Basic xenstore questions (building a watchdog)

[Charles Duffy <cduffy@spamcop.net>]

I'm looking at building a xenstore-based watchdog, as described at 
http://lists.xensource.com/archives/html/xen-users/2005-07/msg00597.html

However, being somewhat new to xenstore, I'd appreciate some pointers.

- What portions of the xenstore namespace should I use? I'm looking for 
at least two settings writable by the DomUs: A flag to enable/disable 
the watchdog for a given domain (ideally, this would be specific to an 
individual run of the instance in question and not persisted across 
runs), and a counter via the modification of which the watchdog may be pet.

   http://wiki.xensource.com/xenwiki/XenStoreReference indicates that 
/tool is likely an appropriate region (perhaps I should create 
/tool/watchdog/<vmid>/{enabled,counter}?), but this doesn't provide 
guidance as to how I can avoid persistance (if this is possible). 
Alternately, are vm GUIDs unique on a per-invocation basis? If so, 
/tool/watchdog/vmid/enabled could enable the watchdog only when it 
contains the current vm's GUID (preventing it from working across 
restarts). (Given some basic playing around with xenstore-list, 
xenstore-read and friends from within a DomU, however, it looks like I 
can't read contents of /vm from the DomU -- so am I actually able to 
find my own GUID such as to be able to use it in this way? For that 
matter, how can a DomU find its own vmid?)

- Are the xend.xenstore classes intended exclusively for xend's use, or 
is it acceptable for them to be used by 3rd-party software as well?

- Any example code I'd be well-advised to look at?

Thanks!