>Message</b></td> <td><input type="text"
                                           name="msg"></td></tr>
    <tr><td colspan=2><input type="submit"
                             name="check" value="Send"></td></tr>  </table>
 </form>
 [$endif$]

Maintaining persistent (session) data

 (Embperl 1.2 or above)

While hidden fields are useful when working with forms, it's often necessary to store persistent data in a more general way. Embperl utilizes Apache::Session to do this job. Apache::Session is caple of storing persistent data in memory, in a textfile or in a database. More storage methods may supported in the future. While you can simply call Apache::Session from an Embperl page, Embperl can do it for you. All you need to do is to put your data in the hash %udat. The next time the same user requests any Embperl page %udat will contain the same data. You can simply use this to keep state information for the user. Depending on your expire settings, the state can also kept between mulitiple sessions. A second hash, %mdat, can be used to keep a state for one page, but for multiple users. A simple example would be a page hit counter:

  The page is requested [+ $mdat{counter}++ +] times
  since [+ $mdat{date} ||= localtime +]

The above example counts the page hits and shows the date when the page is first requested. You don't need to worry about performance - as long as you don't touch %udat or %mdat, no action is taken.

Breaking your code up into components

 (Embperl 1.2 or above)

Subroutines

It is better to write subroutines than to keep placing repetitive pieces of code in your program many times. You can do this with Embperl too. As an example, if you have text input fields with labels, this may work better for you:

 [$ sub textinput $]
    [- ($label, $name) = @_ -]
    [+ $label +]<input type=text name=[+ $name +]>
 [$ endsub $]
 <form>
    [- textinput ('Last Name', 'lname')  -]<p>
    [- textinput ('First Name', 'fname') -]<p>
 </form>

The sub metacommand starts the subroutine and the parameters are passed in the array @_. You can do anything in the subroutine that you would normally be able to do inside normal Embperl pages. Embperl lets you call this subroutine just like any other Perl subroutine: just write its name and, if necessary, the parameter list.

Execute

If you are working on an entire site rather than just a few pages, you are well aware that there are always elements which occur in every page or across many pages. Instead of copying the source code to every page, you can include other Embperl pages in your page - so you have to write the source only once. Such an included page could be a header, a footer, a navigation bar, and so on. Embperl is not only capable of including such partial pages, you can also pass arguments - for example, to tell the navigation bar which of its own element to highlight:

 Example for a simple navigation bar
 [- @buttons = ('Index', 'Infos', 'Search') -]
 <table><tr><td>
     [$if $buttons[$col] eq $param[0]$] <bold> [$endif$]
     <a href="[+ $buttons[$col] +].html"> [+ $buttons[$col] +] </a>
     [$if $buttons[$col] eq $param[0]$] </bold> [$endif$]
 </td></tr></table>
 <hr>

Now if you are on the ``Info'' page you can include the navigation bar this way:

 [- Execute ('navbar.html', 'Infos') -]

This will include the navigation bar, which is stored in the file navbar.html, and pass as its first parameter the string 'Infos'. The navigation bar module itself uses a dynamic table to display one column - which contains the text and a link - for every item in the array @buttons. The text which matches that which is passed as the first parameter is displayed in bold. There is also a long form of the Execute call, which allows you to control all of the details of how the called page is executed.

Creating Component Libraries

Instead of creating a single file for every piece of HTML-code you wish to include, you can pack them together in just one library. To do this, split up every piece of code you want to include separately in one Embperl subroutine (sub-metacommand). Now, you can use the import parameter of the Execute function to import all of the subrountines defined in one file, into the namespace of the current page. Afterwards, you are able to call them just like any other Perl subroutine.

Moreover, if you wish to have some systemwide Embperl subroutines, you can put all the Embperl code in a normal Perl module (a foo.pm file), install it into your Perl system (or a private library path), and use it just like any other Perl module - just by saying

  use mymodule;

Debugging

Embperl log file

The log file is the main source for debugging. It shows you what Embperl does while it processes your page. Depending on the debug flag settings, Embperl logs the following things:

Source
Parsing
Compiling
Environment
Form data
Evals (Source + Result)
Table processing
Input tag processing
HTTP headers
and more

Addtional you can write your own debug informtion to the Embperl logfile by writing to the special filehandle "LOG" which is opend by Embperl.

Database access

Plain DBI

This is another example of using plain DBI within Embperl. In opposition to the example I gave in the chapter about dynamic tables, this example works with explicit loops.

