Spot Instances: We Know They're Cheap, But When Should We Actually Use Them?

In theory, AWS Spot Instances are a cost-saving marvel. They promise savings of up to 90% compared to On-Demand Instances, but despite the appealing price tag, many organizations still hesitate to implement them. The real question is: when is it actually a good idea to use Spot Instances without risking disruptions or compromising performance?

Let’s explore the ins and outs of Spot Instances, uncover the best use cases, and understand when they might not be the right choice for your workloads.

What Are Spot Instances, Anyway?

Spot Instances are AWS's spare computing capacity that’s available at a steep discount. While their price can be enticing—up to 90% less than On-Demand Instances—the trade-off is that they can be terminated with little notice if AWS needs the capacity for more urgent tasks. This makes Spot Instances an excellent tool for certain workloads, but not for others.

AWS helps you assess Spot pricing with tools like the Spot Instance Advisor, allowing you to compare savings and interruption risks based on your region, instance type, and other criteria.

When Should You Use Spot Instances?

Spot Instances are ideal for workloads that don’t require constant uptime and can handle interruptions. Here are some scenarios where they shine:

• Batch Processing & Data Workloads: If your tasks involve processing large amounts of data in batch jobs, Spot Instances can significantly reduce costs. Examples include image rendering, video transcoding, and other data processing tasks that can be paused and resumed without issue.
• Continuous Integration & Testing: CI pipelines can be heavy on resource consumption but are often scheduled and non-constant. Instead of using On-Demand Instances full-time, Spot Instances allow you to scale your testing infrastructure without the hefty price tag.
• Machine Learning & High-Performance Computing: Training machine learning models or running simulations can require extensive compute power, especially for GPU instances. The cost of these instances can be prohibitive when running On-Demand, but Spot Instances offer a cost-effective alternative.
• Supplementing Existing Infrastructure: If you already have critical workloads running on Reserved or On-Demand Instances, Spot Instances can fill in the gaps or provide temporary capacity during peak times.

Where Should You Avoid Spot Instances?

Despite their savings, Spot Instances are not suitable for every workload. Here’s when they should be avoided:

• Critical Systems & Databases: For applications that require constant uptime and availability—like web servers, databases, or transactional systems—Spot Instances are too risky. The unpredictable nature of Spot Instances means that these critical workloads could experience interruptions or failure.
• Stateful Applications: Applications that rely on data persistence or those with strict recovery requirements are not ideal candidates for Spot Instances.
• Development Environments: Developers working on critical tasks may risk losing their work if Spot Instances are terminated. For environments where stability is key, it’s better to rely on On-Demand Instances or Reserved Instances.

Best Practices for Using Spot Instances

If you decide Spot Instances are the right fit for your workloads, here are some best practices to keep in mind:

1. Use Spot Fleets for Scalability: Spot Fleets allow you to manage multiple Spot Instances as a single entity, ensuring that you meet your capacity requirements while staying within budget. It helps manage the fluctuation in Spot pricing and availability.
2. Pick Stable Options: Not all Spot Instances are created equal. Choosing older instance types or those in less congested availability zones can reduce the likelihood of price spikes and interruptions.
3. Consider Hibernation: With EC2 hibernation, you can pause Spot Instances when they are interrupted and resume them when capacity becomes available. This can help mitigate disruptions for certain workloads.
4. Monitor and Adjust Regularly: Make sure to monitor your Spot Instance usage frequently. Use AWS tools to keep track of pricing trends and optimize your instance selection as needed.

Final Thoughts: When to Leverage Spot Instances

In summary, AWS Spot Instances offer great potential for saving on cloud infrastructure costs—but they’re not a one-size-fits-all solution. They are perfect for tasks that are fault-tolerant and can tolerate the occasional interruption, like batch processing, CI/CD, and machine learning training.

However, if you rely on consistent uptime or need to guarantee performance, you’ll want to stick with Reserved or On-Demand Instances. The key is to understand the unique needs of your workloads and align them with the right instance types for maximum efficiency and savings.