Deployment engineering for puma

Puma is software that is expected to be run in a deployed environment eventually. You can certainly use it as your dev server only, but most people look to use it in their production deployments as well.

To that end, this is meant to serve as a foundation of wisdom how to do that in a way that increases happiness and decreases downtime.

Specifying puma

Most people want to do this by putting gem "puma" into their Gemfile, so we'll go ahead and assume that. Go add it now... we'll wait.

Welcome back!

Single vs Cluster mode

Puma was originally conceived as a thread-only webserver, but grew the ability to also use processes in version 2.

Here are some rules of thumb:

MRI

Use cluster mode and set the number of workers to 1.5x the number of cpu cores in the machine, minimum 2.
Set the number of threads to desired concurrent requests / number of workers. Puma defaults to 16 and that's a decent number.

Migrating from Unicorn

If you're migrating from unicorn though, here are some settings to start with:
- Set workers to half the number of unicorn workers you're using
- Set threads to 2
- Enjoy 50% memory savings
As you grow more confident in the thread safety of your app, you can tune the workers down and the threads up.

Worker utilization

How do you know if you're got enough (or too many workers)?

A good question. Due to MRI's GIL, only one thread can be executing Ruby code at a time. But since so many apps are waiting on IO from DBs, etc., they can utilize threads to make better use of the process.

The rule of thumb is you never want processes that are pegged all the time. This means that there is more work to do that the process can get through. On the other hand, if you have processes that sit around doing nothing, then they're just eating up resources.

Watching your CPU utilization over time and aim for about 70% on average. This means you've got capacity still but aren't starving threads.

Daemonizing

I prefer to not daemonize my servers and use something like runit or upstart to monitor them as child processes. This gives them fast response to crashes and makes it easy to figure out what is going on. Additionally, unlike unicorn, puma does not require daemonization to do zero-downtime restarts.

I see people using daemonization because they start puma directly via capistrano task and thus want it to live on past the cap deploy. To this people I said: You need to be using a process monitor. Nothing is making sure puma stays up in this scenario! You're just waiting for something weird to happen, puma to die, and to get paged at 3am. Do yourself a favor, at least the process monitoring your OS comes with, be it sysvinit, upstart, or systemd. Or branch out and use runit or hell, even monit.

Restarting

You probably will want to deploy some new code at some point, and you'd like puma to start running that new code. Minimizing the amount of time the server is unavailable would be nice as well. Here's how to do it:

Don't use preload!. This dirties the master process and means it will have to shutdown all the workers and re-exec itself to get your new code. It is not compatible with phased-restart and prune_bundler as well.
Use prune_bundler. This makes it so that the cluster master will detach itself from a Bundler context on start. This allows the cluster workers to load your app and start a brand new Bundler context within the worker only. This means your master remains pristine and can live on between new releases of your code.
Use phased-restart (SIGUSR1 or pumactl phased-restart). This tells the master to kill off one worker at a time and restart them in your new code. This minimizes downtime and staggers the restart nicely. WARNING This means that both your old code and your new code will be running concurrently. Most deployment solutions already cause that, but it's worth warning you about it again. Be careful with your migrations, etc!

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