Replacing Cron/Anacron/Atd

[Evan]

Hi all.

I've been following upstart development with half an eye for a while
now, and one of the features that has always interested me the most is
the plans for it to eventually replace cron/anacron/atd as the
system-wide task scheduling tool. I don't think there are pre-existing
designs for this beyond what's listed on the FAQ (if there are, I
couldn't find them with Google), so I gave it some thought myself.
Here are a few options I came up with:

My initial thought was to simply have upstart emit standard events
every fixed period of time that jobs could hook into (ie 'starts on
hourly'). However this runs into problems fairly quickly: it's not
very flexible, it would be fairly complicated to add
anacron-capabilities to it, and writing a job to start every x hours
(x > 1) would require some uglyness in a pre-start script.  While this
would certainly work, it isn't great.

My next thought was to have a config file somewhere
(/etc/init-schedule.conf ?) where users could configure custom timed
events. Upstart would read it, and know to emit event X every Tuesday,
event Y every 3 hours, and event Z once a day. While this would be
significantly cleaner than the previous solution, it still has one
problem which I dislike - configuration for time-based jobs ends up in
two different files. If I have job B that has a 'starts on
B-schedule', the user then has to search through init-schedule
separately to figure out when B-schedule is actually emitted. A better
solution, but again not optimal.

My third and final design is in many ways the farthest from the
original 'everything by events' design of upstart, but I believe it
most effectively replaces the existing cron/anacron/atd system.
Upstart will designate a special event (like 'startup' is today)
called 'schedule'. This event will have one parameter called 'job'. If
schedule is referenced in a start or stop stanza without a specified
job parameter, job=$self will be assumed for convenience. Each job
file will have optional 'schedule' stanzas, similar in syntax to atd.
Upstart will parse these stanzas, and when appropriate will emit a
'schedule job=x' event. For the user, anacron-like functionality will
be available via the special-case schedule stanza 'schedule if
missed', and atd-like functionality will be available via 'initctl
schedule'.

There follows a very simple upstart job using this proposed syntax:
#####
start on schedule

schedule every hour
schedule if missed

console output
exec echo "This job runs like an hourly anacron job!"
#####

Of course these are only proposals - I'm not very familiar with the
innards of upstart, so I'm sure I've made some false assumptions.
However, I hope these will serve as starting-points for a proper
design discussion. I'm quite familiar with C, and I've been looking
for a place to give back to Ubuntu for a while now. If a design is
decided upon for this feature, I'd be willing to spend some time
implementing it.

Thanks,
Evan Huus