# 4. FastCGI¶

FastCGI is a deployment option on servers like nginx, lighttpd, and cherokee; see uWSGI and Standalone WSGI Containers for other options. To use your WSGI application with any of them you will need a FastCGI server first. The most popular one is flup which we will use for this guide. Make sure to have it installed to follow along.

Watch Out

Please make sure in advance that any app.run() calls you might have in your application file are inside an if __name__ == '__main__': block or moved to a separate file. Just make sure it’s not called because this will always start a local WSGI server which we do not want if we deploy that application to FastCGI.

## 4.1. Creating a .fcgi file¶

First you need to create the FastCGI server file. Let’s call it yourapplication.fcgi:

#!/usr/bin/python
from flup.server.fcgi import WSGIServer
from yourapplication import app

if __name__ == '__main__':
WSGIServer(app).run()


This is enough for Apache to work, however nginx and older versions of lighttpd need a socket to be explicitly passed to communicate with the FastCGI server. For that to work you need to pass the path to the socket to the WSGIServer:

WSGIServer(application, bindAddress='/path/to/fcgi.sock').run()


The path has to be the exact same path you define in the server config.

Save the yourapplication.fcgi file somewhere you will find it again. It makes sense to have that in /var/www/yourapplication or something similar.

Make sure to set the executable bit on that file so that the servers can execute it:

# chmod +x /var/www/yourapplication/yourapplication.fcgi


## 4.2. Configuring Apache¶

The example above is good enough for a basic Apache deployment but your .fcgi file will appear in your application URL e.g. example.com/yourapplication.fcgi/news/. There are few ways to configure your application so that yourapplication.fcgi does not appear in the URL. A preferable way is to use the ScriptAlias and SetHandler configuration directives to route requests to the FastCGI server. The following example uses FastCgiServer to start 5 instances of the application which will handle all incoming requests:

LoadModule fastcgi_module /usr/lib64/httpd/modules/mod_fastcgi.so

FastCgiServer /var/www/html/yourapplication/app.fcgi -idle-timeout 300 -processes 5

<VirtualHost *>
ServerName webapp1.mydomain.com
DocumentRoot /var/www/html/yourapplication

ScriptAlias / /var/www/html/yourapplication/app.fcgi/

<Location />
SetHandler fastcgi-script
</Location>
</VirtualHost>


These processes will be managed by Apache. If you’re using a standalone FastCGI server, you can use the FastCgiExternalServer directive instead. Note that in the following the path is not real, it’s simply used as an identifier to other directives such as AliasMatch:

FastCgiServer /var/www/html/yourapplication -host 127.0.0.1:3000


If you cannot set ScriptAlias, for example on a shared web host, you can use WSGI middleware to remove yourapplication.fcgi from the URLs. Set .htaccess:

<IfModule mod_fcgid.c>
<Files ~ (\.fcgi)>
SetHandler fcgid-script
</Files>
</IfModule>

<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteCond %{REQUEST_FILENAME} !-f
RewriteRule ^(.*)$yourapplication.fcgi/$1 [QSA,L]
</IfModule>


Set yourapplication.fcgi:

#!/usr/bin/python
#: optional path to your local python site-packages folder
import sys
sys.path.insert(0, '<your_local_path>/lib/python2.6/site-packages')

from flup.server.fcgi import WSGIServer
from yourapplication import app

class ScriptNameStripper(object):
def __init__(self, app):
self.app = app

def __call__(self, environ, start_response):
environ['SCRIPT_NAME'] = ''
return self.app(environ, start_response)

app = ScriptNameStripper(app)

if __name__ == '__main__':
WSGIServer(app).run()


## 4.3. Configuring lighttpd¶

A basic FastCGI configuration for lighttpd looks like that:

fastcgi.server = ("/yourapplication.fcgi" =>
((
"socket" => "/tmp/yourapplication-fcgi.sock",
"bin-path" => "/var/www/yourapplication/yourapplication.fcgi",
"check-local" => "disable",
"max-procs" => 1
))
)

alias.url = (
"/static/" => "/path/to/your/static"
)

url.rewrite-once = (
"^(/static($|/.*))$" => "$1", "^(/.*)$" => "/yourapplication.fcgi$1" )  Remember to enable the FastCGI, alias and rewrite modules. This configuration binds the application to /yourapplication. If you want the application to work in the URL root you have to work around a lighttpd bug with the LighttpdCGIRootFix middleware. Make sure to apply it only if you are mounting the application the URL root. Also, see the Lighty docs for more information on FastCGI and Python (note that explicitly passing a socket to run() is no longer necessary). ## 4.4. Configuring nginx¶ Installing FastCGI applications on nginx is a bit different because by default no FastCGI parameters are forwarded. A basic Flask FastCGI configuration for nginx looks like this: location = /yourapplication { rewrite ^ /yourapplication/ last; } location /yourapplication { try_files$uri @yourapplication; }
location @yourapplication {
include fastcgi_params;
fastcgi_split_path_info ^(/yourapplication)(.*)$; fastcgi_param PATH_INFO$fastcgi_path_info;
fastcgi_param SCRIPT_NAME $fastcgi_script_name; fastcgi_pass unix:/tmp/yourapplication-fcgi.sock; }  This configuration binds the application to /yourapplication. If you want to have it in the URL root it’s a bit simpler because you don’t have to figure out how to calculate PATH_INFO and SCRIPT_NAME: location / { try_files$uri @yourapplication; }
location @yourapplication {
include fastcgi_params;
$/var/www/yourapplication/yourapplication.fcgi  ## 4.6. Debugging¶ FastCGI deployments tend to be hard to debug on most web servers. Very often the only thing the server log tells you is something along the lines of “premature end of headers”. In order to debug the application the only thing that can really give you ideas why it breaks is switching to the correct user and executing the application by hand. This example assumes your application is called application.fcgi and that your web server user is www-data: $ su www-data
$cd /var/www/yourapplication$ python application.fcgi
Traceback (most recent call last):
File "yourapplication.fcgi", line 4, in <module>
ImportError: No module named yourapplication


In this case the error seems to be “yourapplication” not being on the python path. Common problems are:

• Relative paths being used. Don’t rely on the current working directory.
• The code depending on environment variables that are not set by the web server.
• Different python interpreters being used.