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Home Forums AWS AWS Certified Solutions Architect Associate Understanding ASG behaviour

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  • Understanding ASG behaviour

  • Rumman

    Member
    December 12, 2020 at 6:59 am

    For the question

    A company needs to deploy at least 2 EC2 instances to support the normal workloads of

    its application and automatically scale up to 6 EC2 instances to handle the peak load.

    The architecture must be highly available and fault-tolerant as it is processing mission-critical

    workloads.

    As the Solutions Architect of the company, what should you do to meet the above

    requirement?

    The option that says: Create an Auto Scaling group of EC2 instances and set the

    minimum capacity to 2 and the maximum capacity to 6. Use 2 Availability Zones and

    deploy 1 instance for each AZ is incorrect because if an AZ outage occurred, there will

    only be 1 running instance left. The scenario requires you to run at least 2 EC2 instances

    to support the workload of their application.

    But I am wondering if one of the AZ is gone why can’t ASG not been able to spin up two instances in the other AZ as we set a minimum cap to 2?

  • Gerome-TutorialsDojo

    Member
    December 14, 2020 at 7:54 pm

    Hello Rumman,

    Thanks for the feedback.

    But I am wondering if one of the AZ is gone why can’t ASG not been able to spin up two instances in the other AZ as we set a minimum cap to 2?

    — There is a statement in the scenario “processing mission-critical workloads” this means that there shouldn’t be a downtime. If you would recreate this option in the AWS Management Console, it takes a few minutes before ASG can spin up another instance in the remaining AZ. Therefore, if you need to run at least 2 EC2 instances to process mission-critical workloads, you must deploy 2 instances in each AZ. If there is an AZ outage, you would still have 2 instances running.

    Let us know if you need further assistance. The Tutorials Dojo team is dedicated to helping you pass your AWS exam on your first try!

    Regards,

    Gerome @ Tutorials Dojo

  • anthill5060

    Member
    January 23, 2025 at 3:31 am

    Hello, if this thread can be re-lived 🙂
    I just stumbled on the same question. The explanation says that the option with 2-6 instances in 2 AZs “is incorrect. It is required to have 2 instances running all the time. If an AZ outage happened, ASG will launch a new instance on the unaffected AZ. This provisioning does not happen instantly, which means that for a certain period of time, there will only be 1 running instance left.”;
    I don’t fully agree with such an explanation. In such logic, we can imagine that in times of peak load an AZ failure also can happen, leading to only 3 running instances left, which is also insufficient.
    I believe that AWS advocates right-sizing and scaling dynamically, not pre-provisioning excess capacity as in the suggested answer. Manually overprovisioning does not solve this either. A 3-instance loss due to AZ failure would still result in insufficient capacity until ASG compensates.

    While I understand that the question might anticipate a solution with 0 downtime in case of the AZ failure, it would make more sense to provision a minimum of 3 instances in 3 AZs, which will be at least more cost-effective.

    • JR-TutorialsDojo

      Administrator
      January 27, 2025 at 10:46 am

      Hello anthill5060,

      Thank you for sharing your insights on this topic.

      Your point about right-sizing and dynamic scaling is crucial, and it’s always good practice to consider these aspects when designing architectures. However, given the constraints of this specific question, we had to choose the best option among the provided ones.

      Since the scenario requires a minimum of two instances to manage regular traffic, you should always have at least two instances running, even in the event of an Availability Zone (AZ) outage. To achieve this, you can use an Auto Scaling Group to automatically distribute your compute resources across two or more Availability Zones. It is important to set the minimum capacity to four instances and the maximum capacity to six instances. With two instances running in each AZ, your system will maintain at least two operational instances, even if one AZ fails.

      The given question focuses on the need for zero downtime and strong fault tolerance. In this context, configuring the Auto Scaling Group (ASG) with at least four instances across two Availability Zones (AZs) more effectively meets these requirements than other options.

      Let us know if you need further assistance.

      Regards,
      JR @ Tutorials Dojo

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