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examples: update var_service/README again

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Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
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Denys Vlasenko 2016-10-14 18:38:08 +02:00
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1 changed files with 13 additions and 5 deletions
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18
examples/var_service/README
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@ -40,8 +40,8 @@ There are just two key programs in runit. Firstly, runsv supervises the
process for an individual service. Service directories themselves sit process for an individual service. Service directories themselves sit
inside a containing directory, and the runsvdir program supervises that inside a containing directory, and the runsvdir program supervises that
directory, running one child runsv process for the service in each directory, running one child runsv process for the service in each
subdirectory. Out of the box on Debian, for example, an instance of subdirectory. A typical choice is to start an instance of runsvdir
runsvdir supervises services in subdirectories of /var/service/. which supervises services in subdirectories of /var/service/.
If /var/service/log/ exists, runsv will supervise two services, If /var/service/log/ exists, runsv will supervise two services,
and will connect stdout of main service to the stdin of log service. and will connect stdout of main service to the stdin of log service.
@ -94,7 +94,7 @@ dhcp, zeroconf, ppp, openvpn and such. They need to be controlled,
and in many cases you also want to babysit them. and in many cases you also want to babysit them.
They present a case where different services need to control (start, stop, They present a case where different services need to control (start, stop,
restart) eact other. restart) each other.
var_service/dhcp_if var_service/dhcp_if
@ -110,7 +110,9 @@ dynamic network link services (ppp/vpn/zcip).
This is an example of service with has a "finish" script. If downed ("sv d"), This is an example of service with has a "finish" script. If downed ("sv d"),
"finish" is executed. For this service, it removes DHCP address from "finish" is executed. For this service, it removes DHCP address from
the interface. the interface. This is useful when ifplugd detects that the the link is dead
(cable is no longer attached anywhere) and downs us - keeping DHCP configured
addresses on the interface would make kernel still try to use it.
var_service/zcip_if var_service/zcip_if
@ -138,6 +140,9 @@ inteface "if".
var_service/fw var_service/fw
"Firewall" script, although it is tasked with much more than setting up firewall.
It is responsible for all aspects of network configuration.
This is an example of *one-shot* service. This is an example of *one-shot* service.
It reconfigures network based on current known state of ALL interfaces. It reconfigures network based on current known state of ALL interfaces.
@ -147,7 +152,7 @@ Uses conf/*.ipconf (static config) and /var/run/service/fw/*.ipconf
One-shot-ness of this service means that it shuts itself off after single run. One-shot-ness of this service means that it shuts itself off after single run.
IOW: it is not a constantly running daemon sort of thing. IOW: it is not a constantly running daemon sort of thing.
It starts, it configures the network, it shuts down, all done It starts, it configures the network, it shuts down, all done
(unlike infamous NetworkManagers which sit in RAM forever, doing hell knows what). (unlike infamous NetworkManagers which sit in RAM forever).
However, any dhcp/ppp/vpn or similar service can restart it anytime However, any dhcp/ppp/vpn or similar service can restart it anytime
when it senses the change in network configuration. when it senses the change in network configuration.
@ -158,10 +163,13 @@ This is achieved very simply by having
# Make ourself one-shot # Make ourself one-shot
sv o . sv o .
at the very beginning of fw/run script, not at the end. at the very beginning of fw/run script, not at the end.
Therefore, any "sv u /var/run/service/fw" command by any other Therefore, any "sv u /var/run/service/fw" command by any other
script "undoes" o(ne-shot) command if fw still runs, thus script "undoes" o(ne-shot) command if fw still runs, thus
runsv will rerun it; or start it in a normal way if fw is not running. runsv will rerun it; or start it in a normal way if fw is not running.
This mechanism is the reason why fw is a service, not just a script.
System administrators are expected to edit fw/run script, since System administrators are expected to edit fw/run script, since
network configuration needs are likely to be very complex and different network configuration needs are likely to be very complex and different
for non-trivial installations. for non-trivial installations.