Hi, Jiaxin,

Let's handle these cases separately. When you shut down a DEA or a Diego
Cell gracefully (say, as BOSH does when updating or deleting a VM), it will
attempt to get replacements for its app instances scheduled and running on
other VMs before it shuts down the instances it hosts. We call this process
"evacuation". As Yitao pointed out, if you have the spare capacity in your
deployment for those replacement instances, this evacuation procedure
should prevent any downtime for the application. In your case, even if you
have all 4 app instances on the same DEA or Diego Cell when it starts to
shut down, the system will schedule 4 replacement instances on other VMs
and get them running before the original 4 instances are stopped.

The other thing you've asked about is rebalancing to improve instance
anti-affinity when more capacity is available. Currently, neither the DEAs
nor Diego do that, but this kind of active rebalancing is of great interest
to the Diego team and is something we intend to work on in the near future.

If you want to attempt this rebalancing manually, you can determine some
placement information about your instances from the app-stats endpoint
(/v2/apps/:guid/stats) on CC's API. The response will include the IP of the
host for each instance. You could then use the `cf restart-app-instance`
command to restart instances one by one by their index. On the Diego
runtime, at least, instance anti-affinity across availability zones and
hosts is the most important placement factor, so if there is sufficient
capacity, the replacement instances will be placed on VMs with fewer
instances of that app. Of course, this manual rebalancing procedure is less
than ideal, which is one reason the Diego team is quite interested in
having the platform handle it on your behalf. Moreover, the goal would also
be to perform the rebalancing in a safer way, ideally similar to the
"evacuation" concept above except for individual app instances instead of
all the instances on a particular VM.

Thanks,
Eric, CF Runtime Diego PM