DOP-C02 Incident and Event Response Practice Question
A company runs a critical application on EC2 instances in an Auto Scaling group. The application must be highly available across multiple Availability Zones. Which TWO configurations are necessary to achieve this? (Choose TWO.)
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
✓
Place an Application Load Balancer in front of the Auto Scaling group.
To achieve high availability across multiple Availability Zones (AZs), two key configurations are required. First, the Auto Scaling group must be configured to launch instances in multiple AZs (Option E). This ensures that if one AZ fails, the application can continue serving traffic from instances in other AZs. Second, an Application Load Balancer (ALB) should be placed in front of the Auto Scaling group (Option D). The ALB distributes incoming traffic across instances in all AZs where the Auto Scaling group has launched instances. It also performs health checks and automatically routes traffic away from unhealthy instances. Option A is incorrect because using a single AZ would create a single point of failure, undermining high availability. Option B is incorrect because Spot Instances can be terminated with little notice, which is unsuitable for a critical application requiring consistent availability. Option C is incorrect because a Classic Load Balancer lacks advanced features like path-based routing and native cross-zone load balancing, making the ALB a better choice for modern applications.
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 single Availability Zone to reduce latency.
Why it's wrong here
Placing all instances in a single Availability Zone creates a single point of failure; if that AZ experiences an outage, the entire application becomes unavailable. Latency reduction is minor and does not justify the availability risk, and AWS recommends spreading workloads across multiple AZs to achieve high availability.
- ✗
Use Spot Instances to reduce costs.
Why it's wrong here
Spot Instances offer significant cost savings but are interruptible with a 2-minute warning when EC2 reclaims capacity. For a critical application that requires high availability, the risk of termination due to Spot price fluctuations or capacity constraints makes them unsuitable as the primary compute foundation. They can be used for fault-tolerant or stateless workloads, but this application's critical nature demands on-demand or reserved capacity.
- ✗
Use a Classic Load Balancer to distribute traffic.
Why it's wrong here
Classic Load Balancer is a legacy load balancer that operates at both the request and connection level, but it lacks native support for advanced routing, host/path-based rules, and target groups that modern ALB/NLB provide. It also requires more manual configuration for cross-zone load balancing and health checks, and does not integrate as seamlessly with Auto Scaling groups and dynamic scaling policies. While it can distribute traffic, it is not the best practice for a modern, highly available architecture.
- ✓
Place an Application Load Balancer in front of the Auto Scaling group.
Why this is correct
An Application Load Balancer operates at Layer 7 and can distribute HTTP/HTTPS traffic across instances in multiple Availability Zones, providing health checks and automatic registration of instances in the Auto Scaling group. This integration allows the ALB to route traffic only to healthy instances and to scale with the ASG's changes, enhancing availability and fault tolerance. It is the recommended pattern for web applications requiring high availability and advanced routing.
- ✓
Configure the Auto Scaling group to launch instances in multiple Availability Zones.
Why this is correct
Configuring the Auto Scaling group to launch instances across multiple Availability Zones ensures that if one AZ fails or experiences degradation, the remaining instances can continue serving traffic, and the ASG can maintain desired capacity. This design eliminates the single point of failure associated with a single AZ and is a fundamental aspect of building highly available applications. Combined with a load balancer, it provides resilience and automatic recovery.
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Written by Johnson Ajibi, MSc IT Security
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