I have observed it takes 15 data points in 15 minutes to trigger an alarm and start scale in and I could not find an option to change or adjust this behavior.
Can we change this behavior or we will have to define another scaling policy to scale in?
It seems like its the default settings for Target Tracking scaling policy for cpu utilization that
CPUUtilization > 50 for 3 datapoints within 3 minutes [will trigger high alarm]
CPUUtilization < 35 for 15 datapoints within 15 minutes [will trigger low alarm]
Is this true?
Yes, you are correct. These are default settings for the target tracking policy. However, you shouldn't be editing target tracking alarms outside of autoscaling settings.
The AWS specifically writes:
DO NOT EDIT OR DELETE.
If you are not happy with how target tracking policy works, you can use step or simple scaling policies instead.
For more advanced tuning of your scaling, you can use multiple scaling polices.
I'm using the AutoScalingGroup to launch a group of EC2 instances. These instances are acting as workers which are continuously listening to SQS for any new request.
Requirement:
Do upscale on something like throughput (i.e Total number of messages present in SQS by total number instances).
And I want to downscale whenever any instance which is part of ASG is sitting idle (CPUIdle) for let's say more than 15 mins.
Note: I am not looking for any metric which applies as whole to a particular ASG (eg: Average CPU).
One way of doing that could be defining the custom metric and allowing it to trigger a cloudwatch alarm to do that.
Is there a better way to accomplish this?
If you are defining the scaling policy at instance level, then you defeating the entire purposes of ASG. If you need to scale based on changing conditions, such as the queue size, then you can configure ASG based on the conditions specified here
https://docs.aws.amazon.com/autoscaling/ec2/userguide/as-using-sqs-queue.html
A custom metric to send to Amazon CloudWatch that measures the number of messages in the queue per EC2 instance in the Auto Scaling group.
A target tracking policy that configures your Auto Scaling group to scale based on the >custom metric and a set target value. CloudWatch alarms invoke the scaling policy.
If you know a specific time window when the queue size goes up or down, you can also scale based on schedule.
You can always start with a very low instance count in ASG and set the desired capacity as such (say 1) and scale up based on queue, so you can continue using ASG policies.
I have created the following two alerts for an autoscaling group:
Scale up 1 instance if "CPUUtilization >= 75%" changes to state ALARM
Scale down 1 instance if "CPUUtilization >= 30%" changes to state OK
I have chosen to trigger the scale down event on OK to not have a constant ALARM in Cloudwatch if load is below 30%. On the other hand that is exactly the issue. When an upscale happens leaving the group with an average load between 30% and 75% the state is set to ALARM.
Is there any way to configure Cloudwatch to trigger scale up and down events properly without being left with an ALARM state after the scaling happened?
The "Scale down" operation should be set to "CPUUtilization < 75%" (state OK), and don't worry about leaving the group without machines, though you have a min number of instances and it does not go beyond that.
When you setup an Auto Scaling groups in AWS EC2 Min and Max bounds seem to make sense:
The minimum number of instances to scale down to based on policies
The maximum number of instances to scale up to based on policies
However, I've never been able to wrap my head around what the heck Desired is intended to affect.
I've always just set Desired equal to Min, because generally, I want to pay Amazon the minimum tithe possible, and unless you need an instance to handle load it should be at the Min number of instances.
I know if you use ElasticBeanstalk and set a Min to 1 and Max to 2 it sets a Desired to 2 (of course!)--you can't choose a value for Desired.
What would be the use case for a different Desired number of instances and how does it differ? When you expect AWS to scale lower than your Desired if desired is larger than Min?
Here are the explanations for the "min, desired and max" values from AWS support:
MIN: This will be the minimum number of instances that can run in your
auto scale group. If your scale down CloudWatch alarm is triggered,
your auto scale group will never terminate instances below this number
DESIRED: If you trip a CloudWatch alarm for a scale up event, then it
will notify the auto scaler to change it's desired to a specified
higher amount and the auto scaler will start an instance/s to meet
that number. If you trip a CloudWatch alarm to scale down, then it
will change the auto scaler desired to a specified lower number and
the auto scaler will terminate instance/s to get to that number.
MAX: This will be the maximum number of instances that you can run in
your auto scale group. If your scale up CloudWatch alarm stays
triggered, your auto scale group will never create instances more than
the maximum amount specified.
Think about it like a sliding range UI element.
With min and max, you are setting the lower bound of your instance scaling. Withe desired capacity, you are setting what you'd currently like the instance count to hover.
