SIMPLE SOLUTIONS

CRON - reference manual online

Cron-like scheduler for Perl subroutines.

Chapter
2011-06-06
Schedule::Cron(3pm)            User Contributed Perl Documentation            Schedule::Cron(3pm)

NAME Cron - cron-like scheduler for Perl subroutines
SYNOPSIS use Schedule::Cron; # Subroutines to be called sub dispatcher { print "ID: ",shift,"\n"; print "Args: ","@_","\n"; } sub check_links { # do something... } # Create new object with default dispatcher my $cron = new Schedule::Cron(\&dispatcher); # Load a crontab file $cron->load_crontab("/var/spool/cron/perl"); # Add dynamically crontab entries $cron->add_entry("3 4 * * *",ROTATE => "apache","sendmail"); $cron->add_entry("0 11 * * Mon-Fri",\&check_links); # Run scheduler $cron->run(detach=>1);
DESCRIPTION This module provides a simple but complete cron like scheduler. I.e this module can be used for periodically executing Perl subroutines. The dates and parameters for the subroutines to be called are specified with a format known as crontab entry (see "METHODS", "add_entry()" and crontab(5)) The philosophy behind "Schedule::Cron" is to call subroutines periodically from within one single Perl program instead of letting "cron" trigger several (possibly different) Perl scripts. Everything under one roof. Furthermore, "Schedule::Cron" provides mechanism to create crontab entries dynamically, which isn't that easy with "cron". "Schedule::Cron" knows about all extensions (well, at least all extensions I'm aware of, i.e those of the so called "Vixie" cron) for crontab entries like ranges including 'steps', specification of month and days of the week by name, or coexistence of lists and ranges in the same field. It even supports a bit more (like lists and ranges with symbolic names).
METHODS $cron = new Schedule::Cron($dispatcher,[extra args]) Creates a new "Cron" object. $dispatcher is a reference to a subroutine, which will be called by default. $dispatcher will be invoked with the arguments parameter provided in the crontab entry if no other subroutine is specified. This can be either a single argument containing the argument parameter literally has string (default behavior) or a list of arguments when using the "eval" option described below. The date specifications must be either provided via a crontab like file or added explicitly with "add_entry()" ("add_entry"). extra_args can be a hash or hash reference for additional arguments. The following parameters are recognized: file => <crontab> Load the crontab entries from <crontab> eval => 1 Eval the argument parameter in a crontab entry before calling the subroutine (instead of literally calling the dispatcher with the argument parameter as string) nofork => 1 Don't fork when starting the scheduler. Instead, the jobs are executed within current process. In your executed jobs, you have full access to the global variables of your script and hence might influence other jobs running at a different time. This behaviour is fundamentally different to the 'fork' mode, where each jobs gets its own process and hence a copy of the process space, independent of each other job and the main process. This is due to the nature of the "fork" system call. nostatus => 1 Do not update status in $0. Set this if you don't want ps to reveal the internals of your application, including job argument lists. Default is 0 (update status). skip => 1 Skip any pending jobs whose time has passed. This option is only useful in combination with "nofork" where a job might block the execution of the following jobs for quite some time. By default, any pending job is executed even if its scheduled execution time has already passed. With this option set to true all pending which would have been started in the meantime are skipped. catch => 1 Catch any exception raised by a job. This is especially useful in combination with the "nofork" option to avoid stopping the main process when a job raises an exception (dies). after_job => \&after_sub Call a subroutine after a job has been run. The first argument is the return value of the dispatched job, the reminding arguments are the arguments with which the dispatched job has been called. Example: my $cron = new Schedule::Cron(..., after_job => sub { my ($ret,@args) = @_; print "Return value: ",$ret," - job arguments: (",join ":",@args,")\n"; }); log => \&log_sub Install a logging subroutine. The given subroutine is called for several events during the lifetime of a job. This method is called with two arguments: A log level of 0 (info),1 (warning) or 2 (error) depending on the importance of the message and the message