How to upgrade a clustered environment
The upgrade in a clustered environment works nearly the same as in a non-clustered as described in the official documentation.
A general note: it is not possible to use the instance while the upgrade is happening, but the upgrade should be a matter of a few minutes usually.
You should follow some steps:
- (optional) serve via the load balancer a static maintenance page with more information - otherwise the default "Instance is in maintenance mode" page will be shown, which should also be fine
- enable maintenance mode (this is stored in the config.php - so if you have them on a shared storage it only needs to be done on one server and the others should receive this as well - otherwise do it on every web server) - in practice it is helpful to just shut down the web server of each node except the one that handles the upgrade
- disable cron.php execution (it doesn't start new jobs while in maintenance, but maybe there is one running)
- the web based updater is not suited for a clustered environm