—

              
HideShow 391 lines of Pod
=pod
=head1 Contents
=over 4
=item Tips and Tricks
=item Alternative Way To Do Global Variables, using __PACKAGE__
=item Global Variables Via Namespaces
=item Handling Queries in DBI
=item Handling Exits
=item Handling Errors
=item Development and Production Websites
=back
=head1 Tips and Tricks
This document follows on from the Embperl/EmbperlObject introductory
tutorial. As you can see from that, Embperl/EmbperlObject enables extremely
powerful websites to be built using a very intuitive object-oriented
structure. Now, we'll look at some additional, "unofficial" techniques
which may also be useful in certain circumstances.
This is a small collection of personal tricks which I have developed
over the course of months using EmbperlObject in my own websites. I
hope they are useful, or at least spur you on to develop your own
frameworks and share these with others.
If you have any Tips & Tricks you want to share with the public
please send them to richter@dev.ecos.de .
=head1 Alternative Way To Do Global Variables, using __PACKAGE__
In the process of developing a large website I have found it can be a
little onerous at times to use the Request object to pass around
global data. I would like to just create variables like $xxx rather
than typing $req->{xxx} all the time. It may not seem like much, but
after a while your code can start looking a lot more complex because
of all the extra brackets and suchlike. As a typical lazy programmer,
I looked for a way to simplify this.
The method I am going to describe should be used with caution, because
it can increase memory useage rather dramatically if you're not
careful. The way I use it, no extra memory is used, but you do need to
be aware of the issues.
Basically, you change the way you include files from F</base.html>, so
that they are included into the same package as F</base.html>:
        [- Execute ({inputfile => '*', package => __PACKAGE__}) -]
You should only do this with HTML files which are included from
F</base.html>, not with the files such as F<subs.html> - those files
have to be in their own packages in order for Perl inheritance to
work. You can't use this technique with any files which are accessed
via method calls.
So how does this make things better? Well, since all these files now
share the same package, any variables which are created in one of the
files is accessible to any of the other files. This means that if you
create $xxx in F</init.html>, then you can access $xxx in
F</head.html> or any other file. This effectively gives you global
variables across all the files which are included from F</base.html>
into the same package as F</base.html>.
The thing you need to be careful of here is that if one of these files
is included more than once elsewhere on the website, then it will be
seperately compiled for that instance - thus taking up more
memory. This is the big caveat. As a rule, if your files are all just
included once by F</base.html>, then you should be fine. Note that
you'll also need to change any calls to parent files, for example:
F</contact/init.html>
        [- Execute ({inputfile => '../init.html', package => __PACKAGE__}) -]
        [-
                # Do some setup specific to this subdirectory
        -]
This is ok, since F<../init.html> will still be compiled into the same
package as the rest of the files included from F</base.html>, and so
only one version of it will exist in the Embperl cache. Thus memory
usage is not increased.
I like this technique because it simplifies the look of my code, which
is important for projects containing complex algorithms. It is not the
"official" way to implement globals though, and should be used with
care.
=head1 Global Variables Via Namespaces
The previous section described a way to share variables between
different files which are included from F</base.html>, by using the
same package across all the files. However this doesn't help us much
when dealing with the method files such as F<subs.html>, because
these files have to have their own packages - so we are back to square
one.
There is another way to share variables across even different
packages, and that is by using namespaces. For variables that need to
be accessible even from F<subs.html>, you could use a namespace which
is specific to your website. For example, if your website domain is
mydomain.com, then you could create variables using the form
        $mydomain::xxx = "hello";
As long as you then make sure that you only use this namespace on this
website (and other websites on the same Apache web server use their
own namespaces), then you shouldn't get any conflicts. Once again, use
this with caution, since you introduce the possibility of
inadvertently sharing variables between completely different
websites. For example, if you cut and paste some useful code from one
website to another, you will need to make sure you change the
namespace of any globals. Otherwise, you could get some very obscure
bugs, since different requests to the various websites could conflict.
You also need to be careful about variable initialization, since these
globals will now exist between different requests. So, it's possible
that if you don't re-initialize a global variable, then it may contain
some random value from a previous request. This can result in obscure
bugs. Just be careful to initialize all variables properly and you'll
be fine.
Finally, note that Embperl will only clean up variables which don't
have an explicit package (i.e. are in one of the packages
automatically set up by Embperl). Variables in other namespaces are
not automatically cleaned up. As a result, you need to pay closer
attention to cleaning up if you use your own namespaces. The safe way
to clean up a variable is simply to 'undef' it.
