[Linux-ha-dev] Ordering of OCF Start, Stop and Monitor actions

[Alan Robertson]

Doug Knight wrote:
> Yes, Thanks Alan. Let me digest it, and walk through my OCF script. I'll
> see if I have any other questions after that.
> 
> Thanks for getting back to me.
> 
> Doug
> 
> On Wed, 2007-03-14 at 11:41 -0600, Alan Robertson wrote:
>> Doug Knight wrote:
>> > Hi All,
>> > I currently am running a two node cluster (host1 and host2) with version
>> > 2.0.8. I have a resource defined with a place constraint of "#uname eq
>> > host1", so that it will start on host1 (using an OCF RA script,
>> > including all of the required methods). The resource itself has
>> > target_role set to "stopped".
>> > 
>> > Question 1: Is the monitor method called regularly on both nodes to make
>> > sure the resource is not running?
>>
>> No.  It is called on every node when we first start up (we call it a
>> probe operation).  If you ask us to, we will run it periodically to
>> ensure that a running copy continues to run.
>>
>> You can also manually request to run this initial probe again to catch
>> errors made by system administrators (but I don't know of anyone who
>> does that).
>>
>>
>> > Next, I change the target_role to "started" (i.e. I use the GUI and
>> > click the start button).
>> [Better yet, use the "outline" start button]
>>
>> > Question 2: What is the order of OCF methods called to bring up the
>> > resource? Is Monitor called before Start on host1? Does Stop and/or
>> > Monitor ever get called on host2?
>>
>> Monitor gets called when we first start up on every node.
>> It also get called repeatedly on any node that we think is running it --
>> if you ask us to monitor the resource.
>>
>> > Resource is up and running on host1, and I decide to move the resource
>> > to host2. I click the constraint and change it to "#uname eq host2". The
>> > resource stops on host1 and starts on host2.
>> > 
>> > Question 3: Same idea, what are the sequence of method calls to migrate
>> > the resource from host1 to host2?
>>
>> In the past...
>> 	monitor every resource on every node once to see what's already
>> 		running
>> 	start on "some node"
>> 	monitor periodically on "some node" (if requested)
>>
>> Request to move resource arrives...
>> 	stop monitoring on "some node" (if it had been requested)
>> 	stop resource on "some node"
>> 	start resource on "some other" node
>> 	monitor periodically on "some other node" (if requested)
>>
>> > I'm trying to thoroughly understand the sequence of events that occurs
>> > for each phase in support of the Postgres WAL file forwarding
>> > configuration I posted last week ("[Linux-HA] Two node cluster with
>> > Postgres in WAL file fwding mode", started March 1).


Let me offer this caveat:

Monitor might be called at any time.

Stop should work at any time,
	and succeed harmlessly if it's already stopped.

Start should work at any time,
	and succeed harmlessly if it's already started.

http://www.linux-ha.org/OCFResourceAgent gives more details.

-- 
    Alan Robertson <alanr at unix.sh>

"Openness is the foundation and preservative of friendship...  Let me
claim from you at all times your undisguised opinions." - William
Wilberforce