Autoscaling groups (High Availability and Scalability)

Question 1

the goals of ASG

Answer 1

1. scale in (add) or scale out (remove) to match an increased or decreased load
2. we can set the minimum and maximum number of machines running
3. automatically register new instances to a load balancer (ASG is behind the LB)

Question 2

ASGs have the following attributes

Answer 2

1. a launch configuration
2. min size / max size / initial capacity
3. network + subnets info (where can instances be created)
4. load balancer info (or target group)
5. scaling policies

Question 3

Scaling poicies

Answer 3

what will trigger a scale out

what will trigger a scale in

Question 4

Autoscaling alarms

Answer 4

it is possible to scale an ASG based on CloudWatch alarms

we configure scaling policies - when to scale in or out - based on alarms

Question 5

CloudWatch alarms

Answer 5

based on metrics (average CPU) that are computed for the overall ASG instances (averages)
if they go up the alarm goes off - scale out
when they go back down - scale in

Question 6

Auto Scaling NEW Rules

Answer 6

you can say “I want to have a target average CPU usage of 40% in my ASG” and it will scale in and out to meet this goal

• number of requests on the ELB per instance
• average network in our out

Question 7

Auto Scaling based on Custom Metric

Answer 7

for ex., number of connected users

1. we’ll create this custom metric from our application and send it to CloudWatch (PutMetric API)
2. create CloudWatch alarm to react to low or high values
3. use the alarm as the scaling policy for ASG

Question 8

to update an ASG

Answer 8

you must provide a new launch configuration or launch template (newer)

Question 9

if you attach IAM roles to an ASG

Answer 9

they will get assigned to EC2 instances

Question 10

ASG payment

Answer 10

ASGs are free, you only pay for the underlying resources being launched, f. ex. EC2 instances, EBS volumes

Question 11

if an instance in ASG is terminated

Answer 11

ASG will detect this and replace it automatically with a new one

ASG can terminate instances marked as unhealthy by LB and replace them

Question 12

launch configuration vs launch template (newer)

Answer 12

launch configuration allows you to specify only one instance type

launch template allow you to use spot fleet (of both on demand and spot instances, for ex.)

Question 13

Target tracking scaling

Answer 13

scaling policy, the easiest to set up
I want the average ASG CPU to stay at around 40%

if it goes over, it will provision more instances, if it goes down - will scale in

Question 14

scaling policies

Answer 14

1. Target tracking scaling
2. simple / step scaling
3. scheduled actions

Question 15

simple / step scaling

Answer 15

scaling policy

you set CloudWatch alarm and if it is triggered - you can configure how many instances will be added

For ex., if average CPU over the entire group goes over 70% - add 2 units
if it goes below 30% - remove 1 unit

Question 16

scheduled actions

Answer 16

scaling policy

anticipate scaling based on known usage patterns

when you know that on certain day or at certain time you have to increase the number of instances

Question 17

scaling cooldowns

Answer 17

helps to ensure that your ASG doesn’t launch or terminate additional instances before the previous scaling activity taking effect

Question 18

scaling-specific cooldowns

Answer 18

in addition to default cooldown for ASG we can create cooldowns that apply to a specific simple scaling policy

A scaling-specific cooldown period overrides the default cooldown period

Question 19

one common use for scaling-specific cooldowns

Answer 19

is with scale-in policy - a pocily that terminates instances based on a specific criteria or metric.

Because this policy terminates instances, EC2 Auto Scaling needs less time to determine whether to terminate additional instances

if the default cooldown period of 300 seconds is too long - you can reduce costs by applying a scaling-specific cooldown period of 180 seconds to the scale-in policy

Question 20

if your app is scaling up and down multiple times each hour

Answer 20

modify ASG cool-down timers and the CloudWatch alarm period that triggers the scale in

Question 21

ASG default termination policy

Answer 21

1. find AZ which has the most number of instances
2. if there are multiple instances in the AZ to choose from, delete the one with the oldest launch configuration

the idea is that ASG tries to balance the number of instances across AZ

Question 22

Lifecycle hooks

Answer 22

normal life cycle for new instance when it’s added to ASG is
pending (scale out) –> InService –> Terminating (scale in) –> terminated

by adding steps (lifeCycle hooks) pendingWait -> pendingProceed between pending and inService we have the option to configure certain things: install extra software, do extra checks before your instance is declared inService

we can also add terminatingWait -> terminatingProceed between terminating and terminated - to extract information, logs or files out of the instance before it is completely terminated

Question 23

both launch template and launch configuration allow you

Answer 23

to specify

• the AMI ID of your instances
• the instance type
• key pair
• security groups
• tags
• EC2 user-data
• other parameters to launch EC2 instances

Question 24

launch configurations are considered

Answer 24

legacy, not recommended to use because you have to recreate them every time you change a parameter

Question 25

launch templates

Answer 25

are newer and recommended to use 1. can have multiple versions 2. you create parameter subsets and share them across templates (partial configuration for re-use and inheritance) 3. provision using both on-demand and spot instances, or a mix 4. can use T2 unlimited burst feature

Question 26

I have an ASG and an ALB, and I setup my ASG to get health status of instances thanks to my ALB. One instance has just been reported unhealthy. What will happen?

Answer 26

ASG will terminate the instance Because the ASG has been configured to leverage the ALB health checks, unhealthy instances will be terminated

Question 27

Your boss wants to scale your ASG based on the number of requests per minute your application makes to your database.

Answer 27

you create a CloudWatch custom metric and build an alarm on this to scale your ASG The metric "requests per minute" is not an AWS metric, hence it needs to be a custom metric

Question 28

An application is deployed with an Application Load Balancer and an Auto Scaling Group. Currently, the scaling of the Auto Scaling Group is done manually and you would like to define a scaling policy that will ensure the average number of connections to your EC2 instances is averaging at around 1000. Which scaling policy should you use?

Answer 28

Target Tracking