Re: Dependencies in S6

[Guillermo <gdiazhartusch@gmail.com>]

El mié., 21 oct. 2020 a las 7:10, Amaresh Kotekal escribió:
>
> Secnario 1:
> 1. A & B are one shot services in a s6-rc bundle. Service B is dependent on
> A. ( added in dependency file).
> 2. I think Service B will start after the completion of service A ( A then
> B, in serial manner ) is it correct ? or
> 3. Service B will start immediately after the service A as started (in a
> parallel manner) ?

#2. If A starts successfully, B will be started after A, i.e.
serially. If A fails to start, B won't be started.at all.

> Secnario 2:
> 1. A is one shot service and B is a long run service. Service B is
> dependent on Service A.

Same as scenario #1. Whether B is a longrun or a oneshot does not
change the behaviour.

> Secnario 3:
> 1. A is a long run service and B is a one shot service. Service B is
> dependent on Service A.
> [...]
> 3. Do I need to use notification mechanism if I want to start service B
> after A service as reached particular point in execution

Same as scenario #1, service type does not affect the behaviour. What
does depend on service type is the way s6-rc detects that state
transition has completed, and the way it detects transition failure.
For oneshots, transition completion happens when the oneshot's process
terminates. And transition failure is determined based on the process'
exit code, and, if a timeout was specified with a 'timeout-up' file,
also on whether it terminates before the timeout.

For longruns that support readiness notification, transition
completion happens when the longrun's process notifies readiness. And
if a timeout was specified with a 'timeout-up' file, transition
failure is determined based on whether notification happens before the
timeout.

If there is no support for readiness notification, transition
completion happens when the longrun's process has been spawned by its
corresponding s6-supervise process, because there is no better way. If
the process can be polled for readiness, you can use the
s6-notifyoncheck program to hook to s6-supervise's readiness
notification mechanism.

* http://www.skarnet.org/software/s6/s6-notifyoncheck.html

> Secnario 4:
> 1. A & B both are long run services. Service B is dependent on Service A.

Left as an excercise :)

G.