 [-
 # connect to database
 $dbh = DBI->connect($DSN) ;
 # prepare the sql select
 $sth = $dbh -> prepare ("SELECT * from $table") ;
 # excute the query
 $sth -> execute ;
 # get the fieldnames for the heading in $head
 $head = $sth -> {NAME} ;
 -]
 <table>
    <tr>
    [$ foreach $h (@$head) $]
        <th>[+ $h +]</th>
    [$ endforeach $]
    </tr>
    [$ while $dat = $sth -> fetchrow_arrayref $]
        <tr>
            [$ foreach $v (@$dat) $]
                <td>[+ $v +]</td>
            [$ endforeach $]   
        </tr>
    [$ endwhile $]
 </table>

DBIx::Recordset

DBIx::Recordset is a module for easy database access.

Search Example

 [-*set = DBIx::Recordset -> Search ({%fdat,
                                     ('!DataSource'   => $DSN,
                                      '!Table' => $table,
                                      '$max'   => 5,)}) ; -]
 <table>
  <tr><th>ID</th><th>NAME</th></tr>
  <tr>
    <td>[+ $set[$row]{id} +]</td>
    <td>[+ $set[$row]{name} +]</td>
  </tr>
 </table>
 [+ $set -> PrevNextForm ('Previous Records',
                          'Next Records',
                          \%fdat) +]

Search sets up a Recordset object

Search will take the values from %fdat and use them to build a SQL WHERE expression. This way, what you search for depends on what is posted to the document. For example, if you request the document with http://host/mydoc.html?id=5 the above example will display all database records where the field 'id' contains the value 5.

Data can accessed as array or via the current record

The result of the query can be accessed as an array (this does not mean that the whole array is actually fetched from the database). Alternative, you can directly access the current record just by accessing the fields.

    set[5]{id}   access the field 'id' of the sixth found record
    set{id}      access the field 'id' of the current record

Fields can be accessed by name

While normal DBI let you access your data by column numbers, DBIx::Recordset uses the field names. This makes your program easier to write, more verbose and independent of database changes.

PrevNextForm generates no/one/two buttons depending if there are more records to display

The PrevNextButtons function can be used to generate button for showing the previous record or the next records. PrevNextButton generates a small form and includes all necessary data as hidden fields. To get it to work, it's enough to feed this data to the next request to Search.

As for Search there are methods for Insert/Update/Delete

Example for Insert

If %fdat contains the data for the new record, the following code will insert a new record into the database.

 [-*set = DBIx::Recordset -> Insert ({%fdat,
                                      ('!DataSource'   => $DSN,
                                       '!Table' => $table)}) ; -]

Database table can also tied to a hash

DBIx::Recordset can also tie a database table to a hash. You need to specify a primary key for the table, which is used as key in the hash.

    $set{5}{name}    access the name with the id=5
                     (id is primary key)

Security

When running under mod_perl, all Perl code shares the same interpreter. This means that every application can access data from every other application. Embperl maintains a separate namespace for every document, which is enough to avoid accidentally overwriting other applications data, but there is no real security. You can access any data from any other application that runs on the same server process under mod_perl if you explicitly specify a package name. This is not a problem of Embperl itself, but caused by the design of Apache and mod_perl.

Escaping/Unescaping

Input: unescaping

Embperl is able to unescape the source it reads. That is usfull when you use a high level HTML editor, which might add unwanted html tags like <BR> into your Perl code.

This feature if off by default in Embperl 2.0 and can be turned on with the configuration directive EMBPERL_INPUT_ESCMODE.

When turned on it does

convert HTML escapes to characters (e.g. < to <)
remove HTML tags from Perl code (e.g. <br> insert by high level editor)

Output: escaping

It's very important to do a correct escing of your HTML output, to avoid security problems like cross-site-scripting. Therefor Embperl escapes all output that comes from Perl. This escaping is context sensitv and uses HTML or URL escaping, depending on the context. It's also possible to use XML escaping.

The output escaping can be configured by the configuration directive "EMBPERL_ESCMODE" or changed anytime inside any page by setting $escmode

How to continue

There are two addtionaly introduction documents:

``perldoc IntroEmbperlObject'' gives you an tutorial about how to build an object-oriented website.

``perldoc IntroEmbperl2'' describes the advanced features of Embperl 2

For a full documenation read ``perldoc Embperl'' and ``perldoc Config''.

You can find additional information on http://perl.apache.org/embperl