[Info-vax] Quorum strategy

[Marc Van Dyck]

Stephen Hoffman formulated the question :
> On 2019-05-22 08:44:59 +0000, Marc Van Dyck said:
>
>> First question : did I get that right ?
>
> Clustering does have a concept of a primary for the connection manager, but 
> does not otherwise present that to the app-level processing.
>
> In the absence of the possibility to update the code, and in the absence of 
> the possibility of writing some code to automatically perform the app 
> transition from secondary to primary, or to—for instance—scan for the 
> lowest-addressed host when more than one host is present in the cluster...
>
> The whole reason the quorum disk was implemented was to avoid the need for a 
> primary-secondary configuration.  And that primary-secondary configuration is 
> what you want here.
>
> Give one vote to the cluster member host that you're referring to as the 
> primary.  Remove the quorum disk. Give no votes for the secondary.
>
> The secondary is operating as only a little more than a warm standby, here.  
> Yes, it can be configured to run some apps.
>
> Since you reboot to promote the secondary to primary, that reboot can be with 
> an added vote, and this only when the primary can be left hard-down.
>
> Could well be one boot root to boot as secondary, and another boot root to 
> boot as primary.  Or write a manual process that tweaks the relevant system 
> parameters.
>
> Downside: if the failed primary reboots and rejoins the party, you'll have 
> two hosts that both want to be "primary".  Much like adding code for the 
> transition, it's also possible to code some checks at startup to detect and 
> block a second host from becoming primary.

We don't want the secondary to crash or hang if the primary fails. 
First
because each application abmormally interrupted requires some work to
restart it, and second, because it is always easier to trobleshoot if
we still have a vms instance alive. So giving a zero vote to the
secondary is not viewed as a good option. If it was that simple, it
would be implemented already.

-- 
Marc Van Dyck