0

I am about to take my AWS Architect Associate Certification Exam and I have some things on ELB and ASG that I still don't get (or maybe I just did not study enough) and I liked to ask your help to clear things out.

  1. Multi-AZ Autoscaling Group
    what difference does it make when I say I have one ASG that will handle autoscaling for 3 AZs rather than have one ASG for each AZ? If fault tolerance is the answer then the latter should be the standard setup, why have one ASG for three or two AZs?


  1. Multi-AZ ELB
    same kind of question as I had for #1.


    3.
  • Multi-AZ ASG and one ELB for each AZ
  • Multi-Az ASG and one ELB that serves multiple AZs
  • One ASG and One ELB for each AZ


What are the use cases for each?

Hingle McJingleberry
  • 541
  • 4
  • 26

1 Answers1

2

The answer becomes more obvious when you think about the implications and understand what may be some missing details.

If an ASG crosses multiple availability zones, then it can increase capacity in the healthy zones when the instances in a catastrophically failed AZ become unavailable. With one in each, there would be no coordination like this.

The same thing is true for ELB. In both Classic and Application load balancers, when you deploy a single ELB in multiple AZs, you actually get balancer hardware allocated from the beginning in each AZ -- yet the price is the same. If an AZ fails, it fails, and you still have working hardware in the remaining zones.

ELBs and ASGs in a single AZ would not be fault tolerant, and there's no reason to provision separate ones for each AZ, when you can provision just one, and have it handle the failure of an entire availability zone (unlikely, but not impossible) by scaling out (deploying more hardware) capacity in the healthy zones that remain.

Michael - sqlbot
  • 169,571
  • 25
  • 353
  • 427
  • I see. Thanks. So that means there will never be a case where we would need one ASG for each AZ? ASG is tied to a region right? but it resides in an AZ, so if that AZ goes down the ASG goes down as well. – Hingle McJingleberry Apr 23 '19 at 03:44
  • Depends on your needs. Auto Scaling Groups are tied to a region I believe, you can't auto scale across regions, yet. You could auto scale EC2 instances in one AZ if your requirements demand that your EC2 instances are in the same AZ for latency reasons - this would increase performance perhaps but if that AZ goes down, your site will go down as well as there's no longer any fault tolerance. – Jack Marchetti Apr 24 '19 at 02:45