Spot Instances for cost savings in AWS - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding Spot Instances behavior

What happens to an AWS Spot Instance when the spot price exceeds your bid price?

A AWS sends a two-minute warning before terminating the instance.

B The instance is terminated immediately without warning.

C The instance is automatically converted to an On-Demand instance.

D The instance continues running until you manually stop it.

💡 Hint

Think about how AWS manages Spot Instances to allow graceful shutdown.

AWS Spot Instances receive a two-minute warning before termination when the spot price exceeds the bid or capacity is no longer available. This allows applications to save state or gracefully shut down.

❓ Architecture

intermediate

Designing fault-tolerant architecture with Spot Instances

You want to use Spot Instances to save costs but ensure your application remains highly available. Which architectural pattern best supports this goal?

A Deploy Spot Instances in a single Availability Zone to reduce complexity.

B Use only Spot Instances behind a load balancer with no fallback.

C Use Spot Instances with a fixed bid price and no scaling policies.

D Mix Spot Instances with On-Demand Instances in an Auto Scaling group.

💡 Hint

Consider how to maintain availability when Spot Instances are interrupted.

Mixing Spot Instances with On-Demand Instances in an Auto Scaling group provides cost savings while maintaining availability. On-Demand Instances serve as fallback when Spot Instances are interrupted.

❓ Configuration

advanced

Spot Instance interruption handling configuration

Which AWS service or feature allows you to automatically handle Spot Instance interruptions by launching replacement instances?

A AWS Lambda with CloudWatch Events monitoring Spot interruptions.

B AWS Elastic Beanstalk configured with Spot Instances only.

C EC2 Auto Scaling group with mixed instances policy including Spot Instances.

D AWS Systems Manager Automation documents for manual instance replacement.

💡 Hint

Think about automated scaling and instance replacement features.

EC2 Auto Scaling groups with mixed instances policies can automatically replace interrupted Spot Instances by launching new ones, maintaining desired capacity.

❓ security

advanced

Security considerations for Spot Instances

What is a key security best practice when using Spot Instances in your AWS environment?

A Assign broad IAM roles to Spot Instances to simplify access management.

B Use dedicated security groups and restrict network access to Spot Instances.

C Disable encryption on Spot Instances to improve performance.

D Store sensitive data only on Spot Instance local storage for faster access.

💡 Hint

Consider how to protect instances that may be interrupted or replaced frequently.

Using dedicated security groups with restricted network access limits exposure and protects Spot Instances, which can be interrupted and replaced frequently.

❓ service_behavior

expert

Spot Instance interruption notice timing

How long before AWS terminates a Spot Instance does it provide an interruption notice, and how can you detect it programmatically?

A Two minutes; by querying the instance metadata service for the interruption notice URL.

B Ten minutes; by subscribing to SNS notifications configured on the instance.

C Instant termination; no programmatic detection is possible.

D Five minutes; by polling the EC2 metadata endpoint for interruption time.

💡 Hint

Recall the exact timing and how AWS exposes interruption notices to instances.

AWS provides a two-minute interruption notice accessible via the instance metadata service at a specific URL. Applications can poll this URL to detect imminent termination.

Practice

1. What is the main benefit of using Spot Instances in AWS?

easy

A. They offer unlimited storage space for your data.

B. They guarantee 24/7 uptime without interruptions.

C. They automatically scale your application based on traffic.

D. They provide significant cost savings by using spare capacity.

Solution

1. Step 1: Understand Spot Instances purpose

   Spot Instances use spare AWS capacity and are cheaper but can be interrupted.

2. Step 2: Compare options

   Only They provide significant cost savings by using spare capacity. mentions cost savings from spare capacity, others describe unrelated features.

3. Final Answer:

   They provide significant cost savings by using spare capacity. -> Option D

4. Quick Check:

   Spot Instances = Cost savings [OK]

Hint: Spot Instances save money by using spare capacity [OK]

Common Mistakes:

• Thinking Spot Instances guarantee uptime
• Confusing Spot Instances with auto-scaling
• Assuming Spot Instances provide extra storage

2. Which of the following is the correct way to request a Spot Instance using AWS CLI?

easy

A. aws ec2 request-spot-instances --spot-price 0.05 --instance-count 1 --type t3.micro

B. aws ec2 create-spot-instance --price 0.05 --count 1 --instance-type t3.micro

C. aws ec2 spot-instance-request --max-price 0.05 --count 1 --instance t3.micro

D. aws ec2 launch-spot-instance --price 0.05 --number 1 --type t3.micro

Solution

1. Step 1: Identify correct AWS CLI command

   The correct command is aws ec2 request-spot-instances with parameters like --spot-price, --instance-count, and --type.

