Jeff’s Note #
Unlike generic exam dumps, ADH analyzes this scenario through the lens of a Real-World Site Reliability Engineer (SRE).
For SOA-C02 candidates, the confusion often lies in balancing cost reduction with workload reliability guarantees when using Spot Instances. In production, this is about knowing exactly how instance interruption behavior and duration guarantees influence job restart strategies. Let’s drill down.
The Certification Drill (Simulated Question) #
Scenario #
A mid-size biotech research company, GenexBiotech, runs large-scale batch analysis jobs on Amazon EC2. Each job executes independently and typically finishes in under 2 hours. These jobs are stateless—if a job fails or the instance shuts down, the job must be restarted from scratch. Currently, GenexBiotech is running these jobs exclusively on On-Demand EC2 instances, and they want to reduce their compute costs without compromising completeness.
The Requirement: #
Identify the most cost-effective EC2 instance purchasing option to run these batch jobs, ensuring jobs will complete within their time-frame and can restart from scratch if interrupted.
The Options #
- A) Purchase Reserved Instances to run all batch jobs.
- B) Submit a one-time Spot Instance request for each batch job.
- C) Submit a Spot Instance request with a defined duration matching job execution time.
- D) Use a combination of On-Demand and Spot Instances for the batch jobs.
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Correct Answer #
C) Submit a Spot Instance request with a defined duration matching job execution time.
Quick Insight: The SysOps Imperative #
- Spot Instances offer the biggest cost savings but are interrupted when capacity changes.
- One-time Spot requests (B) can be interrupted anytime, risking job failure midway.
- Scheduled (duration-defined) Spot Instances provide a guaranteed runtime up to the specified duration, reducing risk.
- Reserved Instances (A) provide cost savings but require commitment and are less flexible for transient workloads.
- Mixing On-Demand and Spot (D) improves availability but increases cost and complexity.
Content Locked: The Expert Analysis #
You’ve identified the answer. But do you know the implementation details that separate a Junior from a Senior?
The Expert’s Analysis #
Correct Answer #
Option C
The Winning Logic #
By submitting a Spot Instance request with a defined duration (also known as a Spot block), GenexBiotech can request instances that run for a fixed length of time without interruption—up to 1 to 6 hours. This matches well with the < 2-hour job run length, ensuring jobs won’t be interrupted mid-run. Spot blocks offer significant cost savings compared to On-Demand, while eliminating the risk of interruptions that occur with one-time Spot requests.
- Since jobs restart from scratch if interrupted, having a predictable window without interruptions is critical.
- Reserved Instances reduce hourly cost but still pay full price regardless of usage and lack flexibility.
- Mixing On-Demand and Spot adds operational complexity and cost without guaranteed uninterrupted runtime.
The Trap (Distractor Analysis): #
- Why not A? Reserved Instances require upfront commitment and don’t provide the maximum savings possible for workloads that are flexible and stateless.
- Why not B? One-time Spot requests can be reclaimed at any time, so jobs could fail before completion, leading to restarts.
- Why not D? Using mixed instance types increases complexity and cost without guaranteeing uninterrupted runtime required by the job.
The Technical Blueprint #
# Example CLI to request a Spot block for 2 hours (120 minutes)
aws ec2 request-spot-instances \
--instance-count 1 \
--type "one-time" \
--launch-specification file://launch-spec.json \
--block-duration-minutes 120
In launch-spec.json, define your instance type, AMI ID, subnet, security groups, etc.
The Comparative Analysis #
| Option | Operational Overhead | Automation Level | Impact on Cost | Risk of Job Failure |
|---|---|---|---|---|
| A) Reserved Instances | Low | Medium | Moderate cost savings | Low (stable) |
| B) One-time Spot Instances | Low | Medium | High cost savings | High (interruptions) |
| C) Spot Instances with Duration | Medium | High | Highest cost savings | Low (defined duration) |
| D) Mixed On-Demand & Spot | High | High | Moderate savings + complexity | Medium |
Real-World Application (Practitioner Insight) #
Exam Rule #
For the exam, always pick Spot Instances with defined duration when you see short, interruptible batch jobs requiring guaranteed run times.
Real World #
In reality, many teams use Spot blocks combined with automation to restart failed jobs or checkpoint state, balancing cost and resilience effectively.
(CTA) Stop Guessing, Start Mastering #
Disclaimer
This is a study note based on simulated scenarios for the SOA-C02 exam.