=head1 Handling Queries in DBI
If you are like me, you probably use DBI extensively to enable your
dynamic websites. I have found the cleanup of queries to be onerous -
e.g. calling finish() on queries. If you don't do that, then you tend
to get warnings in your error log about unfinished queries.
What I do these days is use a global hash, called e.g. %domain::query
(see the previous section for using namespaces to safely implement
global variables). Then, whenever I create a query, I use this
variable. For example:
        $domain::query{first_page} = $domain::dbh->prepare (qq{
                SELECT *
                FROM pages
                WHERE page = 1
                });
        $domain::query{first_page}->execute();
        my $first_page = $domain::query{first_page}->fetchrow_hashref();
This little pattern, I find, makes all my queries easier to read and
keep track of. You give each one a name in the %domain::query hash
that makes sense. Then, at the end of each request, in the
F</cleanup.html> file, you can do something like this:
        while (($name, $query) = each (%domain::query))
        {
                $query->finish();
        }
        $domain::dbh->disconnect();
Once again, this method is not really the "official" way of doing
things in Embperl. You should use the Request object to pass around
global variables if you're not comfortable with the risks involved
with namespaces (e.g. conflicting websites on the same web server).
=head1 Handling Exits
You will often find that you want to terminate a page before the
end. This doesn't necessarily indicate an error condition; it can be
just that you've done all you want to do. When you do this, it is good
to first clean up, otherwise you can get annoying warnings showing up
in your error logs.
I use the following framework. F</cleanup.html> is Executed from
F</base.html>, and it is the last thing that is done. It calls the
cleanup() function in the F</subs.html> file:
F</cleanup.html>
        [-
                $subs->cleanup ();
        -]
F</subs.html>
        [!
                sub cleanup
                {
                        while (($name, $query) = each (%domain::query))
                        {
                                $query->finish();
                        }
                        $domain::dbh->disconnect();
                }
                sub clean_exit
                {
                        cleanup();
                        exit();
                }
        !]
Now, whenever I want to exit prematurely, I use a call to
$subs->clean_exit() rather than just exit(). This makes sure that the
queries and database connections are shut down nicely.
=head1 Handling Errors
The EMBPERL_OBJECT_FALLBACK directive in F<httpd.conf> allows you to
set a file which will be loaded in the event that the requested file
is not found. This file should be relative to the same directory as
F<base.html>. 
I have found that making a special /errors/ directory is useful,
because it enables that special subdirectory to define its own
F<head.html> file, F<init.html> and so on.  So, I then just put this
in F</notfound.html>:
        [-
                $http_headers_out{'Location'} = "/errors/";
                clean_exit();
        -]
See the previous section, "Handling Exits" for more on clean_exit().
=head1 Development and Production Websites
When I am developing a website, I usually use at least two machines. I
have a workstation where I do developing and testing, and a separate
production server, which is accessed by the public. When I am finished
making changes to the development version of the website, I move it
over to the production server for testing there. However when I do
this, I usually don't copy it immediately over the existing production
version, because there are sometimes issues with Perl modules which
haven't been installed on the server, or other issues which break the
code on a different machine. So I use a separate virtual server and
subdomain (which is easy if you run your own DNS) to test the new
version. For example if the production version of the server is at
www.mydomain.com, then I might do testing on the production server
under test.mydomain.com, or beta. or whatever subdomain you like. This
means you have to create a new virtual server in the httpd.conf
file. You also obviously create a new directory for the test server
(see below for an example).
When you do all this, you end up with a very nice, isolated testing
environment on the same server as production. Obviously you hopefully
did all your major testing on your workstation, where you can crash
the machine and it doesn't matter too much. The production server
testbed is a last staging area before production, to get rid of any
lingering glitches or omissions. When you're sure it's all working
correctly you just copy the files from one directory tree (test) to
another (production) on the same machine. This test server can also be
used as a beta of the new production version. Friendly users can be
given access to the new version, while the old version is still
running.
One issue that comes up when you do this is that of databases. It is
very likely that you will be using a special test database rather than
the live one to test your new version. It would be very unwise to use
a production database for testing. So your production database might
be called "mydatabase", and the test one called
"mydatabase_test". This is fine, but it means that you have to
remember to change the database name in your code when you copy the
files over to production. This is very error prone. The solution is to
set variables like the database name in httpd.conf, by setting an
environment variable. You just add it to the virtual server section.
