I keep toying with the idea of writing a cron that implements a Poisson process. Say I give it a parameter of 3600; our `pcron` would ensure the jobs occur randomly but average out to once per hour, making the timing of the next run independent of the last via the memoryless property of the exponential distribution.
The next sleep interval would be calculated probably as as t = -\lambda \ln(U) (where U is a uniform random variable). This way you ensure that the probability of the job firing in the next 10 seconds is the same whether the last job finished an hour ago or just five seconds ago. But \lambda remains the average amount of time between jobs.
It’s compelling to me because it solves thundering herd problems at the architectural level, and also because it simply seems like a lot of fun to have to code very defensively against such chaos. Switching back to a deterministic schedule after surviving such chaos probably leads to a much more robust system overall.
Does anyone maintain a programmatically accessible list of holidays for their company? Similar to the HOLIDAYS.txt in the article, but it would allow for things like “don’t run this the day before or during a company holiday.”
I work at a company with different holidays in certain countries, which would complicate things, and require something more structured than a list of dates. But having that accessible could be useful.
Has anyone tackled that, or come across a solution?
I think this could be solved quite similar to the OP and better done with systemd. Spitballing, but I think the best thing to do would be to write the timer in a standard file but have the activation time be written in an override file. That way you can ensure you are just editing that file with your scraper (should be able to hit the API if it is something like a google calendar or outlook).
I think the systemd timer would give you the benefit here as you can write the time in varying formats. Timezones, UTC, local, or whatever. That should give you the structure you need, if I'm understanding your problem correctly.
While systemd has more boilerplate than cron I think it has a lot of advantages that make it worth it. Best to just have a skeleton of these jobs (I keep some in my dotfiles) and then you have it. Or have the LLM write it (ironically one of the few instances I'll advocate for letting the AI write the code). You can do everything in the article and so much more.
Ruby has https://github.com/bokmann/business_time but when I looked at it, custom code was needed to calculate holidays that were offset because they are on the weekend.
I had something come up recently that I think sounds similar. That project needs several time-sensitive jobs. When any one of them runs, the first thing it does is check a holidays.json file.
It parses the file using jq and compares its entries with the current time according to GNU date. At the root is the names of the jobs. Each job has its own list of holidays. Each of these holiday items in the job's respective list has keys for the display name of the holiday, the formatted date to compare to, and in a few cases the ISO day-of-week and a string containing a modulo arithmetic function (e.g. don't run the friday before Christmas, etc.).
Sorry, yes that means I call eval on that string and yes that means some of these are repeated in the same file under the arrays for the other jobs. Also, such lists will have to be maintained and the exact observed dates cannot always be known ahead of time beyond about a year since people can change their minds for various reasons (think bank holidays). Depending on your use case you may also want to define a start time and end time for a window of when this should or shouldn't run (i.e. business hours).
I don't know if that helps. I know it's hacky, but I don't think there's a nice way to handle things like "second monday after 4th of july, but if the 4th also happens to be monday then it should instead be the second tuesday". God help you if you also need to handle each holiday being observed in different timezones. At least at the end of the day none of this would be much code, just very terse code dense with meaning.
Great post. And if you want some control support for your cronjobs perl App::Cronjob[1] can provide features such has exclusive locking, so a job won't run if the previous run is still going, or provide a timeout, and some options for sending mail on success or failure
This is great! I'm sure like a lot of programmers, I had been fulfilling the requirement for similar conditional logic by having a simple recurring cron job run other code or database queries with the conditional logic that this post demonstrates can be done directly in cron.
I keep toying with the idea of writing a cron that implements a Poisson process. Say I give it a parameter of 3600; our `pcron` would ensure the jobs occur randomly but average out to once per hour, making the timing of the next run independent of the last via the memoryless property of the exponential distribution.
The next sleep interval would be calculated probably as as t = -\lambda \ln(U) (where U is a uniform random variable). This way you ensure that the probability of the job firing in the next 10 seconds is the same whether the last job finished an hour ago or just five seconds ago. But \lambda remains the average amount of time between jobs.
