Rails - Feb 2006 - RailsCron: Trigger one time

Hi Kyle,

Thanks for your help so far.  Now that I have it up and running, I''ve
some
behavior questions.

I want to trigger an event just one time at a future time.  I''m using
some of
your sample code to try this, but the behavior of some combinations of start,
every, and finish behave in some unpredictable ways.

     3  class RailsMaintainer
     4    include ActsAsBackground
     5    background :remove_sessions, :start => Time.now, :every =>
7.minutes,
:finish => 5.minutes.from_now
     6
     7
     8    def self.remove_sessions
     9      f = File.new("testfile",  "a+")
    10      fmt = "%d %b %Y at %I:%M:%S %p"
    11
    12      f << "The time is: " << Time.new.strftime(fmt)
<< "\n"
    13      f.close
    14    end
    15  end

My observations:
1) start=0 (default), every=1.minute, finish=2.minutes.from.now
   - instead of starting immediately, it waits to the next whole minute, i.e.
     if I start it at 12:23:34, it waits until 12:24:00.  I think it should
     start immediately
   - sometimes it triggers twice, which I think is correct.  sometimes it
     triggers THREE times, which I don''t understand why
2) start=1.minute.from_now, every=1.minute, finish=2.minutes.from.now
   - it started at a second granularity, 12:23:35, instead of waiting to the
     next minute, which is correct but seems inconsistent with the default
     start behavior.  It also triggers twice, which it shouldn''t
because of
     the finish time.
3) start=1.minute.from_now, every=1.minute, finish=1.minute.from.now
   - tried this to just trigger one time.  Sometimes it triggered, sometimes
     it didn''t.  There seems to be a race condition.
4) start=Time.now, every=7.minutes, finish=5.minutes.from_now
   - Presumably, this should execute once.  And indeed, it does.  But then
     if I do RailsMaintainer.new again, it never triggers again.  I have to
     restart the RailsCron process.

Requests:
1) An option to trigger a job 1 time at a particular time
2) When you get the time, a more detailed description of the timing would be
very helpful.  The biggest question for us is how sure can we be of when an
event triggers?  We''d like to say, send an email reminder at 6:23:15
and have
the process kick off at that second, not 6:24:00 or 6:24:15.
3) If we restart the RailsCron process (perhaps because the shared host restarts
their unstable system), it''s unclear when the next events will trigger.
So if I
say update my file every 10 minutes, and 3 minutes after my last update the
server restarts, when it comes up, does the next event occur immediately
(00:13), or at the "correct" interval (00:20), or after 10 more
minutes (00:23).
 In the database, it doesn''t look like you store the last trigger time,
which I
think would be helpful in creating more predictable behavior.

I know I''m asking a lot of questions, but I''m seriously trying
to use your
plugin.  If these kinds of use cases are not in your plans to support, please
let me know.

Thanks,
Jake

On 2/24/06, Jake <vanguardian@netscape.net> wrote:
> Hi Kyle,
>
> Thanks for your help so far.  Now that I have it up and running,
I''ve some
> behavior questions.
>
> I want to trigger an event just one time at a future time.  I''m
using some of
> your sample code to try this, but the behavior of some combinations of
start,
> every, and finish behave in some unpredictable ways.
>
>      3  class RailsMaintainer
>      4    include ActsAsBackground
>      5    background :remove_sessions, :start => Time.now, :every =>
7.minutes,
> :finish => 5.minutes.from_now
>      6
>      7
>      8    def self.remove_sessions
>      9      f = File.new("testfile",  "a+")
>     10      fmt = "%d %b %Y at %I:%M:%S %p"
>     11
>     12      f << "The time is: " <<
Time.new.strftime(fmt) << "\n"
>     13      f.close
>     14    end
>     15  end
>
> My observations:
> 1) start=0 (default), every=1.minute, finish=2.minutes.from.now
>    - instead of starting immediately, it waits to the next whole minute,
i.e.
>      if I start it at 12:23:34, it waits until 12:24:00.  I think it should
>      start immediately
>    - sometimes it triggers twice, which I think is correct.  sometimes it
>      triggers THREE times, which I don''t understand why
:start => 0 starts at the Unix epoch.  You want start => Time.now. 
The three times thing is perhaps because the running range is
inclusive of the finish time, so start => 0, every => 5, :finish =>10,
triggers at 0, 5, 10.
> 2) start=1.minute.from_now, every=1.minute, finish=2.minutes.from.now
>    - it started at a second granularity, 12:23:35, instead of waiting to
the
>      next minute, which is correct but seems inconsistent with the default
>      start behavior.  It also triggers twice, which it shouldn''t
because of
>      the finish time.
See previous.
> 3) start=1.minute.from_now, every=1.minute, finish=1.minute.from.now
>    - tried this to just trigger one time.  Sometimes it triggered,
sometimes
>      it didn''t.  There seems to be a race condition.
Ok, I''ll look at that.
> 4) start=Time.now, every=7.minutes, finish=5.minutes.from_now
>    - Presumably, this should execute once.  And indeed, it does.  But then
>      if I do RailsMaintainer.new again, it never triggers again.  I have to
>      restart the RailsCron process.
>
> Requests:
> 1) An option to trigger a job 1 time at a particular time
If you add a RailsCron model entry manually, and have the called
method delete the entry, that should do it.
> 2) When you get the time, a more detailed description of the timing would
be
> very helpful.  The biggest question for us is how sure can we be of when an
> event triggers?  We''d like to say, send an email reminder at
6:23:15 and have
> the process kick off at that second, not 6:24:00 or 6:24:15.
> 3) If we restart the RailsCron process (perhaps because the shared host
restarts
> their unstable system), it''s unclear when the next events will
trigger.  So if I
> say update my file every 10 minutes, and 3 minutes after my last update the
> server restarts, when it comes up, does the next event occur immediately
> (00:13), or at the "correct" interval (00:20), or after 10 more
minutes (00:23).
>  In the database, it doesn''t look like you store the last trigger
time, which I
> think would be helpful in creating more predictable behavior.
>
> I know I''m asking a lot of questions, but I''m seriously
trying to use your
> plugin.  If these kinds of use cases are not in your plans to support,
please
> let me know.
That''s fine. My personal needs revolve around running
timing-insensitive background processes like clearing sessions, and
sending email out of a queue.  I just want them to run in the
background occasionally.  That''s why I haven''t caught your
issues yet.

I want RailsCron to work for these tight timing issues. I think that
the first step is to establish a unit test suite, but that''s tricky to
do right for this, and I haven''t had time recently.  Let''s
talk over
your concerns.  Send me a email or ping me on AOL IM: rbyhaxr
>
> Thanks,
> Jake
>
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>

--
Kyle Maxwell
Chief Technologist
E Factor Media // FN Interactive
kyle@efactormedia.com
1-866-263-3261