So my first completed web project is an app that notifies users when a new post appears on a craigslist search. The user can have as many searches as they want, and can receive notifications via email or text message. Here is the link to the app, CLAlerts. The app is pretty bare bones, with a very basic html/css front-end (taken from the flaskr tutorial) and a python/flask/sqlalchemy back-end. The github repo is here. Overtime, I may add a few more features or re-design the app, but at the moment I am just happy to have something that is functional that I can show off.
I was fairly familiar with running a simple flask app thanks to tutorials floating around the web, but this app involved regularly scraping craigslist to check for new posts, which as far as I know, is outside of the capabilities of flask. I have used the linux utility cron to run scraping scripts in the past, and a similar solution seemed appropriate for this app. I wanted to keep things as pyhtonic as possible, so I dug around and turns out there is a scheduler python library called APScheduler. I used the library to run the scraping and message sending tasks for the app. You can check out the final product on github, but below is a sample code snippet.
from apscheduler.scheduler import Scheduler
import time
sched = Scheduler()
@sched.interval_schedule(hours=1)
def run_tasks():
task1()
task2()
sched.start()
while True:
time.sleep(1)
pass
It’s pretty straight forward. You use the ‘@sched’ decorator to tell APScheduler what functions you want to run. You can define the interval a couple of ways, in this case I am using the interval scheduler, but you can also use a date based or cron style scheduler. The while loop at the end keeps the process running. I read somewhere that using the ‘time.sleep(1)’ line in the while loop stops the process from hogging up CPU resources.
So far this seems to be working. I have launched the app using a micro aws instance. I’ve setup two tmux sessions and have the flask app running in one session and the scheduler running in another. The scheduler runs once an hour and scrapes craigslist and sends out the alerts using the Twilio API for text messages and the python SMTP library for email. So far it seems to be running fairly reliably. At one point, the scheduler appeared to freeze and I had to kill the process and restart the script. I still haven’t figured out what caused it, my only guess is that the script froze up on a database connection error. I am currently using sqlite which can’t handle simultaneous writing, but at the moment, I don’t have a way to test this theory. If I find the time, I might try and set up a test for this. I’ll make sure to write a post about it if I do.