I finally got around to setting up a more sophisticated deployment system for some of my apps. These apps include some built on a custom PHP framework and others that are Python / Django apps. I figured I'd share my experience...

Why is a high-level deployment infrastructure important? Deployment is something that should be simple, accessible, and repeatable. It should be as close to a "single click" as possible. Previously, for me, it was a bash script that exported some SVN branches. While this worked fine, as projects progress, you want some accountability, history, and the ability to roll back mission critical applications when something goes wrong with a deploy.

Capistrano is an open source, command line, deployment tool that provides all of these features. It's written in Ruby. You leverage a variety of built in "recipes" (Capistrano's term for a deployment script) that execute certain procedures to deploy an app. Out-of-the-box it's ideally built to deploy a Rails app. However, after some minor tweaks it can deploy most anything and do it well. It can restart servers, update symlinks, change permissions - pretty much anything. It assumes you access your POSIX compliant server via SSH via the same password (or have ssh keys setup).

Webistrano is an open source web front-end for Capistrano. It's a convenience layer that abstracts the command line away and provides an interface to perform the same tasks. This interface shows history as well as providing a convenient GUI for creating new deployment projects, stages, and recipes. Highly recommended.

Let's get down to business. This post makes a few assumptions about things you've already installed and used previously.

• Ruby 1.8.5+
• RubyGems
• MySQL 5.0+
• Apache 2+
• Subversion and a repository containing the "production" branch of your app.

Installing Capistrano

Well, this is an easy one (you probably want to do this as root):

gem install capistrano

Installing Webistrano

Also fairly easy, with a little splash of configuration.

# wget http://labs.peritor.com/webistrano/attachment/wiki/Download/webistrano-1.4.zip
# unzip webistrano-1.4.zip
# mv webistrano-1.4 /path/to/where/you/want/webistrano

Setup the database tables and create a new webistrano user (obviously be conscious of your security preferences for access to your database in the host and password portions):

# mysql
mysql> CREATE DATABASE `webistrano`;
mysql> CREATE USER 'webistrano'@'localhost' IDENTIFIED BY 'password';
mysql> GRANT ALL PRIVILEGES ON `webistrano`.* TO 'webistrano'@'localhost' WITH GRANT OPTION;

Now, in the directory where you placed webistrano you're going to want to copy config/database.yml.sample to config/database.yml. Edit this file, in the production area, to match your database settings. By default the file expects a socket to connect, you can chase this by specifying host: and port:. (Keep in mind Webistrano is simply a Rails app).

You should now be able to have Rails migrate the new database you created. In the webistrano directory:

# RAILS_ENV=production rake db:migrate

Finally, copy config/webistrano_config.rb.sample to config/webistrano_config.rb and edit according to your preferred mail settings.

We can now test to see if webistrano is working properly by serving it via mongrel:

# ruby script/server -d -e production -p 3000

This starts a single mongrel daemon, using the production environment, listening on port 3000. You should now be able to hit http://127.0.0.1:3000/ and get the Webistrano login prompt. If this is working, kill that mongrel instance.

For longer term serving I decided to go with Phusion Passenger (essentially mod_rails for Apache). It's a nearly zero configuration solution for serving a rails app and will feel at home to anyone with experience serving PHP apps via Apache and mod_php.

Installing Phusion Passenger

Again, as root:

# gem install passenger
# passenger-install-apache2-module

The second command will invoke an installer which compiled Passenger and provides instructions on integrating it into your Apache config. Essentially, edit your httpd.conf as follows (these were specific to my install, make sure to use the ones provide by the installer for you):

LoadModule passenger_module /usr/lib/ruby/gems/1.8/gems/passenger-2.2.9/ext/apache2/mod_passenger.so
PassengerRoot /usr/lib/ruby/gems/1.8/gems/passenger-2.2.9
PassengerRuby /usr/bin/ruby

Now you can simply add VirtualHost entries to your httpd.conf for any of your Rails apps. Let's add one for Webistrano:

<VirtualHost *:80>
ServerName webistrano.mydomain.com
DocumentRoot /path/to/webistrano/public
</VirtualHost>

Yes, Passenger makes it that simple. Add configuration directives as needed for your environment.

Now Webistrano should be serving from the VirtualHost you specified, seamlessly, via Passenger.

Deploying A Non-Rails App

Now the fun stuff.

