Support Lifecycle Hooks

[vishh]

Capturing requirements for Lifecycle hooks discussed during the summit.

Lifecycle hooks was agreed to be a useful feature and support for a few basic hooks is expected to be in place. Specifically, PreStart, PreStop and PostStop hooks will be introduced to start with. Supporting hooks might require separating the lifetime of namespaces from that of the processes in the container.

Hooks will be binaries that will be exec'ed. The Spec and current State of the container will be provided as arguments to the hook (via fds?). Are the exec hooks executed synchronously?

This issue is meant to define the Spec for Hooks and discuss the hooks that are necessary to begin with.

[LK4D4]

I think makes sense to add PreCheckpoint, PostCheckpoint, PreRestore, PostRestore at least for linux extension. ping @boucher @avagin
I think hooks must be synchronous and receive state as json to stdout.

[wking]

On Thu, Jun 25, 2015 at 11:06:52AM -0700, Alexander Morozov wrote:

I think makes sense to add PreCheckpoint, PostCheckpoint,
PreRestore, PostRestore at least for linux extension…

Hmm, do we even have checkpoint/restore lifecycle actions 1? Maybe
that should be a separate issue before we talk about hooks for them
;).

[boucher]

It would definitely be useful to have pre/post checkpoint/restore.

[philips]

@wking @LK4D4 @boucher Lets concentrate on what we know we must do: prestart and poststop before defining hooks for other stuff first.

@vishh What is a prestop hook used for in your mind? We had a debate on pre-stop and just held off on defining one in appc: appc/spec#188 (comment)

[mrunalp]

I agree that prestart and poststop are what we call agree on as useful and can start with adding those.

[mrunalp]

Should also we define what arguments are passed to these hooks as well? On Linux, we will have to pass the pid of the container process.

[LK4D4]

@mrunalp I thought we will pass whole state to all hooks.

[mrunalp]

@LK4D4 We could do that but wouldn't the state be implementation specific?

[LK4D4]

@mrunalp But it still will be a string, right?

[mrunalp]

Yes, that is one way to make it work; let the hooks interpret whatever is passed down. If we do decide to pass the whole state then it might be better to send it over a pipe rather than as an argument.

[LK4D4]

Totally agree on pipe(maybe stdin). Dunno how it will work in windows, probably the same.