itself. For example, you could use Log4perl (<http://log4perl.sf.net>) for logging purposes for example like in the following code snippet: use Log::Log4perl; use Log::Log4perl::Level; my $log_method = sub { my ($level,$msg) = @_; my $DBG_MAP = { 0 => $INFO, 1 => $WARN, 2 => $ERROR }; my $logger = Log::Log4perl->get_logger("My::Package"); $logger->log($DBG_MAP->{$level},$msg); } my $cron = new Schedule::Cron(.... , log => $log_method); loglevel => <-1,0,1,2> Restricts logging to the specified severity level or below. Use 0 to have all messages generated, 1 for only warnings and errors and 2 for errors only. Default is 0 (all messages). A loglevel of -1 (debug) will include job argument lists (also in $0) in the job start message logged with a level of 0 or above. You may have security concerns with this. Unless you are debugging, use 0 or higher. A value larger than 2 will disable logging completely. Although you can filter in your log routine, generating the messages can be expensive, for example if you pass arguments pointing to large hashes. Specifying a loglevel avoids formatting data that your routine would discard. processprefix => <name> Cron::Schedule sets the process' name (i.e. $0) to contain some informative messages like when the next job executes or with which arguments a job is called. By default, the prefix for this labels is "Schedule::Cron". With this option you can set it to something different. You can e.g. use $0 to include the original process name. You can inhibit this with the "nostatus" option, and prevent the argument display by setting "loglevel" to zero or higher. sleep => \&hook If specified, &hook will be called instead of sleep(), with the time to sleep in seconds as first argument and the Schedule::Cron object as second. This hook allows you to use select() instead of sleep, so that you can handle IO, for example job requests from a network connection. e.g. $cron->run( { sleep => \&sleep_hook, nofork => 1 } ); sub sleep_hook { my ($time, $cron) = @_; my ($rin, $win, $ein) = ('','',''); my ($rout, $wout, $eout); vec($rin, fileno(STDIN), 1) = 1; my ($nfound, $ttg) = select($rout=$rin, $wout=$win, $eout=$ein, $time); if ($nfound) { handle_io($rout, $wout, $eout); } return; } $cron->load_crontab($file) $cron->load_crontab(file=>$file,[eval=>1]) Loads and parses the crontab file $file. The entries found in this file will be added to the current time table with "$cron->add_entry". The format of the file consists of cron commands containing of lines with at least 5 columns, whereas the first 5 columns specify the date. The rest of the line (i.e columns 6 and greater) contains the argument with which the dispatcher subroutine will be called. By default, the dispatcher will be called with one single string argument containing the rest of the line literally. Alternatively, if you call this method with the optional argument "eval=>1" (you must then use the second format shown above), the rest of the line will be evaled before used as argument for the dispatcher. For the format of the first 5 columns, please see "add_entry". Blank lines and lines starting with a "#" will be ignored. There's no way to specify another subroutine within the crontab file. All calls will be made to the dispatcher provided at construction time. If you want to start up fresh, you should call "$cron->clean_timetable()" before. Example of a crontab fiqw(le:) # The following line runs on every Monday at 2:34 am 34 2 * * Mon "make_stats" # The next line should be best read in with an eval=>1 argument * * 1 1 * { NEW_YEAR => '1',HEADACHE => 'on' } $cron->add_entry($timespec,[arguments]) Adds a new entry to the list of scheduled cron jobs. Time and Date specification $timespec is the specification of the scheduled time in crontab format (crontab(5)) which contains five mandatory time and date fields and an optional 6th column. $timespec can be either a plain string, which contains a whitespace separated time and date specification. Alternatively, $timespec can be a reference to an array containing the five elements for the date fields. The time and date fields are (taken mostly from crontab(5), "Vixie" cron): field values ===== ====== minute 0-59 hour 0-23 day of month 1-31 month 1-12 (or as names) day of week 0-7 (0 or 7 is Sunday, or as names) seconds 0-59 (optional) A field may be an asterisk (*), which always stands for ``first-last''. Ranges of numbers are allowed. Ranges are two numbers separated with a hyphen. The specified range is inclusive. For example, 8-11 for an ``hours'' entry specifies execution at hours 8, 9, 10 and 11. Lists