How to use python sched in a daemon process

Let’s assume we want to download a file (or to do some tasks) to every 5 seconds, but the condition is to not do the same task twice or more times at the same moment, even if takes more than 5 seconds. For example, we have a to download a file and this will take 8 seconds. Also, if takes more than 5 seconds, it should not wait until the next iteration, to start again (3 seconds more), but will start the download immediately.

So, the traditional cronjob/lock file combination was not suitable for my case.

I chose to use python language with python-daemon and sched.

To install python-daemon, you can do it with

The code below should be self explanatory, but I will do a short presentation. We start the main()function in the daemon context. After 1 second, we will call run_scheduled() first time. This one will call our “download” function get_file that can take a random time to finish its execution (we setup the random between 1 and 10 seconds). Inside run_scheduled, after the first run, we will schedule the next run depending on how much it takes the download and call again the same method:

where restart can be between 0 and 4 (either the download took 1 to 4 seconds, so we have to wait 5 to 1 seconds, or it took more than 5 seconds, that means to schedule the download immediately).

Of course, there are also other methods to achieve the same result like, threads or simple while True… with sleep between iterations 🙂

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