Example:
You know your application will have heavy load due to a marketing email or product launch...simply scale up your desired capacity beforehand:
aws autoscaling set-desired-capacity --auto-scaling-group-name my-auto-scaling-group --desired-capacity 2 --honor-cooldown
Source
"Desired" is (necessarily) ambiguous.
It means the "initial" number of instances. Why not just "initial" then? Because the number may change by autoscaling events.
So it means "current" number of instance. Why not just "current" then? Because during an autoscaling event, instances will start / terminate. Those instances do not count towards "current" number of instances. By "current", a user expects instances that are operate-able.
So it means "target" number of instance. Why not just "target" then? I guess "target" is just as good (ambiguous) as "desired"...
When you expect AWS to scale lower than your Desired if desired is
larger than Min?
This happens when you set a CloudWatch alarm based on some AutoScaling policy. Whenever that alarm is triggered it will update the DesiredCount to whatever is mentioned in config.
e.g., If an AutoScalingGroup config has Min=1, Desired=3, Max=5 and there is an Alarm set on an AutoScalingPolicy which says if CPU usage is <50% for consecutive 10 mins then Remove 1 instances then it will keep reducing the instance count by 1 whenever the alarm is triggered until the DesiredCount = MinCount.
Lessons Learnt: Set the MinCount to be > 0 or = DesiredCount. This will make sure that the application is not brought down when the mincount=0 and CPU usage goes down.
In layman's terms, DesiredCapacity value is automatically updated on scale-in and scale-out events.
In other words,
Scale-in or Scale-out are done by decreasing or increasing the DesiredCapacity value.
Desired capacity simply means the number of instances that will come up / fired up when you launch the autoscaling. That means if desired capacity = 4, then 4 instances will keep on running until and unless any scale up or scale down event triggers. If scale up event occurs, the number of instances will go up till maximum capacity and if scale down event occurs it will go down till the minimum capacity.
Correct me if wrong, thanks.
I noticed that desired capacity went down and no new instance came up when
I set one of the instances to standby. It kept on running but was detached from ELB ( requests were not forwarded to that particular instance when accessed via ELB DNS ). No new instance has been initiated by AWS. Rather desired capacity was decreased by 1.
When I changed the state of instance ( from standby ) the instance was again attached to ELB ( the instance started to get requests when accessed via ELB DNS ). The desired capacity was increased by 1 and became 2.
Hence it seems no of instances attached to ELB can't cross the threshold limit set by min and max but the desired capacity is adjusted or changed automatically based on the occurrence of scale in or scale out event. It was definitely something unknown to me.
It might be a way to let AWS know that this is the desired capacity required for the respective ELB at a given point in time.
Min and max is self explanatory but desired was confusing until i have attached Target Tracking Auto scaling policy with the ASG where CPU utilization was the target metric. Here, desired instances were scaled out and scaled in based on target CPU utilization. If any desired count are placed through cloudformation/manual, for time being ASG will create same number of instances as desired count. But later ASG policy will automatically adjust the desire instances based on target CPU utilization.
Desired is what we start initially. It will go to min or max depending on the scale-in / scale-out.
I liked the analogy with a slider to understand this - https://stackoverflow.com/a/36272945/10779109
Think of min and max as the maximum allowed brightness on a screen. You probably don't want to min to be 0 in that case (sidenote). The desired quantity keeps changing based on the env (in the case of ASG, it depends on the scaling policies).
For instance, if the following check runs every hour, this is where desired quantity is required.
if low_load(<CPU or Mem etc>) and desired_capacity>= min_capacity:
desired_capacity = desired_capacity-1
Max capacity can also be understood in the same way where you'd want to keep increasing the desired quantity based on a cloudwatch_alarm (or any scaling policy) up to the max capacity.
I have an AWS autoscale group. Is it possible to set an alarm for a percentage increase of CPU? For example, if CPU increases 40% over 1min, trigger the alarm? Thus if CPU is at 0% at 12:51 and 40% at 12:52 the alarm will be triggered.
You can set alarms to check CPU average is greater than predefined value for a period of time.
But to check if it increased with precentage, you might need to implement a custom metrics as mentioned below AWS Blog
Yes You can Set Custom Alarm for CPU Load average Increase for AWS Auto scaling. Just follow the steps below.
You can find this in Auto Scaling Groups select a group for which you want to set alam, further goto Scaling Policies click on Add policy then next to Execute policy when: click on Create new alarm, set alarm for your desired aws auto scaling instance according to your need and set that alarm in Execute policy when: and in Take the action: you can select Add, Remove or Set to options according to your needs and you are done. When auto scaling group reaches to provided threshold which is given in Scaling Policies it will execute policy which you set. For reference you can check AWS Docs