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Deploying a production site

Many things influence the details of how one deploys a web application to a production site. Some people have root access to the machine on which the site will run, some only have ssh access to a user account. Different people use different web servers. Different environments also run on different operating platforms and versions of those platforms. The list goes on...

The high level details are the same though. In short, here’s what you need to do to deploy an application:

  • Package your application and distribute it to the machine where it is to be installed.
  • Create a virtualenv for your application, and install your application and all its dependencies into that virtualenv.
  • Provide configuration for which explicit settings are needed in production (these settings are defaulted for convenience, but their defaults only make sense in development).
  • Set up (or migrate) the database for the application.
  • Provide your application as a WSGI app to the web server.
  • Configure your web server to serve the WSGI app – both a version via http and one served via https.

The rest of this section elaborates on these points by walking you through an example. The example is the simple “hello world” application developed in Getting started with Reahl, but with a few files included for use in a production environment. The example is done in terms of an Apache 2.2.x web server, using mod_wsgi.

Packaging and distributing your application

Since any Reahl project is also a Python Egg, packaging it is a matter of building and distributing the egg. Your .reahlproject file specifies all the formats you want the egg to be packaged in, and for each such format, a list of repositories to which you’d like to distribute it.

This example has the following in its .reahlproject file:

  <distpackage type="wheel">
  </distpackage>

The <distpackage type=”sdist”> tag represents a package to be built as a Python Egg source distribution. In this example, no repositories are defined for distributing the package. Repositories would be defined inside the <distpackage> tag, by listing a <packageindex> for each repository to which this package should be distributed.

A <packageindex> represents a PyPi-style repository. It expects a single attribute repository=”some value” which has exactly the same meaning as the --repository command line argument to the normal setup.py upload command which is normally used to upload eggs to PyPi.

By default, though, all packages are put in a “local repository” when built. This local repository is just a special directory in which all eggs will be dumped. For our example we’ll just copy the egg from there to the target production machine.

The reahl script that is used during development creates a directory for all housekeeping tasks. Normally it would create this directory as .reahlworkspace in your home directory. The local repository for eggs would then be ~/.reahlworkspace/dist-egg.

To build your egg, issue the following command from within the directory containing your component:

reahl build

After issuing that, look for the tar file of your built egg in ~/.reahlworkspace/dist-egg. It will be named helloapache-0.0.tar.gz.

What’s to be installed

Besides your application itself (in an egg), any Reahl application will also need the following:

  • A virtualenv in which the egg and its dependencies will be installed
  • A script that creates and starts your application as a WSGI app
  • The configuration directory for your application, set up for a production environment.
  • A location for static files served by the web server.

Before we can go into the details of these, it is necessary to decide where all these files will go on the production file system. Of course, this is up to you, and may depend on the type of access you have to the production machine. For the purposes of this example, root access is assumed, making the following file locations possible:

├── etc
│   ├── apache
│   │   └── sites-available
│   │       ├── default           <-- An Apache site for http
│   │       └── default-ssl       <-- An Apache site for https
│   └── reahl.d                   <-- A directory where all Reahl config directories will go
│       └── helloapache           <-- The config directory for our example site
│           ├── reahl.config.py
│           ├── web.config.py
│           └── systemaccountmodel.config.py
├── usr
│   └── local
│       └── helloapache           <-- The helloapache program itself
│           ├── helloapache.wsgi  <-- The wsgi script
│           └── virtualenv        <-- The virtualenv for helloapache
└── var
    └── local
        └── helloapache           <-- Data used by the helloapache program
            ├── www               <-- The "DocumentRoot" for static files for this website
            └── helloapache.db    <-- The sqlite database (if you use sqlite)

virtualenv

Production environments can also be run in an isolated environment provided by virtualenv. Doing this has some advantages:

  • If you host many applications, each one in its own virtualenv, you will not impact other applications when upgrading one.
  • You’re not tied to the versions of Python packages shipped by your distribution.

There is also a disadvantage though: since Python packages are installed into the virtualenv using pip, they are installed from source. That means they have to be built – a process which relies on the presence of some non-Python packages. These non-Python packages would need to be installed on the production system.

The process of preparing a virtualenv for production is the same as that of preparing it for development, as explained in the Getting started with Reahl. Only, a virtualenv installed in /usr/local/helloapache should be installed as the root user.

Installing helloapache in the virtualenv

Once the virtualenv has been prepared, and all necessary non-Python packages are available in the system, you can now go ahead and install helloapache into the virtualenv (again, do this as root).

You don’t have to install Reahl explicitly. The helloapache egg should have the correct dependencies declared – which means installing helloapache will also install all its dependencies.

We noted earlier that for the sake of this example you can distribute the application by copying helloapache-0.0.tar.gz to the production box. We’re not going to flood PyPi with helloapache projects!

Assuming you put helloapache-0.0.tar.gz in /tmp, simply install it by issuing (as root):

pip install /tmp/helloapache-0.0.tar.gz

The wsgi script

Reahl is meant to be served via a web server that can serve WSGI applications. Apache can do this using its mod_wsgi. This module requires one to supply a Python script which makes a WSGI application available in its name space as the variable application. Of course, this script is also responsible for initialising said WSGI application.

