Cron

by webjunkie on Cron, DB, Paste, Pyramid, Python, SQLAlchemy

Running Standalone SQLAlchemy Scripts in Pyramid

From time to time there comes the need to run automated scripts managed by either Python or Cron. In a recent project I had to run a standalone script that checks records on a database at certain hours. So here’s what I came up with. Most of the script is based on this post by Daniel Mayer.

from paste.deploy import appconfig
from sqlalchemy import engine_from_config
from sqlalchemy.orm.exc import NoResultFound, MultipleResultsFound
#here is where sqlalchemy objects are imported
from PRJ.models import DBSession as db,DBRecords
#import the session manager. This way commits will be handled automatically by the Zope Transaction Manager
import transaction
# Load Application Configuration and Return as Dictionary
conf = appconfig('config:' + 'development.ini',
        relative_to="/var/www/PRJ",
        name="main")
# Bind Engine Based on Config
engine = engine_from_config(conf, 'sqlalchemy.')
db.configure(bind=engine)
#Query the DB
data = db.query(DBRecords).one()
with transaction.manager:
  for record in data:
    #query or update DB
 
by webjunkie on Cron, Python

Manage cron jobs with python-crontab

Cron is the main time based scheduler for any linux based system and is available in almost every distro. And in a recent project I had the task to manage jobs in cron from python. Searching for a good cron manager I came across python-crontab. It makes it really easy to manage jobs directly from cron, here are some examples:
NOTE: This examples used version 0.9.6, there’s a new version available 1.2 on pypi along with some examples, the main difference is that the API has been changed from slice calls to be properties instead of methods.

Installing python-crontab is easy as pie. First we install our virtual enviroment:

cd /var/www
python virtualenv.py --no-site-packages prj-env
cd prj-env
bin/activate

Then we proceed to install python-crontab

pip install python-crontab
or
easy_install python-crontab

Let’s schedule a job to be executed everyday at 12pm

from crontab import CronTab
"""
Here the object can take two parameters one for setting
the user cron jobs, it defaults to the current user
executing the script if ommited. The fake_tab parameter
sets a testing variable. So you can print what could be
written to the file onscreen instead or writting directly
into the crontab file.
"""
tab = CronTab(user='www',fake_tab='True')
cmd = '/var/www/pjr-env/bin/python /var/www/PRJ/job.py'
# You can even set a comment for this command
cron_job = tab.new(cmd, comment='This is the main command')
cron_job.minute().on(0)
cron_job.hour().on(12)
#writes content to crontab
tab.write()
print tab.render()

It will print out

0 12 * * * /var/www/pjr-env/bin/python /var/www/PRJ/job.py

If we want to schedule a job to be executed every five minutes we could do something like this

from crontab import CronTab
tab = CronTab(user='www',fake_tab='True')
cmd = '/var/www/pjr-env/bin/python /var/www/PRJ/job.py'
cron_job = tab.new(cmd)
cron_job.minute().every(5)
#writes content to crontab
tab.write()
print tab.render()

It will print out

*/5 * * * * /var/www/pjr-env/bin/python /var/www/PRJ/job.py

If we want to schedule a job for a specific range of hours for example only working hours, we could do the following

from crontab import CronTab
tab = CronTab(user='www',fake_tab='True')
cmd = '/var/www/pjr-env/bin/python /var/www/PRJ/job.py'
cron_job = tab.new(cmd)
cron_job.minute().on(0)
cron_job.hour().during(09,18)
#writes content to crontab
tab.write()
print tab.render()

It will print out

0 09-18 * * * /var/www/pjr-env/bin/python /var/www/PRJ/job.py

Now to schedule a job to run twice a day at 11 and 16 hrs, we could do the following

from crontab import CronTab
tab = CronTab(user='www',fake_tab='True')
cmd = '/var/www/pjr-env/bin/python /var/www/PRJ/job.py'
cron_job = tab.new(cmd)
cron_job.minute().on(0)
cron_job.hour().on('11,16')
#writes content to crontab
tab.write()
print tab.render()

it will print out

0 11,16 * * * /var/www/pjr-env/bin/python /var/www/PRJ/job.py

Let’s delete the previous command

from crontab import CronTab
cmd = '/var/www/pjr-env/bin/python /var/www/PRJ/job.py'
tab = CronTab(user='www',fake_tab='True')
cron_job = tab.find_command(cmd)
if len(cron_job) > 0:
    tab.remove_all(cmd)
#writes content to crontab
tab.write()

So there you have it, examples that make python-crontab a great python manager for cron jobs. Now I know there are pure Python implementations like this, an event scheduler named sched and libraries like Kronos. But I decided to keep things simple.