Sleep function uses server resources?

Question

I've got two reasons to use a sleep function: first, to automatically send a confirmation email to a client 20 minutes after they contact us. I don't want to use cron jobs because I want it to be exactly 20 minutes (and I'm sick of my web server sending me emails telling me they initiated a cron job.....a new email every 20 minutes!)

Second reason: I've heard of people sending out mass emails using the sleep function. Since my server will only allow 100 emails an hour, I want to use the sleep function to have the script sleep for an hour then continue where it picked up.

My question is this: does it use server resources? Will it slow things down? Are there any other problems with using the sleep function? Thanks in advance!

Answer 1

While a process is sleeping it will not consume CPU time, but the process' working set still requires physical memory and/or pagefile to support that process. In other words, the PHP interpreter process needs to keep running. So long as your server has enough RAM this shouldn't be a problem though.

Answer 2

Email delivery times are pretty variable, so you're not going to get an email to someone's inbox in exactly 20 minutes, no matter what you do.

I use a long-running background script -- launched from CLI, instead of apache -- to handle email sending. My application dumps emails into a queue table, which the mailer script polls every 15 seconds. It sleep()s between polls. This way, I only have one script trying to connect to the SMTP server, and sleeping.

That portion of the app has been running successfully with no major issues for the last 2 years. The only annoyance is keeping the script running -- if it goes down for any reason, mail doesn't go out til you bring it back up. But worst case, you could just restart it via cron periodically, e.g. daily.

If I were tackling your problem, I'd simply put a "Send time" column on the queue table, and date it 20 minutes out for these emails. The mailer would then SELECT * FROM mail_queue WHERE send_time <= NOW()

Alternately, you could use a real jobqueue like beanstalkd. I chose the queue table solely to keep my application stack simple.