Capistrano breaks things down into projects, stages, and recipes. Each app you want managed by capistrano should be it's own project. Each project should have a stage for at least production and optionally staging and development.

Hosts are added globally and form the targets of a deploy for any given project. Hosts can include web, app, and database servers.

Deployments in Capistrano are done to a child directory under "releases" named via the date and time of the deployment. By default 5 releases are kept and available to rollback to. Upon successful deployment a symlink (default is called "current" and can be modified via the current_path configuration variable) is updated to that release directory. It is this symlink that should be targeted by your webserver (your DocumentRoot in Apache).

Capistrano also creates a "shared" directory that is symlinked to in each release useful for storing logs and other data that should be maintained through each deployment.

For non-rails apps you'll use the "Pure File" project type when creating your new project. Upon project creation you can add configuration variables specific to your project. I recommend using :export instead of :checkout for deploy_via for production subversion deployments as this doesn't expose .svn directories. Use an SSH user that has enough permissions to create directories where your deploy will occur or, specify use_sudo to true and create a new configuration variable admin_runner and set it to the same user as runner.

Add a stage to your new project for "production". In the "Manage Hosts" page add a new host for each of your application servers. Then add each host as a target of your "production" stage of your project.

At this point you should be able to execute the "Setup" task for your "production" stage. This is a one time task that simply creates the directories.

Assuming this went successfully, try doing a "Deploy" and see if that finishes without error. You might have to play around with permissions and other minor issues - post a comment if you have any specific questions.

For my PHP framework there are a couple specific tasks I wanted to run in addition to the default Capistrano tasks. You do this by creating custom recipes in the "Manage Recipes" page in Webistrano. Recipes are simply procedures written in ruby. Here's what my recipe looks like:

namespace :deploy do
	task :setup, :except => { :no_release => true } do
		dirs = [deploy_to, releases_path, shared_path]
		dirs += shared_children.map { |d| File.join(shared_path, d) }
		run "#{try_sudo} mkdir -p #{dirs.join(' ')} && #{try_sudo} chmod g+w #{dirs.join(' ')}"
		run "chmod 777 #{shared_path}/log"
	end

	task :finalize_update, :except => { :no_release => true } do
		run "mkdir -p #{latest_release}/app/tmp"
		run "chmod -R 777 #{latest_release}/app/tmp"
		run "rm -rf #{latest_release}/app/logs"
		run "ln -s #{shared_path}/log #{latest_release}/app/logs"
		run "cp #{latest_release}/public_html/.htaccess-production #{latest_release}/public_html/.htaccess"
		run "cp #{latest_release}/app/config/config-production.php #{latest_release}/app/config/config.php"
		run "cp #{latest_release}/app/config/db-default.php #{latest_release}/app/config/db.php"
		run "cp #{latest_release}/app/config/memcache-default.php #{latest_release}/app/config/memcache.php"
	end
end

If you're not familiar with Ruby - what this code is essentially doing is overwriting two tasks in the :deploy namespace with my custom code.

The first, :setup, simply duplicates the base :setup functionality discussed above (creating the releases and shared directories) and chmods the shared log directory to be writable.

The second, :finalize_update, performs a variety of configuration tasks for a PHP app built with my framework. Also, you'll notice that I'm removing my app's logs directory and symlinking to the shared log directory. This way all releases will log to the same directory, consistently.

In my case all of these procedures are command line instructions. Alternatively, you can do a variety of things leveraging the full breadth of the Ruby language and any gem you'd like to introduce. Things such as accessing your CDN API to clear image, JS, or CSS caching, etc.

Deploying Django Apps

First off it's worth noting that I serve my Django apps via mod_wsgi. To make the deployment process easier here's what my app.wsgi script looks like:

import os
import sys

appdir = os.path.normpath(os.path.join(os.path.realpath(os.path.dirname(__file__)), '..'))
sys.path.insert(0, appdir)
os.environ['DJANGO_SETTINGS_MODULE'] = 'settings'
os.environ['PYTHON_EGG_CACHE'] = os.path.join(appdir, '.python-eggs')
import django.core.handlers.wsgi
application = django.core.handlers.wsgi.WSGIHandler()

This code allows us to avoid having to hardcode paths in the wsgi script (and thus avoid having to change them when we deploy). It assumes the following directory structure:

.python-eggs (egg cache)
apps (apps path is added to python system path in settings.py)
public (where your .wsgi script resides)
site_media
templates
settings.py
settings-production.py (used for deploy)
urls.py
...

