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As the Elastic Load Balancer makes auto scaling decisions, this section describes the Auto Scaling Decision Maker in-depth.
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There are few auto scaling decision making variables, out of which all of the vital variables are configurable using the loadbalancer.conf file. (Sample configuration files are provided at the end of this document [a] [b] .For sample configuration files please see Sample Autoscaling Configurations)
- autoscaler_task_interval (t)
This refers to the time period between two iterations of an 'autoscaling decision making task'. When configuring this value, you are advised to consider the time ‘that a service instance takes to join Elastic Load Balancer (ELB)’. This variable is stated milliseconds and the default value is 30000ms. - max_requests_per_second (Rps)
This refers to the number of requests, a service instance can withstand per second. It is recommended that you determine this value for each service instance and for different scenarios. The ideal way to estimate this value could be by load testing a similar service instance. The default value is 100. - rounds_to_average (r)
This refers to the number of ‘autoscaling decision making’ task iterations that need to take place before an autoscaling decision is made. The default value is 10. - alarming_upper_rate (AUR)
Instead of waiting till the service instance reaches its maximum request capacity (where the alarming_upper_rate = 1), the system will be scaled up when it reaches the request capacity which corresponds to alarming_upper_rate. This value should be 0<AUR<=1. The default value is 0.7. - alarming_lower_rate (ALR)
This refers to the lower bound of the alarming rate. This variable an be used as an indicator to decide when the system should be scaled down. This value should be 0<ALR<=1. The default value is 0.2. - scale_down_factor (SDF)
This refers to the factor that is needed to slow down the scaling down process. To reduce the scaling down due to a false-positive events, the scaling down down process needs to be carried out slowly. This value should be 0<SDF<=1. The default value is 0.25.
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The minimum number of instances and the maximum number of instances of service clusters are always respected. The system always maintains the minimum number of service instance requirements and the system will not scale beyond it's limit.
Calculations
The following is the calculation that is used to calculate the actual load based on the number of requests in-flight:
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Scaling Up
The following is the calculation that is used to calculate the expected number of maximum requests that a service instance can withstand. This in-turn is used to decide when the scaling up process should be carried out:
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Scaling down
The following is the calculation that is used to calculate the imaginary lower bound value. This in-turn is used to decide when the scaling down process should be carried out:
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