Re: how to redirect and let the server do some lengthy job in the background?

On Feb 28, 2011, at 12:42, Tan Cheng wrote:

> Thanks for all the input!!! So much good information to learn. Due to
> the tight budget and maybe the site traffic will not be so high, I am
> still looking at a shard hosting which supports cron, and maybe let it
> run a cake shell every 5 hours to encode and update the database. I
> think I am going to do some testing on my ubuntu localhost first. My
> goal here may to get my feet wet before thinking about vps with a
> queue system.

The advantage of having a single daemon that just keeps on running is that you don't have to worry that there's another copy of the daemon running at the same time, possibly competing for encoding the same video. If you run the script via cron on an interval, it is possible that the script will take longer than that interval to run, which would introduce this problem. So if you're going to do this, then you have to handle it, for example by ensuring only one copy of the script can encode videos at a time. A lock file would be one simple idea that could work. (Designate a directory to contain locks, let's say "$APP/encoder/locks" (hereafter "$LOCKS"). When the daemon starts, create (touch) an empty file "$LOCKS/$UNIQUE" where $UNIQUE is something unique to this encoder instance, for example its process id or better yet a generated a UUID. Then check if there are any files in $LOCKS other than $UNIQUE. If there are, that means there are other encoders running; delete the lock and exit and do no encoding. Otherwise, proceed with encoding videos as discussed before, except instead of looping forever, loop until there are no more videos to encode, then delete the lock and exit.) With this in place, you could schedule the script to run as often as you like (every hour, every half hour, every 5 minutes) to guarantee users' videos are encoded as quickly as you need, without risk of the script competing with another instance of itself.

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