If you follow this convention, the following Capistrano recipe works great:

namespace :deploy do
	task :setup, :except => { :no_release => true } do
		dirs = [deploy_to, releases_path, shared_path]
		dirs += shared_children.map { |d| File.join(shared_path, d) }
		run "#{try_sudo} mkdir -p #{dirs.join(' ')} && #{try_sudo} chmod g+w #{dirs.join(' ')}"
		run "chmod 777 #{shared_path}/log"
	end

	task :finalize_update, :except => { :no_release => true } do
		run "rm -rf #{latest_release}/logs"
		run "ln -s #{shared_path}/log #{latest_release}/logs"
		run "cp #{latest_release}/settings-production.py #{latest_release}/settings.py"
		run "mkdir -p #{latest_release}/.python-eggs"
		run "chmod 777 #{latest_release}/.python-eggs"
	end
end

Fin

This should give you a nice intro to leveraging Capistrano via Webistrano. Feel free to comment with questions, suggestions, or anything else!

• Comments

Many (awesome) changes http://blog.jquery.com/2010/01/14/jquery-14-released/.

View the release notes here: http://jquery14.com/day-01/jquery-14

• Comments

The wonderful folks over at jQuery have released Alpha 2 of version 1.4 for all those testers out there.

1.4 is slated for official launch on January 14th - how cute :)

• Comments

Just wanted to mention that jQuery 1.4 Alpha 1 has been released.

Most of the changes seem to revolve around heavy optimization of some core functionality. Installing this alpha and testing in live applications will help get this release out!

• Comments

Send this to your significant other/parent/relative/friend so, instead of that sweater, you get one of these nuggets of awesome this Christmas.

The Pragmatic Programmer: From Journeyman to Master

Write better, cleaner, more maintainable code. Learn how to manage your projects and focus on shipping your product. With insight that covers the gamut of software development from low level to management
this one is a must have for anyone involved in this industry.

The Passionate Programmer: Creating a Remarkable Career in Software Development

Highly recommended! Read my full review.

Code Complete: A Practical Handbook of Software Construction

Another classic "software construction" book. Sharpen your saw with timeless information that can be applied to any project in any language. Less bugs, more productivity, more programmer happiness.

Coders at Work

This one is different. Written as a set of interview transcripts with 15 legendary industry giants, this book is a fantastic insight into how some of the great minds think. It's inspiring to hear it from the source, must have!

Programming Clojure

A developer should learn at least one new language a year. This year that language should be Clojure. Clojure is a dynamic, general purpose, language targeting the Java virtual machine and designed for multi-threaded use. It's growing popularity, ability to leverage the Java standard library, and its multi-threaded nature make this a must have.

The Mythical Man-Month: Essays on Software Engineering

Another classic. Primarily discusses project management from the perspective of Fred Brooks and his experiences at IBM. Brooks' Law states that "adding manpower to a late software project makes it later".

Don't Make Me Think: A Common Sense Approach to Web Usability

Web developers should always keep in mind the user of the product their creating. Usability becomes increasingly important as applications move to the web. The design and usability of your app can make or break its success. This classic is a must read.

Design Patterns: Elements of Reusable Object-Oriented Software

This classic known most commonly as the "gang of four" book is the definitive reference on design patterns. Covering all of the most common cases and time and time again serving as an invaluable source of information.

• Comments

This is an update to my previous how-to Setup Python 2.5, mod_wsgi, and Django 1.0 on CentOS 5 (cPanel).

The biggest reason why I chose to go with Python 2.5 at the time was because the MySQL Python (MySQLdb) package didn't support Python 2.6. The 1.2.3c1 release does so that roadblock is lifted.

The instructions are identical - nothing has really changed in that regard. Just change the references from Python 2.5 to 2.6. Here are the links to the versions I'm using successfully:

Python 2.6.4: http://www.python.org/ftp/python/2.6.4/Python-2.6.4.tgz

setuptools 0.6c11: http://pypi.python.org/packages/2.6/s/setuptools/setuptools-0.6c11-py2.6.egg#md5=bfa92100bd772d5a213eedd356d64086

MySQLdb 1.2.3c1: http://sourceforge.net/projects/mysql-python/files/mysql-python-test/1.2.3c1/MySQL-python-1.2.3c1.tar.gz/download

mod_wsgi 2.6: http://modwsgi.googlecode.com/files/mod_wsgi-2.6.tar.gz

Django 1.1.1: http://www.djangoproject.com/download/1.1.1/tarball/

• Comments

The Problem

I'm sure many have used PHP's default session handling capabilities. By default, PHP uses the filesystem to store session data naming files with their session id # and putting them in /tmp.