Here is the script for our planned setup for this example (you would probably write a very similar script when deploying to a different server):


# Activate your virtual env
virtualenv_dir = '/usr/local/helloapache/virtualenv'
activate_env="%s/bin/activate_this.py" % virtualenv_dir
execfile(activate_env, dict(__file__=activate_env))

from reahl.web.fw import ReahlWSGIApplication
application = ReahlWSGIApplication.from_directory('/etc/reahl.d/helloapache')
application.start()


The strangest thing done by the script relates to the fact that your application lives in a virtualenv. The first paragraph in the script switches the current Python interpreter to the virtualenv (and, by implication all applications and libraries installed there). Only after having done that, can you import ReahlWSGIApplication and create an instance of it from the configuration directory for your application. Lastly, .start() the application.

Production config

According to the plan laid out above, the configuration directory is created inside /etc/reahl.d. This is just convenient if you run multiple web applications on the same machine. What is important to take not of is the configuration settings that are needed for a production machine. Here they are, in the files where they belong for our example application and our chosen setup planned above:

reahl.config.py


# In production this has to be set, to the name of the egg of your application:
from __future__ import print_function, unicode_literals, absolute_import, division
reahlsystem.root_egg = 'helloapache'   

# If using SQLite:
reahlsystem.connection_uri = 'sqlite:////var/local/helloapache/helloapache.db'  

# If using PostgreSQL:
#reahlsystem.connection_uri = 'postgresql://helloapache:[email protected]/helloapache'  

web.config.py


from __future__ import print_function, unicode_literals, absolute_import, division
from helloapache import HelloUI

# The UserInterface class acting as the root of the URL hierachy of your application:
web.site_root = HelloUI       

# HTTP is served on port 80, and HTTPS on 443, except when in development!
web.default_http_port = 80
web.encrypted_http_port = 443

# Each application has one (and only one) directory where static files can be served from
# (This directory needs to be owned by www-data)
web.static_root = '/var/local/helloapache/www'  

systemaccountmodel.config.py


# In production, you'd want this to be set so something sensible:
from __future__ import print_function, unicode_literals, absolute_import, division
accounts.admin_email = '[email protected]'  

Create the database

If you are using SQLite, the database will be saved in a file that is directly accessed from the running application process. Since this process is running as the www-data user, that file must be created such that it is readable and writable by the www-data user.

The easiest way to ensure that happens is to run the database creation commands as the www-data user. Remember though to also activate the virtualenv!

Here’s how you can create the database, making sure you’ve activated the correct virtualenv, as the correct user:

sudo su - www-data
source /usr/local/helloapache/virtualenv/bin/activate
reahl-control createdbuser /etc/reahl.d/helloapache
reahl-control createdb /etc/reahl.d/helloapache
reahl-control createdbtables /etc/reahl.d/helloapache

Of course, all of this assumes you are installing a new application. If you are just upgrading your application to a new version, you will want to retain the existing database, but migrate it to the new schema with:

sudo su - www-data
source /usr/local/helloapache/virtualenv/bin/activate
reahl-control migratedb /etc/reahl.d/helloapache

Apache config

To keep things simple, we have merely simplified the apache configuration that is installed by default on an Ubuntu machine. You may want to do things a bit differently. For example, to run your application on a NamedVirtualHost. Note though, that each application would need its own IP address when using HTTPS.

You need to set up two “websites” in Apache: one for HTTP requests, and another for HTTPS requests. Here are the apache configuration files for each of these:

The HTTP site

<VirtualHost *:80>
    ServerAdmin [email protected]
    WSGIScriptAlias / /usr/local/helloapache/helloapache.wsgi
    ErrorLog ${APACHE_LOG_DIR}/helloapache.error.log
    CustomLog ${APACHE_LOG_DIR}/helloapache.access.log combined
</VirtualHost>

The WSGIScriptAlias ensures that the entire site will be handled by the WSGI application set up by the script prepared for that purpose. The ErrorLog and CustomLog could probably also have been left at their defaults, but it is nice to have the logs of each site separately, so we configured them as a matter of habit.

The HTTPS site

<IfModule mod_ssl.c>
<VirtualHost _default_:443>
    ServerAdmin [email protected]
    WSGIScriptAlias / /usr/local/helloapache/helloapache.wsgi
    ErrorLog ${APACHE_LOG_DIR}/helloapache.error.log
    CustomLog ${APACHE_LOG_DIR}/helloapache.ssl_access.log combined

    SSLEngine on
    SSLCertificateFile    /etc/ssl/certs/ssl-cert-snakeoil.pem
    SSLCertificateKeyFile /etc/ssl/private/ssl-cert-snakeoil.key
</VirtualHost>
</IfModule>

The HTTPS site is configured exactly the same, except that it needs to be told to use SSL. For the example, we’re just using the self-signed certificates already installed on an Ubuntu machine.

Let it run!

You’re finally ready to start it all up. As root, first enable both sites in Apache, then start Apache itself:

sudo a2enmod default
sudo a2enmod default-ssl
sudo /etc/init.d/apache start

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