NAME

Chronic - A constraints-based, opportunistic, application-level scheduler.

DESCRIPTION

Chronic is a constraints-based, opportunistic, application-level scheduler. Unlike cron, that schedules according to a time specification, Chronic schedules tasks when the specified constraints are met. In other words, Chronic schedules during a window of opportunity when the circumstances are favorable.

The original motivation for Chronic was for me to be able to schedule high-impact, recurring tasks like updatedb, emerge rsync, webalizer when I was away from the computer. This proved to be useful enough to abstract the notion of a constraint and build an opportunistic scheduler that schedules according to combinations of arbitrary constraints. The current version of Chronic natively contains the following constraints: System Inactivity, Disk I/O & Load Average. Other constraints planned for near future include: Network IO activity, cron (which emulates cron), and file alterations under a specified directory.

USAGE

Chronic is implemented as a unix daemon, chronicd that reads a chrontab file and executes tasks specified therein. A system level chrontab is placed in /etc/chrontab and user-specific chrontab's live in $HOME/.chrontab. The only option chronicd accepts at the moment is -fg that instructs the application to run in foreground and print debugging messages.

CHRONTAB FORMAT

chrontab contains a task per line. A task consists of a command to be run, and one or more constraint's that must be met before the command is run. The format of task specification is:

    command = COMMAND; constraint = NAME, PARAMETERS; \ 
        constraint = NAME, PARAMETERS;

    eg: 

    command = "emerge rsync"; constraint = Freq, 86400; \
        constraint = Inactivity, 60;

A task specification can be split on multiple lines using the "\" character. It should be noted that constraint names are case-sensitive (and correspond to chucks of code with the same name).

CONSTRAINTS

Chronic supports the following constraints in the current version. This list is likely to grow with future releases, so you should check this manpage after upgrading Chronic.

The Inactivity Constraint

The Inactivity constraint monitors system activity and is met when the system is inactive for a specified amount of time. Inactivity takes one parameter: the number of seconds for which the system must be inactive before the constraint is met. The following task will run updatedb when the system has been inactive for 120 seconds.

    command = "/usr/bin/updatedb"; constraint = Inactivity, 120;

Inactivity is a wrapper around the DiskIO and Loadavg constraints, described later in this section. The constraint's notion of inactivity should be appropriate for most systems. Otherwise, it is possible to tune the notion of inactivity using the DiskIO and Loadavg constraints.

The Freq Constraint

The Freq constraint specifies the maximum frequency of invocation of a task. Freq requires one parameter, the number of seconds, that is the minimum difference between two consecutive invocations. So, Freq, 86400 implies that the task should not be run more than once a day. Freq is usually coupled with another constraint (eg: Inactivity) to limit the frequency of opportunistic scheduling. For example:

    command = "emerge rsync"; constraint = Inactivity, 120; \ 
        constraint = Freq, 86400;

says, run "emerge rsync" once a day. However, if the system is not inactive for more than one day, "emerge rsync" won't be run till the system is inactive.

The Loadavg Constraint

The Loadavg constraint monitors the load average of the system. It accepts two parameters: a load average threshold and the amount of time the system load should linger at or below the this threshold before the constraint is considered to be met. The syntax is Loadavg, TIME, LOADAVG. eg. Loadavg, 60, 0.02 is met when the system has a load average of 0.02 or lower for 60 consecutive seconds. Here's an example of usage:

    command = "cd /home/user/project/; cvs update; make all"; \ 
        constraint = Loadavg, 60, 0.0; constraint = Freq, 3600;

This task specifies invocation of CVS update under /home/user/project/ and make if the load average is 0.0 for 60 seconds. The Freq constraint additionally limits the task to no more than once in 3600 seconds (one hour).

The DiskIO Constraint

The DiskIO constraint monitors the disk read/write activity through the vmstat program. It accepts three parameters: a theshold for number of blocks read from disk, a threshold for number of blocks written to disk and the amount of time for which read/write linger at or below these thresholds. DiskIO syntax is DiskIO, TIME, BLOCKS WRITTEN, BLOCKS READ. eg. DiskIO, 60, 10, 15 is met when 10 or less blocks are written to disk and 15 or less blocks are read from disk for 60 consecutive seconds. Here's an example of usage:

    command = "/usr/bin/updatedb"; constraint = DiskIO, 60, 0, 0; \
        constraint = Freq, 3600;

DISCLAIMER

Chronic is alpha software and many a things are likely to change, introduced or improve in future releases. If you have anything you'd like Chronic to do (or do differently), please add a feature request on the sourceforge site, http://sourceforge.net/projects/chronic or send mail to the author.

AUTHOR

Vipul Ved Prakash, <mail@vipul.net>