[lxc-users] Autostart: container ordering for various multi-container operations

[Jäkel, Guido]

Dear Bostjan,

I already told my 5ct on this many month ago, but now -- with LXC 1.x -- it might be the time to discuss about it, again.


IMHO one want to describe dependencies and it's up to the computer to derive any order from that. It's the same feature as within all the different init systems.

For a specific container on may want to describe the dependency on others -- in an advanced model as a hard (NEED - MUST be running before us) or soft (USE - will be try to start before us, if not running. But don't care, if this fail) dependency.

In spite of this is enough to describe the dependency tree, it will be handy if on is able to describe also forward dependencies to other containers by the additional set (BEFORE/AFTER - start us before, if (and only if) the other is actually requested to start) because in absence of this one have to anchor is piece of information not in the scope of the actual container but on other, that may even not exist.)

From this, a set dependency graph have to be calculated. Then, independent trees and nodes might be started in any arbitrary order, optional with a configurable parallelism.

Shutdown of any or all containers should respect the dependency graph in the obvious reverse order -- if some Container is called to be stopped that is NEED by others, this others have to be stopped first -- if just USED, then don't care.

And as an implementation gimmick, there is an virtual node of dependencies ("ALL") with an implicit NEED of all ("autostart"-marked) Containers and an implicite NEED for this node for every container. This target may be used to start or stop all Containers in the described order.


But the meta question is still: Should this feature be up to LXC or should it be left to the surrounding environment, because every LXC host will already have an full featured init system and it's to set up the startup of Containers in a specific order is as easy as for every other Unix daemon. This also includes different instruments to define what "Container have started" actually mean -- form "successfully spawned lxc-start" up to "a service is ready to work")


Greetings

Guido

>-----Original Message-----
>From: lxc-users [mailto:lxc-users-bounces at lists.linuxcontainers.org] On Behalf Of Bostjan Skufca
>Sent: Wednesday, March 04, 2015 8:30 PM
>To: LXC users mailing-list
>Subject: [lxc-users] Autostart: container ordering for various multi-container operations
>
>Hi there,
>
>I would like to open a discussion about container ordering regarding to lxc.start.order and lxc-autostart operations.
>
>Currently, let's presume that pull request https://github.com/lxc/lxc/pull/461 is merged and that containers start in
>ascending lxc.start.order fashion.
>
>This makes sense for the following multi-container operations:
>- start
>- reset <--- this may be done in any order actually
>
>What about multi-container shutdown?
>
>So, I would like to know what is your opinion about what feels more natural to you:
>
>a) containers are shut down in the same order as they are started
>b) containers are shut down in reverse order regarding to their startup order
>c) should there be a separate lxc.stop.order setting?
>d) something else?
>
>b.
>
>
>PS: My preference would be to perform shutdown in reverse order, if there is no lxc.stop.order setting (implemented or set).
>