This is done for the sake of simplicity. On a single-server, low load website, this particular setup works fine. It's when you start having multiple simultaneous requests from a single client (identified by a session) that the problems begin to show their ugly heads. Utilizing AJAX multiple simultaneous requests might be the norm, even for a low load website.

Essentially, in order to prevent race conditions PHP internally uses a lock to maintain exclusive access to the file containing the session data for the client connection. This means that as soon as a single request acquires exclusive access to that session file, no other request can access the file until the original request completes. What happens when that second request asks to start the session? It waits.

The Solution

Fortunately there's a solution to all this. Implementing your own custom session handler and moving your session storage backend to another technology (such as a MySQL database or memcache) affords you the ability to handle simultaneous requests in a thread-safe manner. Remember, it's a good thing that PHP prevents race conditions by locking the session file. What we're looking to do is increase the granularity of the lock to the level of individual session data key => value pairs.

For this post we're going to stick to storing sessions in the database with our own custom session save handler. Perhaps in another post I'll talk about doing the same in memcache. It's the theory we're concerned about, not necessarily the exact storage mechanism implementation.

Implementing your own custom session handler is simply a matter of calling session_set_save_handler() with the appropriate callback methods for handling the following scenarios:

• open
  Open function, this works like a constructor in classes and is executed when the session is being opened. The open function expects two parameters, where the first is the save path and the second is the session name.
• close
  Close function, this works like a destructor in classes and is executed when the session operation is done.
• read
  Read function must return string value always to make save handler work as expected. Return empty string if there is no data to read. Return values from other handlers are converted to boolean expression. TRUE for success, FALSE for failure.
• write
  The "write" handler is not executed until after the output stream is closed.
• destroy
  The destroy handler, this is executed when a session is destroyed with session_destroy() and takes the session id as its only parameter.
• gc
  The garbage collector, this is executed when the session garbage collector is executed and takes the max session lifetime as its only parameter.

There's a well known trick for situations like this that allow you to pass a class method (static or instance) for callbacks. Let's take a look at a simple example. This correctly implements the required methods but obviously doesn't do much:

class MySession
{
	public function __construct()
	{
		session_set_save_handler(
						array('MySession', 'sess_open'),
						array('MySession', 'sess_close'),
						array('MySession', 'sess_read'),
						array('MySession', 'sess_write'),
						array('MySession', 'sess_destroy'),
						array('MySession', 'sess_gc')
						);

		ini_set('session.auto_start',					0);
		ini_set('session.gc_probability',				1);
		ini_set('session.gc_divisor',					100);
		ini_set('session.gc_maxlifetime',				604800);
		ini_set('session.referer_check',				'');
		ini_set('session.entropy_file',					'/dev/urandom');
		ini_set('session.entropy_length',				16);
		ini_set('session.use_cookies',					1);
		ini_set('session.use_only_cookies',				1);
		ini_set('session.use_trans_sid',				0);
		ini_set('session.hash_function',				1);
		ini_seT('session.hash_bits_per_character',		5);

		session_cache_limiter('nocache');
		session_set_cookie_params(0, '/', '.mydomainname.com');
		session_name('mySessionName');
		session_start();
	}

	public static function sess_open($save_path, $session_name)
	{
		return true;
	}

	public static function sess_close()
	{
		return true;
	}

	public static function sess_read($id)
	{
		return '';
	}

	public static function sess_write($id, $sess_data)
	{
		return true;
	}

	public static function sess_destroy($id)
	{
		return true;
	}

	public static function sess_gc($maxlifetime)
	{
		return true;
	}
}

SPL and Storing Session Data In MySQL

Adding support for MySQL to this class is fairly trivial. Let's start off by creating a table to store our session data:

CREATE TABLE `sessions` (
  `sesskey` char(32) NOT NULL,
  `timestamp` timestamp NOT NULL default CURRENT_TIMESTAMP on update CURRENT_TIMESTAMP,
  `varkey` varchar(128) NOT NULL,
  `varval` longtext NOT NULL,
  PRIMARY KEY  (`sesskey`,`varkey`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

PHP's SPL provides the ability to create objects that behave as though they were arrays. You can make objects that iterate, respond to accessing them via array notation ($object['key']), and lots of other interesting things.

