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Difficulty: MediumImplementing Auto Scaling and Fault Tolerance

A smart agriculture company operates a real-time soil moisture and environmental monitoring telemetry platform on AWS. The telemetry ingestion service runs on Amazon EC2 instances in an Auto Scaling group (ASG) situated in private subnets across two Availability Zones. Traffic is distributed by an Application Load Balancer (ALB). The EC2 instances process the telemetry messages and require internet connectivity to post aggregated metrics to a third-party SaaS analytics API. During seasonal agricultural cycles, sensor devices wake up simultaneously, causing traffic to surge from 5050 requests per second to 25,00025,000 requests per second within 22 minutes. During these spikes, many requests fail with 502 Bad Gateway502\text{ Bad Gateway} or gateway timeout errors. Additionally, if an Availability Zone fails, outbound traffic to the SaaS API is interrupted for all instances.

Which two actions should a Solutions Architect implement to scale the architecture efficiently and ensure fault tolerance? (Select TWO.)

  1. Deploy a NAT Gateway in each Availability Zone where the Auto Scaling group operates, and configure the private subnet route tables to route outbound traffic through the local NAT Gateway in the same zone.Answer
  2. Configure an Amazon EC2 Auto Scaling warm pool with instances in the Stopped state, and request Application Load Balancer pre-warming from AWS Support prior to the seasonal cycle.Answer
  3. C
    Deploy a single NAT Gateway in one public subnet, and configure a transit gateway to route outbound internet traffic from all private subnets through this NAT Gateway to simplify management.
  4. D
    Rely on the Application Load Balancer's automatic scaling and decrease the default Auto Scaling group cooldown period to 30 seconds to trigger rapid scaling of new instances during the load surge.
  5. E
    Implement a target tracking scaling policy based on the RequestCountPerTarget metric, and depend on the Application Load Balancer's default scaling behavior to automatically handle the sudden influx of requests.

Answer

The correct actions are to deploy a NAT Gateway in each Availability Zone and configure the Auto Scaling group warm pool along with requesting ALB pre-warming.
Deploying a dedicated NAT Gateway per Availability Zone removes the single point of failure for outbound traffic. Using an Auto Scaling group warm pool keeps instances in a stopped state ready to scale out rapidly, bypassing bootstrapping delays, while Application Load Balancer pre-warming prepares the load balancer for the immediate load spike.

Step-by-Step Solution

1
Analyze the traffic profile and provisioning delays.
The traffic spikes from 5050 to 25,00025,000 requests per second in 22 minutes, while EC2 instances take 55 minutes to bootstrap.
Normal auto-scaling is too slow to react to this flash traffic pattern, necessitating a mechanism like a warm pool to bypass bootstrapping, and pre-warming of the Application Load Balancer to prevent gateway errors.
2
Assess the NAT Gateway architecture for Availability Zone fault tolerance.
A single NAT Gateway creates a single point of failure for all instances in private subnets across all zones.
To ensure fault tolerance, a NAT Gateway must be deployed in each Availability Zone where the private subnets are situated.
3
Select the combination of solutions that resolves the issues.
Combining multi-AZ NAT Gateways, an Auto Scaling group warm pool, and ALB pre-warming satisfies all scaling and reliability constraints.
This satisfies the requirements without introducing single points of failure or runaway scaling issues caused by incorrect cooldown configurations.

Key Concept

Implementing Auto Scaling and Fault Tolerance
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Implementing Auto Scaling and Fault Tolerance