DOP-C02 Resilient Cloud Solutions Practice Question
A company is deploying a critical microservice on Amazon ECS with Fargate. They need to ensure that the service can tolerate an Availability Zone failure. What is the BEST approach?
⚠ Common exam trap
Watch out — candidates often confuse 'spreading tasks across instances' (Option A) or 'across instance types' (Option B) with the correct concept of spreading across Availability Zones, or they mistakenly think auto scaling (Option D) alone provides AZ failure tolerance, when in fact it only scales based on load and does not guarantee multi-AZ distribution.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Define the service to spread tasks across multiple Availability Zones
Amazon ECS with Fargate allows you to define a service with a 'spread across Availability Zones' strategy. By setting the service's placement strategy to spread tasks across multiple Availability Zones, the service automatically distributes tasks evenly across the specified AZs. If one AZ fails, the remaining tasks in other AZs continue to serve traffic, ensuring high availability. This is the most direct and effective method for tolerating an AZ failure in a Fargate-based deployment.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Use a cluster placement constraint to spread tasks across instances
Why it's wrong here
Cluster placement constraints are only applicable to the EC2 launch type, where you can control task placement relative to container instances. On AWS Fargate, there are no underlying EC2 instances to constrain, so this option is invalid. Even for EC2, a placement constraint such as 'distinctInstance' spreads tasks across instances but does not guarantee distribution across Availability Zones, which is the key requirement for surviving an AZ failure.
- ✗
Use EC2 launch type and spread tasks across instance types
Why it's wrong here
Choosing the EC2 launch type and spreading tasks across instance types improves resilience to instance-level failures, but it does not address Availability Zone failures. All instances, regardless of type, reside within specific AZs; if an entire AZ goes down, tasks on those instances fail. Moreover, the question context implies a serverless approach with Fargate, where you do not manage instances at all, making this option irrelevant.
- ✓
Define the service to spread tasks across multiple Availability Zones
Why this is correct
Defining the service to spread tasks across multiple Availability Zones ensures that the microservice remains available even if a single AZ fails. In Amazon ECS, you can use a placement strategy with 'spread' across 'attribute:ecs.availability-zone' for EC2, or for Fargate you can use the `AvailabilityZoneSpread` placement strategy (or specify AZs in the service definition) to distribute tasks evenly across AZs. This directly mitigates the risk of an AZ outage by maintaining running tasks in other zones.
- ✗
Configure service auto scaling to add tasks when CPU is high
Why it's wrong here
Auto scaling based on CPU utilization addresses changes in demand by adding or removing tasks, but it does not prepare for a sudden AZ failure. If an AZ fails, existing tasks in that AZ are lost; auto scaling might eventually add tasks, but it will only place them in AZs that have capacity and may not restore the desired AZ spread. The primary objective is high availability, not dynamic scaling, so this option is insufficient.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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