We're going to enhance the MySession class with a couple SPL interfaces that will allow the object, when instantiated, to behave like an array. We can then override the $_SESSION superglobal with an instance of our new MySession class. It will provide an identical interface to access session data while internally storing session data to the database. Also, internally it will use methods for row-level locking via MySQL's advisory locks (GET_LOCK() and RELEASE_LOCK()).

The next step is to implement the required methods for the interfaces we're using. These methods allow the object to behave as though it's an array.

class MySession implements Countable, ArrayAccess, Iterator
{
	private $index;
	private $curElement;
	private $locks = array();
	private $sessionName = 'sessionId';
	private $serialize = 'serialize';
	private $unserialize = 'unserialize';
	private $session_id = null;

	public function __construct()
	{
		session_set_save_handler(
						array('MySession', 'sess_open'),
						array('MySession', 'sess_close'),
						array('MySession', 'sess_read'),
						array('MySession', 'sess_write'),
						array('MySession', 'sess_destroy'),
						array('MySession', 'sess_gc')
						);

		ini_set('session.auto_start',					0);
		ini_set('session.gc_probability',				1);
		ini_set('session.gc_divisor',					100);
		ini_set('session.gc_maxlifetime',				604800);
		ini_set('session.referer_check',				'');
		ini_set('session.entropy_file',					'/dev/urandom');
		ini_set('session.entropy_length',				16);
		ini_set('session.use_cookies',					1);
		ini_set('session.use_only_cookies',				1);
		ini_set('session.use_trans_sid',				0);
		ini_set('session.hash_function',				1);
		ini_seT('session.hash_bits_per_character',		5);

		session_cache_limiter('nocache');
		session_set_cookie_params(0, '/', '.mydomainname.com');
		session_name('mySessionName');
		session_start();
	}

	public function destroy()
	{
		$sessionName = session_name();
		$cookieInfo = session_get_cookie_params();
		$cookieExpires = time() - 3600;
		if((empty($cookieInfo['domain'])) && (empty($cookieInfo['secure']))) {
			setcookie($sessionName, '', $cookieExpires, $cookieInfo['path']);
		} elseif(empty($cookieInfo['secure'])) {
			setcookie($sessionName, '', $cookieExpires, $cookieInfo['path'], $cookieInfo['domain']);
		} else {
			setcookie($sessionName, '', $cookieExpires, $cookieInfo['path'], $cookieInfo['domain'], $cookieInfo['secure']);
		}
		unset($_COOKIE[$sessionName]);

		$dbo = DBO::getInstance();
		$q = "DELETE FROM `sessions` WHERE `sesskey` = '".$this->session_id."'";
		$dbo->query($q);

		session_destroy();
	}

	private function lockName($k)
	{
		return 'sesslock'.$this->session_id.$k;
	}

	public function locked($k)
	{
		$k = $this->lockName($k);

		return isset($this->locks[$k]);
	}

	public function acquire($k, $timeout = 0)
	{
		$k = $this->lockName($k);

		if(!isset($this->locks[$k])) {
			$dbo = DBO::getInstance();
			$q = "SELECT GET_LOCK('".$k."', ".$timeout.")";
			$rs = $dbo->query($q);
			$this->locks[$k] = $dbo->result($rs, 0);
			$dbo->fr($rs);

			return $this->locks[$k];
		}

		return false;
	}

	public function release($k)
	{
		$k = $this->lockName($k);

		unset($this->locks[$k]);

