Merge pull request #111312 from erjosito/patch-17

Included a section on "autoscaling and HA"

Author: PRMerger6

articles/application-gateway/application-gateway-autoscaling-zone-redundant.md

@@ -7,6 +7,7 @@ ms.service: application-gateway
 ms.topic: article
 ms.date: 03/24/2020
 ms.author: victorh
+ms.custom: fasttrack-edit
 ---
 
 # Autoscaling and Zone-redundant Application Gateway v2 
@@ -127,9 +128,17 @@ Total price = $267.84 + $85.71 = $353.55
 
 Application Gateway and WAF can be configured to scale in two modes:
 
-- **Autoscaling** - With autoscaling enabled, the Application Gateway and WAF v2 SKUs scale up or  down based on application traffic requirements. This mode offers better elasticity to your application and eliminates the need to guess the application gateway size or instance count. This mode also allows you to save cost by not requiring the gateway to run at peak provisioned capacity for anticipated maximum traffic load. You must specify a minimum and optionally maximum instance count. Minimum capacity ensures that Application Gateway and WAF v2 don't fall below the minimum instance count specified, even in the absence of traffic. Each instance counts as 10 additional reserved Capacity Units. Zero signifies no reserved capacity and is purely autoscaling in nature. Please note that zero additional minimum instances still ensures high availability of the service which is always included with fixed price. You can also optionally specify a maximum instance count, which ensures that the Application Gateway doesn't scale beyond the specified number of instances. You'll continue to be billed for the amount of traffic served by the Gateway. The instance counts can range from 0 to 125. The default value for maximum instance count is 20 if not specified.
+- **Autoscaling** - With autoscaling enabled, the Application Gateway and WAF v2 SKUs scale up or  down based on application traffic requirements. This mode offers better elasticity to your application and eliminates the need to guess the application gateway size or instance count. This mode also allows you to save cost by not requiring the gateway to run at peak provisioned capacity for anticipated maximum traffic load. You must specify a minimum and optionally maximum instance count. Minimum capacity ensures that Application Gateway and WAF v2 don't fall below the minimum instance count specified, even in the absence of traffic. Each instance is roughly equivalent to  10 additional reserved Capacity Units. Zero signifies no reserved capacity and is purely autoscaling in nature. You can also optionally specify a maximum instance count, which ensures that the Application Gateway doesn't scale beyond the specified number of instances. You will only be billed for the amount of traffic served by the Gateway. The instance counts can range from 0 to 125. The default value for maximum instance count is 20 if not specified.
 - **Manual** - You can alternatively choose Manual mode where the gateway won't autoscale. In this mode, if there is more traffic than what Application Gateway or WAF can handle, it could result in traffic loss. With manual mode, specifying instance count is mandatory. Instance count can vary from 1 to 125 instances.
 
+## Autoscaling and High Availability
+
+Azure Application Gateways are always deployed in a highly available fashion. The service is made out of multiple instances that are created as configured (if autoscaling is disabled) or required by the application load (if autoscaling is enabled). Note that from the user's perspective you do not necessarily have visibility into the individual instances, but just into the Application Gateway service as a whole. If a certain instance has a problem and stops being functional, Azure Application Gateway will transparently create a new instance.
+
+Please note that even if you configure autoscaling with zero minimum instances the service will still be highly available, which is always included with the fixed price.
+
+However, creating a new instance can take some time (around six or seven minutes). Hence, if you do not want to cope with this downtime you can configure a minimum instance count of 2, ideally with Availability Zone support. This way you will have at least two instances inside of your Azure Application Gateway under normal circumstances, so if one of them had a problem the other will try to cope with the traffic, during the time a new instance is being created. Note that an Azure Application Gateway instance can support around 10 Capacity Units, so depending on how much traffic you typically have you might want to configure your minimum instance autoscaling setting to a value higher than 2.
+
 ## Feature comparison between v1 SKU and v2 SKU
 
 The following table compares the features available with each SKU.