PHP Cron parser documentation
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This is some rudimentary documentation to explain the purpose and
intended use of the Cron Parser class. Currently, the class exists
in isolation, but when I’ve worked more on an implementation, I’ll
post concrete examples and perhaps a complete cron replacement system.
This class does *not* read in your crontab. It relies on being told a
schedule from a crontab and parsing that. A typical implementation would
be to use a database to store data about cron jobs. Typically, in
a two-server environment, serverB's crons would be set to run 2 minutes
after serverA's crons were due to run, first checking if the cron on serverA
actually ran, thus providing cron redundancy.
Current functionality
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The class has various internal methods, but it is not intended that
these be invoked. Instead, just one method should be used, as follows:
$cron_schedule = "0 5 * * *";//example schedule, (5am each day)
//Constructor - takes cron schedule as an argument
$cron = new CronParser($cron_schedule);
//getLastRan method - returns array (0=minute, 1=hour, 2=dayOfMonth, 3=month, 4=week, 5=year)
$lastRan = $cron->getLastRan();
//Make a timestamp from the array
$cron_ran = mktime ( $lastRan[1] ,$lastRan[0],0 , $lastRan[3] ,$lastRan[2], $lastRan[5]);
So, in this example, it’s obvious that the last ran time was 5am
on the current day. However, this calculation becomes more complex
when some other cron schedules are involved, like:
$cron_schedule = "0,12,30-35 5,7-9 * 9-12 0-3,6";
This example asks cron to run a program at a variety of different
times, and the last ran time might be last yesterday, last week,
last month, or even last year. So, if you need to know if a cron ran,
you can add logging to your app, and then create a CronParser instance
to determine when the cron job was due to run last and use fuzzy time
comparison between that time and the timestamp of your log entry in your
database.
Using this system, you can add an environment variable to your crontab
like (HOSTNAME="serverA") and use this in your logging to determine
which server ran the cron. Use logic to run the cron on serverB if
serverA failed to log success.
Future expansion of this project
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The basic parsing of cron schedules has been cracked, and there’s no
reason why a whole host of PHP applications to control scheduled execution
of scripts can’t be created, for sysadmin and web management use. I plan
to write some apps that I can release under GPL / LGPL for these purposes.
Mick Sear (http://www.ecreate.co.uk, mick@ecreate.co.uk)
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