How Does Cron Internally Schedule Jobs

How does cron internally schedule jobs?

A few crickets heard in this question. Good 'ol RTFC with some discrete event simulation papers and Wikipedia:

http://en.wikipedia.org/wiki/Cron#Multi-user_capability

The algorithm used by this cron is as
follows:

1. On start-up, look for a file named .crontab in the home directories of
   all account holders.
2. For each crontab file found, determine the next time in the future
   that each command is to be run.
3. Place those commands on the Franta-Maly event list with their
   corresponding time and their "five
   field" time specifier.
4. Enter main loop:
   
   
   1. Examine the task entry at the head of the queue, compute how far in the
      future it is to be run.
   2. Sleep for that period of time.
   3. On awakening and after verifying the correct time, execute the task at
      the head of the queue (in background)
      with the privileges of the user who
      created it.
   4. Determine the next time in the future to run this command and place
      it back on the event list at that time

How does a task scheduler fire a job?

The intent of this questions was not about CRON, but task scheduling in general using cron as an example, sorry if this was not clear in the question statement.

I wanted to know how the the lowest level software does time-based scheduling, if it must poll the hardware clock or if there is some sort of hardware interrupt for time based events.

It turns out there is actually a hardware interrupt. From wilipedia:

One typical use is to generate interrupts periodically by dividing the
output of a crystal oscillator and having an interrupt handler count
the interrupts in order to keep time. These periodic interrupts are
often used by the OS's task scheduler to reschedule the priorities of
running processes. Some older computers generated periodic interrupts
from the power line frequency because it was controlled by the
utilities to eliminate long-term drift of electric clocks.

http://en.wikipedia.org/wiki/Interrupt

So, although cron does polling (thanks @joshua-nelson), it is possible not to and the OS does not.

How to list the node cron's scheduled jobs

cron and node-cron have nothing in common except the job done.

• node-cron is as pure js library
• and cron is a linux service

the sentence you quote seem to come from cron itself while node-cron documentation is here : https://www.npmjs.com/package/node-cron

in node-cron jobs are not stored in any file. only in memory.

technically : node-cron only uses clever setTimeout to start programmatically registered jobs.

Laravel Scheduler and Cron - how does Laravel know not to run the scheduled jobs based on daily(), hourly() etc

Under the hood, the Laravel Scheduler uses https://github.com/dragonmantank/cron-expression to determine if a command or job is scheduled to run at the given minute the schedule:run is called.

Each task you schedule translates to a cron expression, which is then passed into the package. A method called isDue is then run against that expression to determine whether or not it should run. So, if you set a task to run hourly, then isDue will yield true at the top of the hour, and Laravel will execute to the task within the cron cycle.

As such, the information does not need to be stored anywhere, as determination is done on the fly.

This might also lead you to wonder what might happen if you have a long-running task that might take longer than the interval. This is where withoutOverlapping comes into the picture. When called, it creates what is known as a mutex, which is similar to a 'lock' of sorts (see What is a mutex? for more information), when the task is initially run. If a mutex already exists for a particular task on subsquent cycles, it means that task is currently running in another cycle, and should not be triggered again in this one.

Where are mutexes stored? Simple: Laravel stores them in a cache, and when a mutexed task is finished, the mutex is removed from the cache. And so the cycle continues.

I could go into much further detail here, but I think this answers your question for the most part.

How to schedule Crontab in Linux to run for every minutes of X?

I have found the solution:

If X is 5, I will just do it this way in crontab -e:

5,15,25,35,45,55 * * * * run anytask

If X is 3, then:

3,13,23,33,43,53 * * * * run anytask

Firebase schedule crontab limitations

Due to Firebase crontab limitations, I had to solve my issue by giving the exact minutes I want the function to be triggered.

5,15,25,35,45,55 * * * *

Firebase do support that format.

A good use-case, to know if your cron is supportad by Firebase before deploying your function, is to validate your cron on crontab.guru. You will be notify wether your cron is standard or not. Firebase support only standard crons.

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