are allowed. A list is a set of numbers (or ranges) separated by commas. Examples: ``1,2,5,9'', ``0-4,8-12''. Step values can be used in conjunction with ranges. Following a range with ``/<number>'' specifies skips of the numbers value through the range. For example, ``0-23/2'' can be used in the hours field to specify command execution every other hour (the alternative in the V7 standard is ``0,2,4,6,8,10,12,14,16,18,20,22''). Steps are also permitted after an asterisk, so if you want to say ``every two hours'', just use ``*/2''. Names can also be used for the ``month'' and ``day of week'' fields. Use the first three letters of the particular day or month (case doesn't matter). Note: The day of a command's execution can be specified by two fields -- day of month, and day of week. If both fields are restricted (ie, aren't *), the command will be run when either field matches the current time. For example, ``30 4 1,15 * 5'' would cause a command to be run at 4:30 am on the 1st and 15th of each month, plus every Friday Examples: "8 0 * * *" ==> 8 minutes after midnight, every day "5 11 * * Sat,Sun" ==> at 11:05 on each Saturday and Sunday "0-59/5 * * * *" ==> every five minutes "42 12 3 Feb Sat" ==> at 12:42 on 3rd of February and on each Saturday in February "32 11 * * * 0-30/2" ==> 11:32:00, 11:32:02, ... 11:32:30 every day In addition, ranges or lists of names are allowed. An optional sixth column can be used to specify the seconds within the minute. If not present, it is implicitely set to "0". Command specification The subroutine to be executed when the the $timespec matches can be specified in several ways. First, if the optional "arguments" are lacking, the default dispatching subroutine provided at construction time will be called without arguments. If the second parameter to this method is a reference to a subroutine, this subroutine will be used instead of the dispatcher. Any additional parameters will be given as arguments to the subroutine to be executed. You can also specify a reference to an array instead of a list of parameters. You can also use a named parameter list provided as an hashref. The named parameters recognized are: subroutine sub Reference to subroutine to be executed arguments args Reference to array containing arguments to be use when calling the subroutine eval If true, use the evaled string provided with the "arguments" parameter. The evaluation will take place immediately (not when the subroutine is going to be called) Examples: $cron->add_entry("* * * * *"); $cron->add_entry("* * * * *","doit"); $cron->add_entry("* * * * *",\&dispatch,"first",2,"third"); $cron->add_entry("* * * * *",{'subroutine' => \&dispatch, 'arguments' => [ "first",2,"third" ]}); $cron->add_entry("* * * * *",{'subroutine' => \&dispatch, 'arguments' => '[ "first",2,"third" ]', 'eval' => 1}); @entries = $cron->list_entries() Return a list of cron entries. Each entry is a hash reference of the following form: $entry = { time => $timespec, dispatch => $dispatcher, args => $args_ref } Here $timespec is the specified time in crontab format as provided to "add_entry", $dispatcher is a reference to the dispatcher for this entry and $args_ref is a reference to an array holding additional arguments (which can be an empty array reference). For further explanation of this arguments refer to the documentation of the method "add_entry". The order index of each entry can be used within "update_entry", "get_entry" and "delete_entry". But be aware, when you are deleting an entry, that you have to refetch the list, since the order will have changed. Note that these entries are returned by value and were opbtained from the internal list by a deep copy. I.e. you are free to modify it, but this won't influence the original entries. Instead use "update_entry" if you need to modify an exisiting crontab entry. $entry = $cron->get_entry($idx) Get a single entry. $entry is either a hashref with the possible keys "time", "dispatch" and "args" (see "list_entries()") or undef if no entry with the given index $idx exists. $cron->delete_entry($idx) Delete the entry at index $idx. Returns the deleted entry on success, "undef" otherwise. $cron->update_entry($idx,$entry) Updates the entry with index $idx. $entry is a hash ref as descibed in "list_entries()" and must contain at least a value "$entry->{time}". If no "$entry->{dispatcher}" is given, then the default dispatcher is used. This method returns the old entry on success, "undef" otherwise. $cron->run([options]) This method starts the scheduler. When called without options, this method will never return and