Welcome back to this walkthrough series
of exam questions and technique lessons.
Now it's time for question eight,
so let's jump in and get started.
This is the question we're reviewing in this lesson.
Catagram enterprise is a business critical application
and any form of outage is unacceptable.
It uses EC2 instances provisioned by an Auto Scaling group,
connected to an application,
load balancer and uses DynamoDB for data storage.
The application currently uses nine EC2 instances
in one availability zone,
and you've been asked to ensure it can continue operating
with a hundred percent capacity while coping
with the failure of any one availability zone.
Which of the following options meets the requirements
in a cost effective way?
And you're asked to choose one.
Now this is a really common scenario,
which you will almost certainly encounter in the exam
dealing with the high availability of applications in AWS.
So go ahead and pause the video,
review the question yourself and try to pick out which bits
matter and which don't.
Now that you've done that let's look at what I identified
and you can compare.
The key to answering this style of question,
which focuses on the ability of a system to recover
and operate through failure is to understand
the raw values in the question.
Any form of outage is unacceptable.
That's the first thing that jumps out of the question text.
The question tells us that the application uses nine EC2
instances in one availability zone.
And given no other information,
you can safely assume that this is the minimum operational
level for the application.
Now you'll also see that nine instances are all in one
availability zone, which is risky,
but that's what the question states.
The key part of this question,
which you need to understand, is that the scenario requires
a solution which can continue operating at 100% capacity,
which is nine EC2 instances.
And it needs to do that while coping with the failure of one
availability zone and then it needs to do so
in a cost effective way.
So there might be multiple correct answers,
but you have to choose one, the most cost effective answer.
So with that being said,
let's look at the answers that we have available.
Now in this particular case, there are a total
of five answers.
So why don't you go ahead and pause the video,
review the answers and try to spot the key elements of them.
And then when you resume,
we can step through them all together and compare
what you found to what I found.
At this point I hope you've attempted to find
the key points in the answers yourself,
but this is what I located in the answers.
Now we know what the question wants at this point.
So all we have to do with each answer is to identify the
pieces which matter.
So the number of instances in each availability zone,
the number of availability zones
and the total number of instances.
We need to pick the answer with the least number
of total instances, which meet the high availability
requirements of the question.
Now looking at these answers,
we have everything that we need because each answer
tells us the number of instances
and the number of availability zones.
So let's step through each answer
and review it based on the requirement of the question.
Answer one proposes to increase the number
of EC2 instances to 18,
providing full one-to-one resilience.
Now this is wrong, can you spot why?
Well, it mentions nothing about changing the number
of availability zones.
The question says that the current instances
are all in one availability zone and without changing this,
it stays the same.
This unfortunately means, if this one availability zone
fails, then all of the instances within that
availability zone also fail.
This answer is the most expensive to implement.
It has the highest number of EC2 instances,
but it doesn't meet the question requirements.
Because if the one availability zone fails
with the instances in, then the whole application fails
and that breaches the requirements in the question.
Now answer two, this is an interesting one,
because it's a wrong answer,
but not in the way that you think.
It's only wrong, because it's not the cheapest way
of delivering the desired outcome.
With this solution, you will have 18 instances,
nine in each of two availability zones.
Now this means if one availability zone fails,
you would still have nine instances.
Meaning it meets the objective of the question,
but it's not the most cost effective answer.
And we'll see why as we go through the remaining answers.
Now note how the starting words of all
but the top answer are the same.
That's an example of word fluff.
So adjust the Auto Scaling group because it's shared in all
of the bottom four answers.
You can safely ignore those words if you're evaluating
between those four answers.
Because it's shared in all of the text,
it essentially has no meaning.
Now the third answer suggests that we change
the Auto Scaling group, so it has three instances
in three availability zones.
So A, B and C for a total of nine EC2 instances.
Now this is cheaper than answers one or two,
but does it actually meet the question requirements?
If one availability zone fails,
that means we only have six instances remaining.
And since nine is the minimum, this answer is also wrong.
The last answer proposes five instances
in availability zone A and availability zone B
for a total of 10.
Now this answer is not the cheapest,
nor is it the most expensive,
but it doesn't meet the question requirements.
If one of those availability zones fail,
there are only five instances remaining,
which is below the minimum required.
So again, this is another wrong answer.
Now the correct answer is answer four.
This has us deploying three instances
into four different availability zones,
A B, C, and D for a total of 12 instances.
If one availability zone fails,
then the other three availability zones provide
three instances each for a total of nine instances.
And that's the minimum capacity of the application,
which means that the system can continue operating.
Now compare answer four with answer two,
both of them work, but answer four
does it with the less instances.
And so it's more cost effective.
And that's why it's the correct answer.
With that being said,
that's everything I wanted to cover in this lesson.
I hope it's been useful in giving you some exposure
to a different style of exam question.
Go ahead and complete the lesson, and when you're ready,
I look forward to you joining me in the next.