UoL LAMP HowTo Install Python Flask Applications

Tags: HowTo Install Pain in the proverbial UoL LAMP Server

Difficulties

This installations is made more difficult by 3 things:

1. Suse Linux does not have mod_wsgi in its repositories, so you're going to have to compile it.
2. mod_wsgi seems picky about where it picks python apps and libraries from
3. The LAMP servers don't put things where you'd expect them to be.
4. Some other stuff that I don't quite understand

Requirements

Before installation can start, the following packages will have to be installed by IT services:

• libmysqlclient-dev
• python-dev

Start Services

Start the Apache and MySQL and make sure that they are restarted when the server is rebooted.

sudo /sbin/chkconfig uol.apache2 on
sudo /etc/init.d/uol.apache2 start
sudo /sbin/chkconfig uol.mysql on
sudo /etc/init.d/uol.mysql start

Proceduce

1. Install mod_wsgi.so into the directory /local/apache2/etc/
   1. Compile mod_wsgi. See: HowTo Compile mod_wsgi for LAMP servers
   2. Copy mod_wsgi.so from the directory /local/apache2/etc/ of an existing UoL LAMP python Flask application.
2. Clone the application from the git repository into /local/ directory.
3. Install virtualenv:

easy_install --install-dir=/local/python virtualenv

For some reason that I can't work out (point 4. above), it would only pick up some of the python libraries from a virtual env and not when they were installed in /local/python/. Maybe this was because of the order that I installed things, but I couldn't get it to work without the virtual environment.

4. Create a virtual environment in the application directory.

cd /local/{application directory}
/local/python/virtualenv --no-site-packages BASELINE

BASELINE is the name of the virtual environment. It could be called anything, but it's called that! Also note that you need to use the full path to the virtualenv utility, since /local/python isn't in $PATH.

5. Activate the environment, install the required packages, then deactivate. This example shows some common requirements:

source BASELINE/bin/activate
easy_install flask
easy_install flask-sqlalchemy
easy_install mysql-python
easy_install flask-wtf
easy_install WTForms-Components
deactivate

6. Load the WSGI module into Apache, by editing the file /local/apache2/etc/loadmodule.conf by adding this line at the end.

LoadModule wsgi_module /local/apache2/etc/mod_wsgi.so

7. Add the WSGI config to the Apache config file /local/apache2/etc/httpd.conf:

<Directory /local/{application directory}>
    WSGIProcessGroup {application name}
    WSGIApplicationGroup %{GLOBAL}
    Order deny,allow
    Allow from all
</Directory>

WSGIDaemonProcess {application name} user=wwwrun threads=5 python-path=/local/{application directory}/BASELINE/lib/python2.6/site-packages:/local/python:/usr/lib64/python2.6/site-packages:/usr/share/doc/packages/ home=/local/{application directory}/
WSGIScriptAlias / /local/{application directory}/{path to *.wsgi}

The two hard won things here are the values for the python-path and home arguments. The python-path must contain every directory where python libraries are installed, including a sub-directory of the virtualenv directory you created earlier. The home must point to your application directory, or you won't be able to pick up your own modules.

8. Restart apache:

sudo /etc/init.d/uol.apache2 restart

9. Create any databases that are required and given the appropriate user permissions.

10. Update the application settings in /local/{application directory}

10. Pray.

Other Possible Stuff (Not required last time used**)

1. You may need to give wwwrun extra permissions to the /local/{application directory} directory.
2. You might need to pray a bit more.

setfacl -m u:wwwrun:rwx /local/{application directory}
setfacl -m d:u:wwwrun:rwx /local/telomere/{application directory}

Gotchas

1. Import of database module fails

This could actually be an error that the application does not have access to the database. It fails with a 'cannot access database' error in the log when it processes the first request after restarting Apache, but subsequent requests cause an 'database module not found' error.

Error: Macro BackLinks(None) failed

'Environment' object has no attribute 'get_db_cnx'