[Maas-devel] Node state and pserv

[Jeroen Vermeulen]

Hi all,

I was thinking the time may have come to streamline how maasserver and 
pserv communicate about nodes.

Right now we have a function hooked into maasserver's Node.save() that 
updates pserv (and thus, Cobbler) and blocks for that to complete.  We 
always wanted to try the simplest thing that could work, see how we'd 
end up driving Cobbler in practice, and then refactor based on that 
knowledge.  We wanted the resulting protocol to be simple, terse, as 
asynchronous as feasible, untainted by Cobbler details, and light in state.

Now, looking at the pserv API, I see that we only have two real 
operations on a node: start it, and stop it.  Everything else is state 
manipulation.

And so my question is: what state manipulations are useful in and of 
themselves, as opposed to as preparation for starting or stopping a 
node?  What does Cobbler really need to know about a node when it's not 
being asked to start or stop the thing?  My imagination stops at its 
DHCP and DNS properties (and as the UI stands, that means only changing 
its hostname).

If that's more or less correct, I'd like to reform the nodes part of the 
pserv API to the following operations.  It may seem wasteful in terms of 
how much goes over the wire per request, but it's also less chatty. 
Plus it would allow a future pserv implementation to be a borderline 
amnesiac, apart from DHCP and DNS state and what's needed to walk a node 
through procedures like commissioning.

  * Start nodes.

Asynchronous.  Takes the nodes' full definitions as arguments: 
networking properties, profile, power method, netboot setting.  Each 
node might be new to pserv, or already known; it'd be entirely up to 
pserv to figure that out.

  * Stop nodes.

Likewise asynchronous.  This would again take full node definitions. 
Some of the nodes being started may be new to pserv.  The netboot 
setting here applies only to the case where somebody turns on the node 
by hand; the next time MAAS starts the node it will specify the netboot 
behaviour it wants at that point.

  * Set host's DHCP/DNS properties.

This looks to me like a completely separate thing from all the others, 
although of course pserv may also have to run through this internally 
when starting or stopping a node.

It'd be up to pserv to figure out whether it's updating a node it knows 
about, or learning about a new one.  For example, if the host is new, 
pserv could pick an arbitrary profile — the next “start” call will 
provide the correct one when the time comes.

Since reconfiguring running services could be expensive, we may need to 
treat that in a similar way to node commissioning timeouts: leave some 
time for a burst of requests to pile up, then come back a little while 
later and service all pending changes at once.  Starting and stopping 
nodes are already asynchronous so no worries if they need to wait for a 
DHCP/DNS sync, but we need to keep that sync delay low enough so as not 
to give rise to artificial commissioning timeouts.

  * Delete host's DHCP/DNS properties.

Maybe.  Or maybe we consider this an offline garbage-collection process. 
  Meh.


And that's the list.  Your average node change (status changes in 
particular) would not directly result in pserv interaction.  They might 
trigger the maasserver into making further requests, such as “good, now 
boot this node.”  But those events are inherently limited in frequency.


…Comments?


Jeroen