Question

I’m betting that someone has already solved this and maybe I’m using the wrong search terms for google to tell me the answer, but here is my situation.

I have a script that I want to run, but I want it to run only when scheduled and only one at a time. (can’t run the script simultaneously)

Now the sticky part is that say I have a table called ‘myhappyschedule’ which has the data I need and the scheduled time. This table can have multiple scheduled times even at the same time, each one would run this script. So essentially I need a queue of each time the script fires and they all need to wait for each one before it to finish. (sometimes this can take just a minute for the script to execute sometimes its many many minutes)

What I’m thinking about doing is making a script that checks myhappyschedule every 5 min and gathers up those that are scheduled, puts them into a queue where another script can execute each ‘job’ or occurrence in the queue in order. Which all of this sounds messy.

To make this longer – I should say that I’m allowing users to schedule things in myhappyschedule and not edit crontab.

What can be done about this? File locks and scripts calling scripts?

Answer 1

add a column exec_status to myhappytable (maybe also time_started and time_finished, see pseudocode)

run the following cron script every x minutes

pseudocode of cron script:

[create/check pid lock (optional, but see 'A potential pitfall' below)] get number of rows from myhappytable where (exec_status == executing_now) if it is > 0, exit begin loop   get one row from myhappytable     where (exec_status == not_yet_run) and (scheduled_time <= now)     order by scheduled_time asc   if no such row, exit   set row exec_status to executing_now (maybe set time_started to now)   execute whatever command the row contains   set row exec_status to completed   (maybe also store the command output/return as well, set time_finished to now) end loop [delete pid lock file (complementary to the starting pid lock check)] 

This way, the script first checks if none of the commands is running, then runs first not-yet run command, until there are no more commands to be run at the given moment. Also, you can see what command is executing by querying the database.

A potential pitfall: if the cron script is killed, a scheduled task will remain in ‘executing_now’ state. That’s what the pid lock at beginning and end is for: to see if the cron script terminated properly. pseudocode of create/check pidlock:

if exists pidlockfile then   check if process id given in file exists   if not exists then     update myhappytable set exec_status = error_cronscript_died_while_executing_this          where exec_status == executing_now     delete pidlockfile   else (previous instance still running)     exit   endif endif create pidlockfile containing cron script process id