Here is a real example of two virtual servers on the same production
machine, which use two different directories, separate log files and
different databases. The website is crazyguyonabike.com, which is a
journal of a bicycle ride I did across America in 1998. I decided to
expand the site to allow other cyclists to upload their own journals,
which resulted in substantial changes to the code. I wanted to keep
the original site up while testing the new version, which I put under
new.crazyguyonabike.com. Here are the relevant apache settings:
F</etc/apache/httpd.conf>
        # The production server
        <VirtualHost 10.1.1.2:80>
                ServerName www.crazyguyonabike.com
                SSLDisable
                ServerAdmin neil@nilspace.com
                DocumentRoot /www/crazyguyonabike/com/htdocs
                DirectoryIndex index.html
                ErrorLog /www/crazyguyonabike/com/logs/error_log
                TransferLog /www/crazyguyonabike/com/logs/access_log
                ErrorDocument 403 /
                ErrorDocument 404 /
                PerlSetEnv WEBSITE_DATABASE crazyguyonabike
                PerlSetEnv WEBSITE_ROOT /www/crazyguyonabike/com/htdocs
                PerlSetEnv EMBPERL_DEBUG 0
                PerlSetEnv EMBPERL_ESCMODE 0
                PerlSetEnv EMBPERL_OPTIONS 16
                PerlSetEnv EMBPERL_MAILHOST mail.nilspace.com
                PerlSetEnv EMBPERL_OBJECT_BASE base.html
                PerlSetEnv EMBPERL_OBJECT_FALLBACK notfound.html
        </VirtualHost>
        <VirtualHost 10.1.1.2:80>
                ServerName crazyguyonabike.com
                Redirect / http://www.crazyguyonabike.com
        </VirtualHost>
        # Set EmbPerl handler for main directory
        <Directory "/www/crazyguyonabike/com/htdocs/">
                <FilesMatch ".*\.html$">
                        SetHandler  perl-script
                        PerlHandler HTML::EmbperlObject
                        Options     ExecCGI
                </FilesMatch>
        </Directory>
        # The test server
        <VirtualHost 10.1.1.2:80>
                ServerName new.crazyguyonabike.com
                SSLDisable
                ServerAdmin neil@nilspace.com
                DocumentRoot /www/crazyguyonabike/com/new
                Alias /pics /www/crazyguyonabike/com/pics
                DirectoryIndex index.html
                ErrorLog /www/crazyguyonabike/com/logs/new_error_log
                TransferLog /www/crazyguyonabike/com/logs/new_access_log
                ErrorDocument 401 /user/register/
                ErrorDocument 403 /
                ErrorDocument 404 /
                PerlSetEnv WEBSITE_DATABASE crazyguyonabike_new
                PerlSetEnv WEBSITE_ROOT /www/crazyguyonabike/com/new
                PerlSetEnv EMBPERL_DEBUG 0
                PerlSetEnv EMBPERL_ESCMODE 0
                PerlSetEnv EMBPERL_OPTIONS 16
                PerlSetEnv EMBPERL_MAILHOST mail.nilspace.com
                PerlSetEnv EMBPERL_OBJECT_BASE base.html
                PerlSetEnv EMBPERL_OBJECT_FALLBACK notfound.html
        </VirtualHost>
        # Set EmbPerl handler for new directory
        <Directory "/www/crazyguyonabike/com/new/">
                <FilesMatch ".*\.html$">
                        SetHandler  perl-script
                        PerlHandler HTML::EmbperlObject
                        Options     ExecCGI
                </FilesMatch>
        </Directory>
        # Restrict access to test server
        <Directory /www/crazyguyonabike/com/new>
                AuthType Basic
                AuthName CrazyTest
                Auth_MySQL_DB http_auth
                Auth_MySQL_Encryption_Types Plaintext
                require valid-user
                PerlSetEnv EMBPERL_OPTIONS 16
                PerlSetEnv EMBPERL_MAILHOST mail.nilspace.com
        </Directory>
Note that the test and production servers each get their own
databases, directories and log files.
You can also see that I restrict access to the test server (which is
generally wise, unless you actually like hackers potentially screwing
with your head while testing). For basic authentication I use
mod_auth_mysql, which is available from the MySQL website. It is nice
because it allows you to authenticate based on a MySQL database.
When you use PerlSetEnv to pass in variables, you access these
variables in your code as follows:
        $db_name = $ENV{WEBSITE_DATABASE};
If you move those constants which differ between the test and
production versions of the same code into the httpd.conf file, then
you can just copy the files over from the test directories to the
production directory without any alterations. This cuts down on
editing errors and also documents specific constants in one place.
=head1 Author
Neil Gunton neil@nilspace.com