Task Statement 4.2: Design cost-optimized compute solutions.
📘AWS Certified Solutions Architect – (SAA-C03)
Overview
When you use compute services in AWS, like EC2 (Elastic Compute Cloud) or Lambda, you pay for the computing capacity you use. AWS offers multiple purchasing options to help optimize cost depending on your workload patterns. Choosing the right one is a key skill for the exam.
The main options are:
- On-Demand Instances
- Reserved Instances (RIs)
- Savings Plans
- Spot Instances
Let’s go through each in detail.
1. On-Demand Instances
What it is:
- You pay for compute capacity by the hour or second without any long-term commitment.
- You can launch or terminate instances at any time.
Key features:
- Flexible, no upfront cost.
- Ideal for short-term, unpredictable workloads.
IT-focused example:
- A development team needs a test server for a few hours to test new code. They can launch an EC2 instance, run tests, and terminate it immediately. They pay only for the exact hours used.
Pros:
- No commitment.
- Simple to use.
- Good for variable or unpredictable workloads.
Cons:
- More expensive per hour than other options.
- Not ideal for steady, long-running workloads.
2. Reserved Instances (RIs)
What it is:
- You commit to using an EC2 instance for 1 or 3 years.
- In return, you get significant discounts compared to On-Demand pricing.
Types of RIs:
- Standard Reserved Instances: Highest discount (up to 72%) but less flexible.
- Convertible Reserved Instances: Slightly lower discount (up to 54%), but you can change the instance type, OS, or tenancy during the term.
Payment options:
- All Upfront (AU): Pay the entire term at once. Maximum discount.
- Partial Upfront (PU): Pay part upfront, rest monthly. Moderate discount.
- No Upfront (NU): Pay monthly over the term. Lowest discount, still cheaper than On-Demand.
IT-focused example:
- A company runs a production web application on a consistent EC2 instance type (say, t3.medium) 24/7. Purchasing a 3-year Standard RI reduces the cost drastically compared to paying On-Demand every month.
Pros:
- Significant cost savings.
- Predictable billing for steady workloads.
Cons:
- Less flexible (especially Standard RIs).
- Commitments may be risky if workload changes.
3. Savings Plans
What it is:
- A newer, flexible way to get EC2 and other compute discounts by committing to a specific hourly spend ($/hour) for 1 or 3 years, instead of a specific instance.
Types of Savings Plans:
- Compute Savings Plans:
- Apply to any EC2 instance type, region, or OS.
- Most flexible.
- EC2 Instance Savings Plans:
- Apply to a specific instance family in a region.
- Slightly higher discount than Compute Savings Plans.
IT-focused example:
- A company estimates it will spend $5/hour on compute resources for the next 3 years. By purchasing a Compute Savings Plan, it gets discounts automatically, even if it changes instance types or regions in the future.
Pros:
- Flexible: discounts follow your usage.
- Applies across multiple instance types, regions, and OS.
- Easier to manage than RIs if workload changes.
Cons:
- Requires a commitment to spend a certain amount.
- Slightly lower discount than Standard RIs if you know exactly what you need.
4. Spot Instances
What it is:
- AWS offers unused EC2 capacity at a very low price (up to 90% off On-Demand).
- However, AWS can terminate the instance with 2 minutes’ notice if capacity is needed elsewhere.
Key features:
- Great for flexible, fault-tolerant workloads.
- You can bid for a maximum price, or just use the current Spot price.
IT-focused example:
- Running batch processing jobs or data analysis tasks that can tolerate interruption.
- Example: A company runs video rendering or big data processing on Spot Instances. If an instance is terminated, the job can resume later or continue on another instance.
Pros:
- Extremely cost-effective.
- Good for scalable, non-critical workloads.
Cons:
- Instances can be interrupted at any time.
- Not suitable for critical, always-on services.
Comparing AWS Purchasing Options
| Option | Commitment | Cost Savings | Flexibility | Best Use Case |
|---|---|---|---|---|
| On-Demand | None | None | High (launch anytime) | Short-term, unpredictable workloads |
| Reserved Instances | 1 or 3 years | Up to 72% | Low to Medium | Steady, predictable workloads |
| Savings Plans | 1 or 3 years ($/hr spend) | Up to 66% | High | Flexible long-term workloads |
| Spot Instances | None | Up to 90% | Low (can be terminated) | Batch jobs, fault-tolerant workloads |
Exam Tips for SAA-C03
- Remember Spot = cheap but interruptible → good for batch or background jobs.
- RIs = predictable discount, good for steady workloads.
- Savings Plans = flexible discount across instance types → more modern than RIs.
- On-Demand = pay for what you use, no commitment → short-term or unpredictable workloads.
- Questions often test matching workloads to the correct purchasing option.
✅ Key Takeaways:
- Use On-Demand for flexibility.
- Use Reserved Instances for long-term, predictable workloads.
- Use Savings Plans for flexible long-term commitments.
- Use Spot Instances for interruptible, cost-sensitive workloads.
