From which user does the scheduled task start?
From user cron.
If two crones run at the same time, will they run sequentially on the same CPU core or simultaneously on different cores (if available)?
On different — in different processes.
If there are more tasks running at the same time than there are processor cores available, in what order will they be executed?
In arbitrary order. Do not rely on the order of cron execution within the same time slot.
Is the execution timeout for crowns the same as for other scripts?
Yes. It is set in Conf.php in the root folder of the installation.
If the cron runs longer than the start interval, what does that result in?
The crown of the next interval will overlap with the previous one.
An example of using global variables
To prevent a cron job from overlapping with an unfinished previous one, a combination of global variables and the functions globVar and tryCatch is used.
The general approach is as follows:
When the cron job starts, it writes a control value to a global variable.
When it finishes, it removes this control value from the variable.
When the next cron job starts in its time slot, it checks the variable. If there is a blocking value, it terminates. If the variable is empty, it writes the control value and starts executing:
=catch: if(condition: $var0 = 0; condition: $control[value] != true; then: $start; else: $control_time)
var0: var(name: "inner"; value: 0)
~control: globVar(name: "contol_crone"; block: 10; date: true)
control_time: if(condition: $dateadd < $#ndt; then: $start)
dateadd: dateAdd(date: $df; minutes: 10; format: "Y-m-d H:i")
df: strPart(str: $control[date]; length: 16)
start: while(preaction: $set_control; preaction: $var1; action: "...."; postaction: $clear_control)
set_control: globVar(name: "contol_crone"; value: true)
var1: var(name: "inner"; value: 1)
clear_control: globVar(name: "contol_crone"; value: false)
catch: if(condition: $#inner = 1; then: $clearGlob)
clearGlob: globVar(name: "contol_crone"; value: false)
The globVar entry is outside the transactional model, so to prevent the variable from getting stuck in case of an error, we set it to false using tryCatch!