CODEWITHSUNDEEP
windowsapiazureazure-web-roles
sharptooth
sharptooth

Reputation: 170499

How do I scale my Azure application without having a temporary outage?

I'm toying with Windows Azure Management API for scaling my Azure web role. At some point I have one instance and decide that I want to go from one instance to two instances. I send an HTTP POSt request to

https://management.core.windows.net:443/<my-subscription-id>/services/hostedservices/<my-service-name>/deployments/<my-deployment-name>/?comp=config

with an XML specifying the same configuration as deployment currently has and instances count set to two. The call succeeds and the change starts. Now for about 30 seconds the web role will not accept HTTP calls - the caller will get 

10061 connection refused

in browser. Which means the role is not serving client requests. That's a problem.

How do I scale the web role in such way that it serves client requests at all times?

Upvotes: 2

Views: 1148

Answers (2)

Sandrino Di Mattia
Sandrino Di Mattia

Reputation: 24895

Please note that, when you have 2 instances or more, you might still experience an outage when you modify the service configuration (like changing the number of instances using the Service Management API). Any configuration change will trigger a reboot of your instances.

To prevent this you'll need to implement the following code in your WebRole.cs/WorkerRole.cs (and as a result you won't have an outage when you change the number of instances):

public override bool OnStart()
{
    RoleEnvironment.Changing += RoleEnvironmentChanging;
    return base.OnStart();
}

private void RoleEnvironmentChanging(object sender, RoleEnvironmentChangingEventArgs e)
{
    if (e.Changes.Any(change => change is RoleEnvironmentConfigurationSettingChange))
    {
        e.Cancel = false;
    }
}

Edit: See astaykov's comment below.

Upvotes: 0

astaykov
astaykov

Reputation: 30903

As per SLA (Service Level Agreement - Compute):

We guarantee that when you deploy two or more role instances in different fault and upgrade domains your Internet facing roles will have external connectivity at least 99.95% of the time.

This means that having one instance is not supported case for SLA, so you may (or will) have downtime. If scale from 2 or more, or from more to 2, there shall not be any outage.

This blog post outlines a good explanation about fault and upgrade domains. Before all, scaling means "upgrade" - you are changing configuration, this configuration change needs to be propagaded through all roles and instances. The only way to do that witout downtime (currently) is to have at least two instances, each of which lives in separate domain.

Upvotes: 7

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