S6-INSTANCED-SERVICE(7)

📦 version 2.15.1.0, in nixpkgs unstable

💅 s6-instanced-service(7) — dynamic instantiation

ok so this is from the Miscellaneous Information Manual, page s6-instanced-service(7), and bestie it's literally all about running a bunch of copies of the same service. let's get into it 🎀

✨ NAME ✨

s6-instanced-service — dynamic instantiation, aka "make copies of a thing on demand"

🌸 DESCRIPTION 🌸

So picture this: an instanced service is a parameterized service — you wanna run several copies of it, and the only thing that changes between copies is the parameter. Each one of those copies? That's called an instance. iconic, right? 💅

Here's the thing though: with s6, an s6-service-directory(7) can only handle one process at a time. so yes, if you want instanced services, there's gotta be one service directory per instance, always. no way around it, fr fr.

Static instantiation means the set of possible instances is finite and known in advance. With s6, that means all the service directories for all possible instances get created — typically by a preprocessor — and instances are just treated like regular services. very pre-planned, very scheduled.

Dynamic instantiation means instances get created on demand instead of preallocated. and starting with version 2.11.2.0, s6 ships a few tools to help you set up and manage dynamically instanced services. slay ✨

🦋 How to make a dynamically instanced service under s6

  • Write a template for a service directory that would run under s6-supervise(8). The run script should take the name of the instance as its first argument; the finish script, if present, should take the name of the instance as its third argument. (yes, third. don't ask, just trust 🫶)
  • Call the s6-instance-maker(8) program with this template as first argument, and a path dir as second argument. s6-instance-maker(8) will create a service directory in dir. Big note: this is an offline tool — it does not interact with any currently active services or supervision trees. it stays in its lane, respectfully.
  • Supervise dir by adding it to your regular s6-scan-directory(7). This is your instanced service now, but it's not running any instances yet. Instead it's a nested supervision tree — the instanced service is an s6-svscan(8) process that will supervise all the instances. main character energy honestly 👑
  • Create and delete instances at will with the s6-instance-create(8) and s6-instance-delete(8) programs; you can list all the available instances with s6-instance-list(8). These tools are online — they work with live service directories, i.e. ones that are being supervised by s6-supervise(8). live and in person, no cap.
  • Instances are regular supervised processes. You can control individual instances with s6-instance-control(8), and check their status with s6-instance-status(8). These tools are online as well. 💖

🧚 Internal workings

Okay so this section is not normative — users should not rely on it. It's only here for informational purposes. so like, cute to know, but don't build your house on it 🏠

💀 Notes

ok so this implementation of dynamic instances may seem expensive: it creates one s6-svscan(8) process per instanced service, and one s6-supervise(8) process per instance. and you're like "bestie that's so many processes?? 😭"

BUT — remember that these processes use very little private memory, so having additional copies of them is far less expensive than it looks. it's really just a convenient way to implement the feature by reusing existing code. vibes immaculate, overhead minimal. 🍓

🌈 SEE ALSO 🌈

s6-instance-control(8), s6-instance-create(8), s6-instance-delete(8), s6-instance-list(8), s6-instance-maker(8), s6-instance-status(8)

👑 AUTHORS 👑

  • Laurent Bercot
  • Alexis <flexibeast@gmail.com> (man page port)

aaand that's the whole page, bestie, you ate that 💅✨ This gem is from Debian, dated February 19, 2023 — go forth and instantiate responsibly 🫶🎀

📦 where this page comes from