Do you mean to update a live system? I think the best way to perform an upgrade of a clustered system is to shutdown the entire system and then perform the upgrade to SR12. IMHO in that case the order of upgrades is not important. To maximize uptime, you can upgrade every individual service and let the installer shutdown and restart the service. See Chapter 3.3 Multi-machine Preparation in the QlikView Server Reference Manual.
See also Upgrading QlikView Server may restart disabled services for some additional points of interest
Thanks for the quick response on this, and thanks for the reference manual tip. The manual does indeed specify a sequence of upgrade to maximise uptime, I just tested it (albeit on the dev instance which is not in cluster mode) and it seems Access Point still works when QMS is disabled (which is the first step on the recommended procedure).
I am guessing this will extend to the cluster scenario as well!
1.Perform a backup as described in Upgrade Procedure (page 21).
2.Stop QMS (which means QMC becomes unavailable).
3.Upgrade in the following order (let the installer restart the services):
a) Web servers
b) Directory Service Connector (DSC)
c) QlikView Server (QVS)
d) QlikView Distribution Service (QDS)
4.Start QMS (which means QMC becomes available again).
But I am still not clear, since DSC and QVS are running on separate machines, would the temporary disparity in releases not cause an issue overall? So once the webserver is upgraded, publisher is on SR8 but webserver is on SR12 albeit for a short time.
Well, your last remark applies to any multi-platform upgrade, whether QlikView is clustered or not. And the task list you mentioned appears in a chapter titled Multi-platform preparations, so I guess Qlik acknoledges the fact that it is possible, but only if you execute the upgrade steps in that specific order.
If you want to know for sure, please contact Qlik Support, I guess your QlikView installation is large enough to get an answer from them rather quickly. And Qlik Support is known for adding any and all additional information that is relevant (or essential) for your planned upgrade.