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some notes on controlled services

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Daniel Barlow 2024-07-28 22:57:23 +01:00
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doc/configuration.rst
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@ -72,10 +72,46 @@ domains, or you want to run two SSH daemons on different ports.
Services
********
We use the `s6-rc service manager <https://www.skarnet.org/software/s6-rc/overview.html>`_ to start/stop/restart services and handle
service dependencies. Any attribute in `config.services` will become
part of the default set of services that s6-rc will try to bring up on
boot.
In Liminix a service is any kind of long-running task or process on
the system, that is managed (started, stopped, and monitored) by a
service supervisor. A typical SOHO router might have services to
* answer DHCP and DNS requests from the LAN
* provide a wireless access point
* connect using PPPoE or L2TP to an upstream network
* start/stop the firewall
* enable/disable IP packet forwarding
* mount filesystems
(Some of these might not be considered services using other definitions
of the term: for example, this use of L2TP would be a "client" in the
client/server classification; and enabling packet forwarding doesn't
require any long-lived process - just a setting to be toggled.
However, there is value in being able to use the same abstractions for
all the things to manage them and specify their dependency
relationships - so in Liminix "everything is a service")
Any attribute in `config.services` will become part of the default set
of services that s6-rc will try to bring up.
Services are usually started at boot time, but **controlled services**
are those that are required only in particular contexts. For example,
a service to mount a USB backup drive should run only when the drive
is attached to the system. Liminix currently implements two kinds of
controlled service:
* "uevent-rule" service controllers use sysfs/uevent to identify when
particular hardware devices are present, and start/stop a controlled
service appropriately.
* the "round-robin" service controller is used for service failover:
it allows you to specify a list of services and runs each of them
in turn until it exits, then runs the next.
Writing services
================
For the most part, for common use cases, hopefully the services you
need will be defined by modules and you will only have to pass the