Azure Autoscale Restarts Running Instances

Question

I've been using Autoscale to shift between 2 and 1 instances of a cloud service in a bid to reduce costs. This mostly works except that from time to time (not sure what the pattern seems to be here), the act of scaling up (1->2) causes both instances to recycle, generating a service outage for users.

Assuming nothing fancy is going on in RoleEntry in response to topology changes, why would scaling from 1->2 restart the already running instance?

Additional notes:

• It's clear both instances are recycling by looking at the Instances tab in Management Portal. Outage can also be confirmed by hitting the public site.
• It doesn't happen consistently but I'm not sure what the pattern is. It feels like when the 1-instance configuration has been running for multiple days, attempts to scale up recycle both. But if the 1-instance configuration has only been running for a few hours, you can scale up and down without outages.
• The first instance always comes back much faster than the 2nd instance being introduced.

Answer 1

This has always been this way. When you have 1 server running and you go to 2+, the initial server is restarted. In order to have a full SLA, you need to have 2+ servers at all time.

Answer 2

Nariman, see my comment on Brent's post for some information about what is happening. You should be able to resolve this with the following code:

public class WebRole : RoleEntryPoint
{
    public override bool OnStart()
    {
        // For information on handling configuration changes
        // see the MSDN topic at http://go.microsoft.com/fwlink/?LinkId=166357.
        IPHostEntry ipEntry = Dns.GetHostEntry(Dns.GetHostName());
        string ip = null;
        foreach (IPAddress ipaddress in ipEntry.AddressList)
        {
            if (ipaddress.AddressFamily.ToString() == "InterNetwork")
            {
                ip = ipaddress.ToString();
            }
        }

        string urlToPing = "http://" + ip;
        HttpWebRequest req = HttpWebRequest.Create(urlToPing) as HttpWebRequest;
        WebResponse resp = req.GetResponse();
        return base.OnStart();
    }
}

Answer 3

You should be able to control this behavior. In the roleEntrypoint, there's an event you can trap for, RoleEnvironmentChanging.

A shell of some code to put into your solution will look like...

RoleEnvironment.Changing += RoleEnvironmentChanging;

private void RoleEnvironmentChanging(object sender, RoleEnvironmentChangingEventArgs e)
{
}

RoleEnvironment.Changed += RoleEnvironmentChanged;

private void RoleEnvironmentChanged(object sender, RoleEnvironmentChangedEventArgs e)
{
}

Then, inside the RoleEnvironmentChanged method, we can detect what the change is and tell Azure if we want to restart or not.

if ((e.Changes.Any(change => change is RoleEnvironmentConfigurationSettingChange)))
{
    e.Cancel = true; // don't recycle the role
}