New & Improved Remote Builders


We’ve decided to rethink our remote builders. They’re the machines that build your app when deploying without a local Docker daemon or when using --remote-only. We had been using AWS CodeBuild, but it was too slow for our taste. It didn’t feel good when we forgot to start our local Docker daemon before deploying.

Starting with flyctl v0.0.164, the new remote builders should just work for your remote builds.

So what’s better about them?

  • They’re much faster to boot up
  • They should be faster when building
  • They stream the Docker build output as it happens, not just at the end
  • They’re private to each organization, these are not shared which allows for layers reuse for your builds

Technical details

  • They run on Fly, like any other app, except they’re ephemeral
  • They use our volumes feature to cache Docker layers
  • They turn off automatically after 10 minutes of inactivity
  • We’re proxying Docker connections to a Docker daemon running inside a VM on Fly
  • These VMs are currently set to use our dedicated-cpu-1x tier

We’re still ironing out a few performance issues around them, but they’re already faster than our current remote building solution.

For your first remote build for an organization you’re part of, the boot might take up to a minute. While it stays up, it should be able to accept build jobs without delay. As it cools down and is turned off, turning it back on should only take a few seconds.

These remote builds will appear in your apps as fly-builder-..., but you won’t be billed for them right now. We’re still working out a reasonable billing plan for them, but in the future you’ll have hours of free remote build time per month. Even if you did overflow on this free build time, you will wouldn’t break the bank.

Upgrade your flyctl to at least v0.0.164 and let us know what you think!


I’ve been using these instead of running a local Docker daemon and it’s super nice. Docker on Mac sucks. Remote Fly docker is wicked fast and my fans don’t come on. :slight_smile:


There was a small bug preventing apps from exiting with the proper exit code. If the exit code is not 0, then a restart is automatically attempted. This intereferred with the remote builder’s ability to stop itself gracefully.

This has now been fixed.

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It seems my builders are not being deleted after builds. It says that the app has crashed.

They’re not being deleted on purpose. We need to keep them around for better build caches.

They appear “crashed”, but they’re really just shut down.

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