It’s compelling to me because it solves thundering herd problems at the architectural level, and also because it simply seems like a lot of fun to have to code very defensively against such chaos. Switching back to a deterministic schedule after surviving such chaos probably leads to a much more robust system overall.
Does anyone maintain a programmatically accessible list of holidays for their company? Similar to the HOLIDAYS.txt in the article, but it would allow for things like “don’t run this the day before or during a company holiday.”
I work at a company with different holidays in certain countries, which would complicate things, and require something more structured than a list of dates. But having that accessible could be useful.
Has anyone tackled that, or come across a solution?
I think this could be solved quite similar to the OP and better done with systemd. Spitballing, but I think the best thing to do would be to write the timer in a standard file but have the activation time be written in an override file. That way you can ensure you are just editing that file with your scraper (should be able to hit the API if it is something like a google calendar or outlook).
I think the systemd timer would give you the benefit here as you can write the time in varying formats. Timezones, UTC, local, or whatever. That should give you the structure you need, if I'm understanding your problem correctly.
While systemd has more boilerplate than cron I think it has a lot of advantages that make it worth it. Best to just have a skeleton of these jobs (I keep some in my dotfiles) and then you have it. Or have the LLM write it (ironically one of the few instances I'll advocate for letting the AI write the code). You can do everything in the article and so much more.
https://man.archlinux.org/man/systemd.time.7
At our company we have enough systems reliant on holiday dates that we have a Holiday system that emits events when there are changes.
This happens surprisingly often, given that religious dates change and there are holidays/closures for storms in some regions.
Ruby has https://github.com/bokmann/business_time but when I looked at it, custom code was needed to calculate holidays that were offset because they are on the weekend.
I had something come up recently that I think sounds similar. That project needs several time-sensitive jobs. When any one of them runs, the first thing it does is check a holidays.json file.
It parses the file using jq and compares its entries with the current time according to GNU date. At the root is the names of the jobs. Each job has its own list of holidays. Each of these holiday items in the job's respective list has keys for the display name of the holiday, the formatted date to compare to, and in a few cases the ISO day-of-week and a string containing a modulo arithmetic function (e.g. don't run the friday before Christmas, etc.).
Sorry, yes that means I call eval on that string and yes that means some of these are repeated in the same file under the arrays for the other jobs. Also, such lists will have to be maintained and the exact observed dates cannot always be known ahead of time beyond about a year since people can change their minds for various reasons (think bank holidays). Depending on your use case you may also want to define a start time and end time for a window of when this should or shouldn't run (i.e. business hours).
I don't know if that helps. I know it's hacky, but I don't think there's a nice way to handle things like "second monday after 4th of july, but if the 4th also happens to be monday then it should instead be the second tuesday". God help you if you also need to handle each holiday being observed in different timezones. At least at the end of the day none of this would be much code, just very terse code dense with meaning.
Great post. And if you want some control support for your cronjobs perl App::Cronjob[1] can provide features such has exclusive locking, so a job won't run if the previous run is still going, or provide a timeout, and some options for sending mail on success or failure
[1]https://metacpan.org/pod/App::Cronjob https://metacpan.org/dist/App-Cronjob/view/bin/cronjob
Also check out the 'chronic' command from moreutils. No more dev nulls.
I learned something cool about cron filtering and a nice api I didn't know existed - date.nager.at
This is great! I'm sure like a lot of programmers, I had been fulfilling the requirement for similar conditional logic by having a simple recurring cron job run other code or database queries with the conditional logic that this post demonstrates can be done directly in cron.
Cool. Had no idea you could run commands inside a CRON expression.
Running a command is the main idea of cron. In this case, the author runs composite commands like:
Where 'test' is another shell command that returns 0 or 1. This is not a special cron syntax, it's just the inherent capability of the Unix shell.
In any case, this whole approach is very clever and shows the beauty of The Unix Way.