		$dbo = DBO::getInstance();
		$q = "SELECT RELEASE_LOCK('".$k."')";
		$rs = $dbo->query($q);
		$ret = $dbo->fetch($rs);
		$dbo->fr($rs);

		return true;
	}

	public function count()
	{
		$dbo = DBO::getInstance();
		$q = "SELECT COUNT(*) FROM `sessions` WHERE `sesskey` = '".$this->session_id."'";
		$rs = $dbo->query($q);
		$ret = $dbo->result($rs, 0);
		$dbo->fr($rs);

		return $ret;
	}

	public function rewind()
	{
		$this->index = 0;
		$this->getCurElement();
	}

	private function getCurElement()
	{
		$dbo = DBO::getInstance();
		$q = "SELECT `varkey`, `varval` FROM `sessions` WHERE `sesskey` = '".$this->session_id."' LIMIT ".$this->index.",1";
		$rs = $dbo->query($q);
		$row = $dbo->fetch($rs);
		$dbo->fr($rs);
		if(is_array($row) && (count($row) == 2)) {
			$this->curElement = $row;
		} else {
			$this->curElement = array(null, null);
		}
	}

	public function key()
	{
		return $this->curElement[0];
	}

	public function current()
	{
		return call_user_func($this->unserialize, $this->curElement[1]);
	}

	public function next()
	{
		$this->index++;
		$this->getCurElement();
	}

	public function valid()
	{
		return ($this->curElement[0] !== null);
	}

	public function offsetSet($k, $v)
	{
		$dbo = DBO::getInstance();
		$q = "REPLACE INTO `sessions` (`sesskey`, `varkey`, `varval`) VALUES ('".$this->session_id."', '".$k."', '".$dbo->sanitize(call_user_func($this->serialize, $v))."')";
		$dbo->query($q);
	}

	public function offsetGet($k)
	{
		$dbo = DBO::getInstance();
		$q = "SELECT `varval` FROM `sessions` WHERE `sesskey` = '".$this->session_id."' AND `varkey` = '".$k."'";
		$rs = $dbo->query($q);
		if($ret = $dbo->result($rs, 0)) {
			$ret = call_user_func($this->unserialize, $ret);
		}
		$dbo->fr($rs);

		return $ret;
	}

	public function offsetUnset($k)
	{
		$dbo = DBO::getInstance();
		$q = "DELETE FROM `sessions` WHERE `sesskey` = '".$this->session_id."' AND `varkey` = '".$k."'";
		$dbo->query($q);
	}

	public function offsetExists($k)
	{
		$dbo = DBO::getInstance();
		$q = "SELECT `varval` FROM `sessions` WHERE `sesskey` = '".$this->session_id."' AND `varkey` = '".$k."'";
		$rs = $dbo->query($q);
		$ret = $dbo->result($rs, 0);
		$dbo->fr($rs);

		return (bool)$ret;
	}

	public static function sess_open($save_path, $session_name)
	{
		return true;
	}

	public static function sess_close()
	{
		return true;
	}

	public static function sess_read($id)
	{
		return '';
	}

	public static function sess_write($id, $sess_data)
	{
		return true;
	}

	public static function sess_destroy($id)
	{
		return true;
	}

	public static function sess_gc($maxlifetime)
	{
		$dbo = DBO::getInstance();
		$q = "DELETE FROM `sessions` WHERE `timestamp` < '".date('Y-m-d H:i:s', time() - $maxlifetime)."'";
		$dbo->query($q);

		return $dbo->query($q);
	}
}

The $dbo object is just an example of an interface to the database through the use of a singleton. Replace the $dbo object with your preferred mysql database interface and you'll be set to go!

Starting your session is now as simple as:

$_SESSION = new MySession;

• Comments

Just wanted to post this quick bash script to iterate over the repositories in a directory, perform an svnadmin hotcopy, and tar/gzip the output.

By using hotcopy this can be performed on a live subversion repository and will produce a pristine backup.

#!/bin/bash
REPOS_PATH=/var/repos
mkdir -p /backups/weekly
rm -rf /backups/tmp
mkdir -p /backups/tmp/repos
for i in $(ls $REPOS_PATH); do
        /path/to/svnadmin hotcopy $REPOS_PATH/$i /backups/tmp/repos/$i
done
FN=svn.weekly.`date '+%Y%m%d'`.tar.gz
tar -czf /backups/weekly/$FN -C /backups/tmp .

• Comments

Read my previous two posts on Django and Python - Part I and Part II

I've been working on a project management tool suite in Django. It's been a great side project to really experiment with Django in real-world scenarios.

Forms

At times I feel like I fight with newforms. In particular, it lacks the ability to specify basic class or style attributes for a given form field from within the template. I'd like to be able to more finely tune the display of the field, directly within the template, with a style attribute or a class. Is it suggested you write your own custom form field widget for a single element? I've been getting around this by doing the following:

<input type="text" name="{{ todo_form.item.name }}" style="width: 720px;"/>

This gets more complicated if you want to set a style attribute for a form field that's a select box (for a ForeignKey model field, for example).

