Hi Julian,
I am assuming you are referring to upgrading a Moodle install on a AWS configuration using auto-scaling. The procedure isn't much different then upgrading a single server.
Though I have not used auto-scaling for Moodle, the approach I would take is:
- Change the configuration of auto-scaling to 1 EC2
- Put Moodle into Maintenance mode
- Make a backup of Moodle install and data. Exactly how depends on your configuration. You just want to be able to restore everything back to the way it was should something break. Snapshots are usually a simple solution.
- Perform the Moodle upgrade and test thoroughly.
- Create a new AMI to be used for auto-scaling
- Modify your auto-scaling config to use the new AMI
- Start up a new EC2 instance with that AMI
- Place the new EC2 in the auto-scaling group and remove the old one
- Adjust your auto-scaling group back to the number of EC2 instances you need
Again, the above is a general guideline and you will want to verify all the steps are correct for your particular install.
Just an FYI - The configuration I use has a single medium EC2, RDS to offload the database functions, and a Load Balancer to offload SSL. This easily handles between 25-50 users being signed on at once. I have not really monitored to determine exactly home many concurrent connections this handles or if I really need the RDS or Load Balancer.
Hope this helps.
Ron