2) it should be possible to register hooks for a resource I want to watch,
hook could be just an URL, when the resource is changed data are POSTed to
the registered URL.
That is a very nice idea however we will need to think about that more:
1. What will happen with the hook itself when it deliver results.
A registered hook would be deleted when the associated instance is
deleted from Tracker or when a user de-registers the hook.
An instance would be deleted from Tracker when it stops exist
('finished' state) or if a user de-registers/deletes the instance from
Tracker.
2. What will be reported if a failure occurs on backend side while pooling.
We need to know about the errors, for example if an instance
"disappears" from a provider or if a provider is not accessible.
This might be done by setting specific state to the instance: 'vanished'
for disappeared instances, 'unknown' if there is a connection error.
Jan