2. Step 2: Check options for syntax correctness

   Only aws ec2 request-spot-instances --spot-price 0.05 --instance-count 1 --type t3.micro uses the correct command and parameter names as per AWS CLI documentation.

3. Final Answer:

   aws ec2 request-spot-instances --spot-price 0.05 --instance-count 1 --type t3.micro -> Option A

4. Quick Check:

   Correct CLI syntax = aws ec2 request-spot-instances --spot-price 0.05 --instance-count 1 --type t3.micro [OK]

Hint: Use 'aws ec2 request-spot-instances' with correct flags [OK]

Common Mistakes:

• Using wrong command names like create-spot-instance
• Incorrect parameter names like --price instead of --spot-price
• Mixing instance count and type flags

3. Consider this AWS CLI command output snippet after requesting Spot Instances:

{
  "SpotInstanceRequests": [
    {
      "SpotInstanceRequestId": "sir-12345",
      "State": "active",
      "Status": {"Code": "fulfilled", "Message": "Your Spot request is fulfilled."},
      "InstanceId": "i-67890"
    }
  ]
}

What does this output indicate?

medium

A. The Spot Instance request is active and the instance is running.

B. The Spot Instance request was denied due to price limits.

C. The Spot Instance request is pending and waiting for capacity.

D. The Spot Instance request was cancelled by the user.

Solution

1. Step 1: Analyze the 'State' and 'Status' fields

   The state is "active" and status code is "fulfilled", meaning the request succeeded and instance launched.

2. Step 2: Interpret the meaning of 'InstanceId'

   The presence of an InstanceId means the Spot Instance is running.

3. Final Answer:

   The Spot Instance request is active and the instance is running. -> Option A

4. Quick Check:

   State active + fulfilled = running instance [OK]

Hint: Active state and fulfilled status mean instance is running [OK]

Common Mistakes:

• Confusing 'active' with 'pending'
• Ignoring the 'fulfilled' status message
• Assuming InstanceId is missing means failure

4. You wrote this JSON snippet to request a Spot Instance but it fails:

{
  "InstanceCount": 1,
  "Type": "one-time",
  "LaunchSpecification": {
    "InstanceType": "t3.micro",
    "ImageId": "ami-0abcdef1234567890"
  }
}

What is the likely error causing the failure?

medium

A. Wrong Type value; should be "persistent" only.

B. Missing the SpotPrice field to set max price.

C. Incorrect InstanceCount value; must be zero.

D. Invalid ImageId format; must be numeric only.

Solution

1. Step 1: Check required fields for Spot Instance request

   Spot requests require a SpotPrice to specify max price willing to pay.

2. Step 2: Validate other fields

   InstanceCount of 1 is valid, Type can be "one-time", and ImageId format is correct.

3. Final Answer:

   Missing the SpotPrice field to set max price. -> Option B

4. Quick Check:

   SpotPrice required for request [OK]

Hint: SpotPrice is mandatory in Spot Instance requests [OK]

Common Mistakes:

• Omitting SpotPrice field
• Using invalid Type values
• Misunderstanding ImageId format

5. You want to run a batch job on AWS using Spot Instances to save costs. The job can be interrupted and resumed later. Which strategy best balances cost savings and job completion?

hard

A. Use Reserved Instances for guaranteed availability.

B. Request On-Demand Instances only to avoid interruptions.

C. Request Spot Instances with a low max price and use checkpointing to save progress.

D. Request Spot Instances with the highest max price to avoid interruptions.

Solution

1. Step 1: Understand workload tolerance

   The batch job can handle interruptions and resume, so Spot Instances are suitable.

2. Step 2: Choose cost-effective and reliable approach

   Setting a low max price saves money but risks interruptions; checkpointing allows resuming work safely.

3. Step 3: Evaluate other options

   On-Demand and Reserved Instances cost more and don't leverage Spot savings; highest max price reduces savings.

4. Final Answer:

   Request Spot Instances with a low max price and use checkpointing to save progress. -> Option C

5. Quick Check:

   Interruptible job + low price + checkpointing = best cost savings [OK]

Hint: Use low max price + checkpointing for interruptible jobs [OK]

Common Mistakes:

• Choosing On-Demand or Reserved ignoring cost savings
• Setting max price too high losing savings
• Not implementing checkpointing for interruptions