title: Azure app-gateway failover when default backend pool is unhealthy
body: <p>I have the following setup in AzureRM.</p>
  
  <pre><code>                                                ----------  
  -----------     ----------     -----------      |app srv1|
  | App     |     | Varnish|     | Load    | ---&gt; ----------
  | Gateway | --&gt; | Cache  | --&gt; | Balancer|  
  |         |     | server |     |         | ---&gt; ----------     
  -----------     ----------     -----------      |app srv2|
                                                  ----------
  </code></pre>
  
  <p>I would like to direct all requests to the Varnish server by default, but if Varnish dies, direct them directly to the Load Balancer.</p>
  
  <p>I read that failover can be achieved with custom probes in the app gateway, but if I add the Load Balancer into the backend pools, if my thinking is right I believe it will do Round-Robin between the Varnish and the LB.</p>
  
  <p>The reason to have the App Gateway is to offload the SSL cert.</p>
  
  <p>Is it possible to achieve this infrastructure with Azure services?</p>