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Difficulty: MediumImplementing Auto Scaling and Fault Tolerance

A logistics company hosts a fleet telemetry tracking application on Amazon EC2 instances in an Auto Scaling group (ASG) behind an Application Load Balancer (ALB). The instances reside in private subnets across two Availability Zones, Zone A and Zone B. The instances must download routing updates daily from an external server over the internet. Currently, a single NAT Gateway is deployed in Zone A to provide outbound internet access.

The company has identified two primary failure patterns:
1. Outbound internet connectivity fails for all instances in both zones if Zone A experiences an outage.
2. At the start of daily delivery shifts at 8:00 AM, a sudden, massive surge in telemetry connections causes the ALB to drop requests and return HTTP 503 (Service Unavailable) errors, even though dynamic CPU-based auto-scaling is configured.

Which combination of actions will resolve these issues? (Select TWO.)

  1. Deploy a NAT Gateway in a public subnet in each Availability Zone, and update the route table of each private subnet to route outbound traffic through the local NAT Gateway in its respective zone.Answer
  2. Create a Scheduled Scaling policy to scale out the Auto Scaling group before 8:00 AM, and request AWS Support to pre-warm the Application Load Balancer to handle the anticipated connection surge.Answer
  3. C
    Reduce the Auto Scaling group cooldown period to 30 seconds to allow the tracking instances to launch and bootstrap more rapidly during the traffic surge.
  4. D
    Deploy a single NAT Gateway in a public subnet in Zone B, and configure the route tables of both private subnets to use this new NAT Gateway as a secondary failover route for internet traffic.
  5. E
    Configure dynamic scaling policies with a CPU utilization target threshold of 90% to trigger rapid auto-scaling of both the Application Load Balancer and the Auto Scaling group when the traffic surge occurs.

Answer

Deploy a NAT Gateway in each Availability Zone, update private subnet route tables to use local NAT Gateways, implement scheduled scaling before 8:00 AM, and request ALB pre-warming from AWS Support.
Deploying a NAT Gateway in each Availability Zone ensures high availability and eliminates the single point of failure, as outbound traffic stays local to the zone. For sudden, predictable spikes in traffic (like the start of shifts at 8:00 AM), relying on dynamic scaling is insufficient because Application Load Balancers (ALBs) and Auto Scaling groups take time to scale out. Pre-warming the ALB ensures it has the capacity immediately, and scheduled scaling proactively launches instances before the spike occurs.

Step-by-Step Solution

1
Analyze the outbound internet connectivity path to eliminate the single point of failure.
Discovered that a single NAT Gateway in Zone A creates a dependency for Zone B. A failure in Zone A disrupts all internet access.
To build a fault-tolerant architecture, outbound NAT resources must be deployed in each Availability Zone so that failures are isolated within the zone.
2
Address the Application Load Balancer capacity issues during sudden spikes.
Determined that the sudden traffic surge at 8:00 AM requires ALB pre-warming since standard scaling cannot react quickly enough to flash traffic.
ALB requires pre-warming by AWS Support to handle rapid, massive increases in connection rates.
3
Address the EC2 instance capacity scaling delays.
Identified that scheduled scaling is required to scale the ASG before the predictable 8:00 AM shift starts.
Scheduled scaling ensures capacity is online and bootstrapped before the traffic arrives, preventing initial connection drops.

Key Concept

Fault-tolerant network architecture and scaling strategies for predictable flash traffic.
Estimated Time:3m 0s
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Topics

Implementing Auto Scaling and Fault Tolerance