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Difficulty: MediumConfigure Azure App Service Web Apps

You are configuring autoscale rules for an Azure App Service plan named AppServicePlan1 that hosts a critical API. The plan currently runs a single instance. You define a scale-out rule that increases the instance count by 11 when the average CPU utilization exceeds 80%80\%. You need to configure a scale-in rule to decrease the instance count by 11 when the load decreases. You must ensure that scaling in does not immediately trigger a scale-out event (flapping) when the resource count is reduced from 22 instances to 11 instance. Which CPU utilization threshold should you specify for the scale-in rule?

  1. Less than 35%35\%Answer
  2. B
    Less than 50%50\%
  3. C
    Less than 65%65\%
  4. D
    Less than 85%85\%

Answer

Less than 35%35\%
The correct option is the one specifying a threshold of less than 35%35\%. When average CPU utilization across 22 instances is below 35%35\%, the total combined workload is less than 70%70\%. Reducing the instance count to 11 results in a single instance carrying a load of less than 70%70\%, which is below the scale-out threshold of 80%80\% and prevents immediate re-scaling (flapping).

Step-by-Step Solution

1
Analyze the scale-out rule configuration and determine the relationship between instance counts and workload distribution.
The scale-out rule is triggered when CPU utilization exceeds 80%80\% on a single instance, and scaling out distributes the load across 22 instances.
To understand the resource state before a scale-in event is evaluated.
2
Calculate the maximum safe CPU utilization on 22 instances before scaling down to 11 instance without exceeding the scale-out threshold.
The maximum safe load on 11 instance is just below the scale-out threshold (80%80\%). On 22 instances, this corresponds to an average CPU utilization of less than 80%2=40%\frac{80\%}{2} = 40\%.
To find the threshold where the combined workload of both instances can fit onto a single instance without triggering a scale-out.
3
Select the option with a threshold value below the calculated maximum safe average CPU utilization of 40%40\%.
The threshold of less than 35%35\% is below 40%40\% and is the only option that prevents flapping.
To choose the correct rule parameter that ensures stable autoscale operations.

Key Concept

Preventing flapping in App Service autoscale configurations by ensuring the scale-in threshold accounts for load distribution changes when instance count decreases.
Estimated Time:1m 30s
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Topics

Configure Azure App Service Web Apps

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