executes the scheduled subroutine calls as needed. Alternatively, you can detach the main scheduler loop from the current process (daemon mode). In this case, the pid of the forked scheduler process will be returned. The "options" parameter specifies the running mode of "Schedule::Cron". It can be either a plain list which will be interpreted as a hash or it can be a reference to a hash. The following named parameters (keys of the provided hash) are recognized: detach If set to a true value the scheduler process is detached from the current process (UNIX only). pid_file If running in daemon mode, name the optional file, in which the process id of the scheduler process should be written. By default, no PID File will be created. nofork, skip, catch, log, loglevel, nostatus, sleep See "new()" for a description of these configuration parameters, which can be provided here as well. Note, that the options given here overrides those of the constructor. Examples: # Start scheduler, detach from current process and # write the PID of the forked scheduler to the # specified file $cron->run(detach=>1,pid_file=>"/var/run/scheduler.pid"); # Start scheduler and wait forever. $cron->run(); $cron->clean_timetable() Remove all scheduled entries $cron->check_entry($id) Check, whether the given ID is already registered in the timetable. A ID is the first argument in the argument parameter of the a crontab entry. Returns (one of) the index in the timetable (can be 0, too) if the ID could be found or "undef" otherwise. Example: $cron->add_entry("* * * * *","ROTATE"); . . defined($cron->check_entry("ROTATE")) || die "No ROTATE entry !" $cron->get_next_execution_time($cron_entry,[$ref_time]) Well, this is mostly an internal method, but it might be useful on its own. The purpose of this method is to calculate the next execution time from a specified crontab entry Parameters: $cron_entry The crontab entry as specified in "add_entry" $ref_time The reference time for which the next time should be searched which matches $cron_entry. By default, take the current time This method returns the number of epoch-seconds of the next matched date for $cron_entry. Since I suspect, that this calculation of the next execution time might fail in some circumstances (bugs are lurking everywhere ;-) an additional interactive method "bug()" is provided for checking crontab entries against your expected output. Refer to the top-level README for additional usage information for this method.
DST ISSUES Daylight saving occurs typically twice a year: In the first switch, one hour is skipped. Any job which which triggers in this skipped hour will be fired in the next hour. So, when the DST switch goes from 2:00 to 3:00 a job which is scheduled for 2:43 will be executed at 3:43. For the reverse backwards switch later in the year, the behaviour is undefined. Two possible behaviours can occur: For jobs triggered in short intervals, where the next execution time would fire in the extra hour as well, the job could be executed again or skipped in this extra hour. Currently, running "Schedule::Cron" in "MET" would skip the extra job, in "PST8PDT" it would execute a second time. The reason is the way how Time::ParseDate calculates epoch times for dates given like "02:50:00 2009/10/25". Should it return the seconds since 1970 for this time happening 'first', or for this time in the extra hour ? As it turns out, Time::ParseDate returns the epoch time of the first occurence for "PST8PDT" and for "MET" it returns the second occurence. Unfortunately, there is no way to specify which entry Time::ParseDate should pick (until now). Of course, after all, this is obviously not Time::ParseDate's fault, since a simple date specification within the DST backswitch period is ambigious. However, it would be nice if the parsing behaviour of Time::ParseDate would be consistent across time zones (a ticket has be raised for fixing this). Then Schedule::Cron's behaviour within a DST backward switch would be consistent as well. Since changing the internal algorithm which worked now for over ten years would be too risky and I don't see any simple solution for this right now, it is likely that this undefined behaviour will exist for some time. Maybe some hero is coming along and will fix this, but this is probably not me ;-) Sorry for that.
LICENSE Copyright 1999-2011 Roland Huss. This library is free software; you can redistribute it and/or modify it under the same terms as Perl itself.
AUTHOR ... roland
perl v5.18.2 2011-06-06 Schedule::Cron(3pm)
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refer to crontab(5)