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Could you estimate the average time a TCP request waits in the surge queue of an AWS CLB from CloudWatch metrics?

To estimate the average time a TCP request waits in the surge queue of an Amazon Web Services (AWS) Classic Load Balancer (CLB), you can use the SurgeQueueLength and RequestCount metrics from Amazon CloudWatch.

SurgeQueueLength is the number of requests in the surge queue, and RequestCount is the total number of requests received by the load balancer.

  1. First, get the average SurgeQueueLength for a specific time period. To do this, use the avg statistic for the SurgeQueueLength metric in CloudWatch.
Namespace: AWS/ELB
MetricName: SurgeQueueLength
Statistic: Average
Dimensions: LoadBalancerName=<your_load_balancer_name>
  1. Next, get the total RequestCount for the same time period. Use the sum statistic for the RequestCount metric in CloudWatch.
Namespace: AWS/ELB
MetricName: RequestCount
Statistic: Sum
Dimensions: LoadBalancerName=<your_load_balancer_name>
  1. Divide the average SurgeQueueLength by the total RequestCount to get the average time a TCP request waits in the surge queue.
AverageTimeInSurgeQueue = AverageSurgeQueueLength / TotalRequestCount

Keep in mind that this is an estimate, and the actual time a request spends in the surge queue may vary depending on factors such as request rate, backend instance performance, and load balancer configuration.

Additionally, you may want to monitor the SpilloverCount metric, which represents the number of requests that have been rejected due to the surge queue being full. A high SpilloverCount could indicate that the load balancer is struggling to handle incoming traffic, and you may need to take action, such as adding more backend instances or increasing the surge queue size.

avg statistics of SurgeQueueLength is averaged across individual load balancer nodes. Should the SampleCount statistics be used to calculate the total length of surge queue?

Yes, you’re correct. Since the avg statistic for SurgeQueueLength is an average value across individual load balancer nodes, it’s not ideal for calculating the total length of the surge queue. Instead, you can use the SampleCount statistic, which gives you the number of data points used for the computation of the average.

To calculate the total length of the surge queue for the desired time period, you can multiply the avg statistic by the SampleCount:

TotalSurgeQueueLength = AverageSurgeQueueLength * SampleCount

Now you can use the TotalSurgeQueueLength to estimate the average time a TCP request waits in the surge queue:

AverageTimeInSurgeQueue = TotalSurgeQueueLength / TotalRequestCount

This approach will give you a more accurate estimation of the average time a TCP request waits in the surge queue of an AWS Classic Load Balancer.

Could you explain the theory behind this formula?

Certainly! The formula I provided earlier is based on the Little’s Law, which is a principle used in queuing theory. Little’s Law states that the average number of items in a queuing system (L) is equal to the average arrival rate of items (λ) multiplied by the average time an item spends in the system (W):

L = λ * W

In the context of an AWS Classic Load Balancer (CLB), we can apply Little’s Law to estimate the average time a TCP request waits in the surge queue:

  • L: Total length of the surge queue (TotalSurgeQueueLength)
  • λ: Arrival rate of TCP requests (RequestCount)
  • W: Average time a TCP request waits in the surge queue (AverageTimeInSurgeQueue)

By rearranging Little’s Law, we can calculate the average waiting time (W):

AverageTimeInSurgeQueue (W) = TotalSurgeQueueLength (L) / TotalRequestCount (λ)

To obtain TotalSurgeQueueLength, we need to consider the AverageSurgeQueueLength and SampleCount from CloudWatch metrics, as discussed earlier:

TotalSurgeQueueLength = AverageSurgeQueueLength * SampleCount

Here, AverageSurgeQueueLength represents the average number of requests in the surge queue across all load balancer nodes during a specific time period, and SampleCount is the number of data points used for computing the average.

By using the modified Little’s Law formula, we can estimate the average time a TCP request waits in the surge queue of an AWS CLB, taking into account the queuing behavior and request arrival rate. However, keep in mind that this is still an approximation and may not precisely represent the actual waiting time for every request.

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