Request for a specific Azure Web App instance

In Microsoft Azure Web Apps you have the ability to scale your site by adding more instances to it where each instance is running on a different VM.

When you have more than one instance a request made to your site can go to any of them using a load-balancer that will decide which instance to route the request to based on how busy each instance is at the time.

One feature of this load-balancer is that once a request from your browser is made to the site, it will add a "cookie" to it (with the response) containing the specific instance id that will make the next request from this browser go to the same instance.

We can use this feature to send a request to a specific instance of our site.

The name of the cookie we're going to use is: ARRAffinity


    private static async Task<HttpResponseMessage> GetFromInstance(Uri url, string instanceId)
        var cookieContainer = new CookieContainer();
        using (var handler = new HttpClientHandler() { CookieContainer = cookieContainer })
            using (var httpClient = new HttpClient(handler))
                cookieContainer.Add(url, new Cookie("ARRAffinity", instanceId));
                return await httpClient.GetAsync(url);

The problem that we have now is getting this instance id, in the update below I'll show how, but a specific site can find out it's own instance id by looking at the environment variable called: WEBSITE_INSTANCE_ID.

So one application for this is that we can create a WebJob that is able to call the Website it is hosted on.

WebJob Code

    private static void Main(string[] args)
        string instanceId = Environment.GetEnvironmentVariable("WEBSITE_INSTANCE_ID");
        string siteName = Environment.GetEnvironmentVariable("WEBSITE_SITE_NAME");
        var url = new Uri("http://" + siteName + "");
        var response = GetFromInstance(url, instanceId).Result;


Azure Web Sites now provides an API to get all instances (IDs) for your website, you can either do it programmatically or using the Azure CLI tools.

Get instance IDs for a web site - sample code

First thing to do is install the Azure Web Apps Management Library from nuget, this is the SDK for managing your Azure Web Site from code.

Now all you need is this code:

internal class Program
    private static void Main(string[] args)
        var cert = new X509Certificate2();
        var client = new WebSiteManagementClient(new CertificateCloudCredentials("subscription_id_guid", cert));

        var instanceIds = client.WebSites.GetInstanceIds("westuswebspace" /*webspace name*/, "somesite" /*web site name*/);
        Console.WriteLine(String.Join(", ", instanceIds));

Azure CLI tools

Azure has CLI tools for both PowerShell (for windows users) and xplat using node.js under the cover (for all users including mac, unix and windows).

To get these tools you can go to this link.

To install the xplat tool you can simply write the following command: npm install azure-cli -g

For more information on using the CLI tools you can go to these links: Managing the Cloud from the Command Line and Azure PowerShell - MSDN

Tip for PowerShell - Start by using the following command: Add-AzureAccount.

In both tools you get the website's instance ids by getting/showing the website.


Get-AzureWebsite sitename

Instances                   : {6d016e86bc41ff8e2fcf5d66da0116e929b41609a8cace17b40b6c5e4eb15b44}
NumberOfWorkers             : 1


> azure site show sitename

info:    Executing command site show
info:    Showing details for site
+ Getting site information
+ Getting site config information
+ Getting repository settings
+ Getting diagnostic settings
+ Getting site instances information
+ Getting locations
data:    Web Site Name:  sitename
data:    Site Mode:      Standard
data:    Enabled:        true
data:    Availability:   Normal
data:    Last Modified:  Mon Jun 16 2014 18:46:58 GMT-0700 (Pacific Daylight Time)
data:    Location:       West US
data:    Host Name
data:    ------------------------
data:    Instance Id
data:    ----------------------------------------------------------------
data:    6d016e86bc41ff8e2fcf5d66da0116e929b41609a8cace17b40b6c5e4eb15b44

Hope this helps.

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