spec/types: clarify only one eventHandler per type

[jonboulle]

No description provided.

[mpasternacki]

1. Why use an array here rather than a JSON object? It would seem more natural to me, and it would intuitively ensure only one handler of a given type.
2. Why explicitly limit event names? Can't an implementation define additional events/handlers? Examples of such handlers would be: ad-hoc maintenance tasks triggered manually (like running chef-server-ctl commands for Chef server); cron-like scheduled tasks; handling situations in multi-application containers when one of the processes exits. Spec may require that if an implementation handles events outside of the spec, the event names should be namespaced (e.g. rocket:cron or jetpack:task) to avoid possible collisions.

[jonboulle]

Why use an array here rather than a JSON object? It would seem more natural to me, and it would intuitively ensure only one handler of a given type.

As discussed here and here, this is for consistency with other types, and unfortunately using an object (map) doesn't guarantee us one handler of a given type when it comes to actually unmarshalling from JSON, so I am inclined to stick with the array model.

Why explicitly limit event names? Can't an implementation define additional events/handlers?

We certainly want a small set of very well defined eventHandlers that all executor implementations must support. As for extensions like ad-hoc tasks, is that worth codifying in image manifests at all, rather than just leaving it up to implementations? I think it's a bit of a grey area. Namespacing could be a good compromise and we could define this as a common approach for dealing with other fields like isolators (optional; small set of well-known fields; other fields accepted, but please namespace; commonly-used fields may be promoted to well-known in future versions of the spec)

[jonboulle]

I am going to merge this as it reflects the current state of the spec; let's continue the discussion about opening up eventHandler types in #58 /cc @philips