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System XVI

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Overview

A forewarning: System XVI is not yet useable, not even in a limited fashion. It remains still prototypical, and will require significant work before making generally available: completion of basic features, making robust the code, and creation of tests. The repository is made available to the public only under this proviso.

System XVI, or S16, is a modern take on service management. It aims to be as or more functional than its alternatives, while retaining a modular design in the UNIX tradition.

We have an IRC channel; find it at irc.freenode.net #systemxvi. You might also want to read the System XVI Manual. This is nowhere near complete, but you may find the work in progress here.

S16 aspires to be both soundly designed and practical: it should be capable of handling as many use cases as possible, yet achieve this on the basis of its simplicity and modularity. The twin temptations of under-flexibility and bloatedness must be avoided.

The Four Motives of S16
  • Interface Orientation: the system should be designed to fit a clean and stable interface. A well-designed interface makes for an obvious implementation.
  • Separation of Concerns: individual components should not do much alone, but work in concert to create a grand system.
  • Modularity: components should be easily replaceable and extensible.
  • Self-healing: components that crash should be able to restart without forgetting system state or otherwise causing breakage.

The Four Motives are a distillation of the factors underpinning the success of many other systems. From these four motives emerges a naturally clean, lightweight, and flexible system.

Design

System XVI is comprised of several binaries which each do one thing well.

Daemons
  • init is responsible for System Initialisation: it has very few tasks to perform, which include reaping processes and starting s16.restartd.
  • s16.configd is charged with maintaining the Service Repository, which keeps a record of the state of services, and notifies interested subscribers, such as s16.graphd when changes to service configuration or state occur.
  • s16.graphd is the Graph Engine, which creates and maintains a graph of services. Using this graph it orders startup, sending requests to s16.restartd for services to start when their dependencies are satisfied.
  • s16.restartd is the Master Restarter: its task is to start and keep services running in response to requests from s16.graphd. If a service fails, it notifies s16.configd about this.

Some services need different forms of supervision, such as those which are managed by inetd. Rather than bloat its core components with complexity, System XVI prefers to let other programs handle this. s16.graphd entrusts supervision of these kinds of services to Delegated Restarters, such as inetd.

Utilities
  • svccfg is used to modify entries in the Service Repository. It can read JSON files in to import new services, as well as edit the configuration of existing services.
  • svcadm enables, disables, or restarts a service as requested.
  • svcs lists services and their status, with in-depth information available on request, such as why a service failed to start.

All these daemons and utilities communicate with each other through a JSON-RPC interface. The interface is simple and stable; anyone who wants to reimplement a part of System XVI is welcome to do so. They need only to implement the protocol that is specified for that part.

FAQ

On what platforms is System XVI supported?

These systems are regularly tested to ensure they work:

  • GNU/Linux
  • FreeBSD
  • NetBSD
  • OpenBSD

The other two BSDs (DragonFly and macOS) are also supported but have not been tested yet. On these platforms all core features should be available. Acting as a system manager on macOS is nontrivial to implement and out of scope, but as an auxiliary service manager, it should work.

Any other platform which is reasonably UNIX-like and has a Kernel Queues available (either natively supported or via a library such as libkqueue) should work to a lesser degree. On those platforms, no tracking of processes across forks is available by default. You may write a process-tracker driver if the platform provides a means of tracking the forking and exits of processes.

Under what licence is System XVI made available?

System XVI is released under the Common Development and Distribution License (CDDL) version 1.1 only. Code under external/ and usr/src/vendor/ may be licensed otherwise, as these are sources from other projects. You may instead use System XVI under the terms of the GPLv3 if you wish, with the exception of graph.c (see within for reasoning.)


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