High Availability/Scaling

Question 1

Vertical Scaling

Answer 1

• works only up to a certain point
• resize processor size (ex: t3.micro to x1.xL)
• appropriate only in certain circumstances

Question 2

Horizontal Scaling

Answer 2

• spin up limitless EC2 instances

- increases high availability

Question 3

3 W’s of Scaling

Answer 3

What - what do we scale, where is the template coming from?
Where - where in the VPC does it make sense to scale? Which load balancer
When - When do I scale? Cloud Watch Alarms can tell us when to scale.

Question 4

Launch template

Answer 4

• a collection of settings that go into building an EC2 instance
• so you don’t have to use the wizard every time
• save the wizard results to a template
• supports versioning
• more granularity
• recommended by AWS

Question 5

Launch configuration

Answer 5

• only for autoscaling
• immutable (can’t change it)
• limited config options
• don’t use them

Question 6

When given a choice between Launch Templates and Launch Configurations which do you choose?

Answer 6

Launch Templates

Question 7

What goes into a Launch template?

Answer 7

• AMI
• EC2 instance size
• security groups
• potentially networking info

Question 8

Why don’t you want to put a VPC inside a launch template?

Answer 8

it would prohibit you doing a autoscaling group later

Question 9

Can you change the bootstrap script in a Launch template

Answer 9

only if you create a new version of it

Question 10

Auto-scaling Groups

Answer 10

• a collection of EC2 instances that are treated as a collective group in terms of management

Question 11

Steps for creating an auto scaling group

Answer 11

1) pick your launch template
2) pick your VPC and network and purchasing options
3) configure your Elastic Load Balancer
4) set scaling policies (min, max, capacity)
5) set up notifications via SNS

Question 12

When do you indicate that you want to use the load balancer health checks for auto scaling group?

Answer 12

Must choose the setting during configuration

Question 13

Autoscaling limits

Answer 13

Minimum - the lowest # of EC2 instances you’ll ever have on line
Maximum - the highest # of EC2 instances
Desired Capacity - how many instances do you want right now - constantly changing.

Question 14

Spot Instances in an auto scaling group

Answer 14

• to save money

- can combine OnDemand and spot instances

Question 15

How to get high availability?

Answer 15

• Multiple AZs
• use a load balancer
• use an autoscaling group
• set autoscaling group to use health checks

Question 16

How auto-scaling group can handle networking

Answer 16

select multiple subnets to spread across AZs

Question 17

How can an auto scaling group notify you?

Answer 17

via SNS

Question 18

Step Scaling

Answer 18

when there’s a check in based on a target value (ex: memory) and if it has 0 - 40% of the target it scales in, and if it has 60% - 100% of target, it scales out)

Question 19

Instance Count scaling

Answer 19

when there’s a check in and it sees how many instances are currently healthy compared to the target number of instances.

Question 20

Scaling Policies - Scaling out example

Answer 20

• Add 10 instances when memory is between 60 - 80%

- Add 15 instances when memory usage is b/w 80 - 100%

Question 21

Scaling Policies - Scaling in example

Answer 21

• terminate 10 instances when memory usage is between 20 - 40%
• terminate 15 instances when memory usage is between 0 - 20%

Question 22

Warm up period

Answer 22

auto scaling hangs out and does not check the new instances

- auto scaling will continue to check necessary capacity, but takes into account that the new instances are warming up

Question 23

Scaling tips

Answer 23

• Scaling out takes longer than scaling in.

- Scale out aggressively Scale in conservatively.

Question 24

Cool Down

Answer 24

• default: 5 minutes
• pauses autoscaling for a set amount of time
• helps prevent runaway scaling events

Question 25

Thrashing

Answer 25

when you’re spinning up new instances and terminating them in an uncontrolled manner

Question 26

Steady State autoscaling group

Answer 26

• can change its AZ and put itself back online if it gets terminated
• when min = max = desired = 1, but you still need to be highly available
• ex: legacy app that cannot be copied

Question 27

Auto Scaling exam tips

Answer 27

• choose reasonably sized min and max without over engineering
• bake things into AMIs so warmup takes less time
• use Reserved Instances to cover the minimum
• use spot instances to fill in

Question 28

Scaling non-relational databases (Dynamo DB)

Answer 28

AWS does all of the Dynamo DB scaling for you

Question 29

Provisioned model

Answer 29

use case: generally predictable workload

• effot to use: need to review past usage to set upper and lower scaling bounds
• cost: most cost-effective model
• most predictive scaling model

Question 30

On Demand model

Answer 30

• use case: sporadic workload
• effort to use: select ‘on Demand’
• cost: small cost per read and write; less cost effective

Question 31

Dynamo exam tips

Answer 31

• favor cost savings over scaling
• predictable workload -> choose provisioned
• sporadic workload -> choose On Demand
• Avoid keys with similar values to get better performance
• can switch between provisioned and on demand every 24 hours
• most questions will be about cost