<label for="category">Category:</label> <select name="{{ category_form.project.name }}" style="width: 221px;">
{% for choice_val, choice_label in category_form.project.field.choices %}
	<option value="{{ choice_val }}">{{ choice_label }}</option>
{% endfor %}

Is this a good use case for template tags? I feel like I'm missing something here.

On the positive side, it was an absolute pleasure to work with multiple forms on a single page submitted to and processed by a single view. This is primarily thanks to prefixes. Excellent, that's how easy it should be.

Ternary Operator

Update: It's been pointed out in comments (thanks!) that Python 2.5 introduced a ternary operator. It's syntax is as follows:

label = "true" if booleanVariable else "false"

I also ran into a minor Python syntax issue. I love the ternary operator in languages that offer it. It's a concise, one-line, syntax for an if-else clause. Consider the following PHP code:

$label = $booleanVariable ? 'true' : 'false';

// the above is identical to the following block:
if($booleanVariable) {
   $label = 'true';
} else {
   $label = 'false';
}

Python unfortunately lacks this syntactic sugar. Fortunately, however, you can effectively accomplish the same thing by doing this:

label = (booleanVariable and 'true' or 'false')

# the above is equivalent to the following block:
if booleanVariable:
   label = 'true'
else:
   label = 'false'

Sessions

Django has built in support for sessions. By default, sessions last longer than the lifecycle of the user's browser. I personally think it should be the other way around. It's easily changed though (in your settings.py):

SESSION_EXPIRE_AT_BROWSER_CLOSE = False

Views

In one of my views I wanted to test whether a filtered result set was empty or not. I was curious whether this was the "pythonic" way to accomplish this:

account = get_object_or_404(Account, pk=account_id)
if account.useraccount_set.filter(user__exact=request.user) != []:

Also, with respect to views and passing context to the response, sometimes it's an efficient shortcut to use locals() instead of explicitly typing out all the variables you'd like to expose. locals() returns a dictionary of all variables defined within the local scope.

def myview(request, id):
    account = get_object_or_404(Account, pk=id)
    new_account_form = NewAccountForm()

    return render_to_response('myview.html', locals())

More soon!

• Comments

Motivation: SD News is a "social news site" (basically a Hacker News clone), written in Rails, that I work on as part of my efforts with a Christian publishing company I run with some friends.  As part of the administrative backend, I wanted to be able to send posts to our Twitter profile.  The site is still young, and the community still growing, so I wanted the admins to have complete control over what gets sent to Twitter.  I had thought of automating this process based on which items have the most votes in a given time period, but trust is easy to lose and all it would take is 1 or 2 irrelevant, or irreverent. posts to lose that trust.

Methodology: I would first need a good Twitter gem for Ruby, and I'd need to decide which URL shortening service I'd use.  Ruby Twitter seemed to be the simplest gem for Twitter.  For the URL shortening I chose bit.ly, because the bitly gem seemed like the easiest, and the documentation was good.  My plan of attack was:

1. Grab one item from the queue, that has not been Twittered
2. Shorten the URL via bitly
3. Send the item's title and shortened URL to Twitter
4. Save the shortened URL in the database so I could retrieve stats later

The script that did this would be run every hour.

Implementation: The two gems made this an almost trivial implementation.

require 'twitter'
require 'bitly'

@item = Item.find(:first, :conditions => "send_to_twitter = 1 and twitterd = 0", :order => "posted_on desc")

if !@item.nil?
   b = Bitly.new(username, password)
   @url = b.shorten("http://news.sensusdivinitatis.com/item/#{@item.id}").short_url

   httpauth = Twitter::HTTPAuth.new(username, password)
   base = Twitter::Base.new(httpauth)
   base.update("#{@item.title[0...110]} - #{@url}") #shorten the title if it's too long

   Item.update(@item.id, :twitterd => 1, :bitly_url => @url) #save the bit.ly url
end

That's pretty much it.  Incidentally, the bitly gem makes it very easy to grab the stats for any URL.  For instance, if you wanted to see how many clicks a given URL has received:

require 'twitter'
require 'bitly'

@item = Item.find(id)
b = Bitly.new(username, password)
@clicks = b.stats(i.bitly_url).stats["clicks"]

Done.

• Comments