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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 Services
******** ********
We use the `s6-rc service manager <https://www.skarnet.org/software/s6-rc/overview.html>`_ to start/stop/restart services and handle In Liminix a service is any kind of long-running task or process on
service dependencies. Any attribute in `config.services` will become the system, that is managed (started, stopped, and monitored) by a
part of the default set of services that s6-rc will try to bring up on service supervisor. A typical SOHO router might have services to
boot.
* 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 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 need will be defined by modules and you will only have to pass the