EC2 Reserved Instances let you commit to using specific instance types for 1 or 3 years in exchange for significant discounts (up to 72% off), while Spot Instances let you bid on unused EC2 capacity at up to 90% off, but AWS can terminate them with just a 2-minute warning when they need the capacity back.
Key Takeaways
Reserved Instances require a 1 or 3-year commitment and guarantee capacity, making them ideal for steady-state workloads like databases and web servers. Spot Instances offer deeper discounts but can be interrupted anytime, making them suitable for fault-tolerant workloads like batch processing, data analysis, and CI/CD jobs. Reserved Instances provide cost predictability, while Spot Instances require architectural resilience to handle sudden terminations.
Reserved Instances: Commit and Save
When you purchase a Reserved Instance, you’re making a capacity reservation with AWS. You commit to a specific instance type, operating system, and region for either one or three years. In return, AWS gives you a discount compared to On-Demand pricing.
You have three payment options: All Upfront (pay everything now for maximum discount), Partial Upfront (pay some now, the rest monthly), or No Upfront (pay monthly with a smaller discount). The All Upfront option gives you the best rates, but ties up your capital.
Gotcha: Reserved Instances aren’t actual instances. They’re billing discounts that apply to matching On-Demand instances you launch. I’ve seen teams buy Reserved Instances thinking they’re pre-provisioned servers sitting ready to use. They’re not. You still launch instances normally, and the discount applies automatically.
Standard Reserved Instances lock you into a specific instance family and size. Convertible Reserved Instances cost slightly more but let you change instance families, operating systems, or tenancy during the term. This flexibility matters when your workload requirements evolve.
Use Reserved Instances for workloads that run continuously: production databases, domain controllers, monitoring systems, or always-on application servers. If you know you’ll need specific capacity for the next year or three, Reserved Instances make financial sense.
Spot Instances: High Risk, High Reward
Spot Instances use AWS’s unused capacity. You specify the maximum price you’re willing to pay, and as long as the Spot price stays below your maximum, your instances run. When demand increases and the Spot price exceeds your maximum (or AWS needs the capacity), you get a 2-minute warning before termination.
That 2-minute warning is critical. Your application needs to handle it gracefully. AWS sends a termination notice through instance metadata and CloudWatch Events. You should checkpoint your work, save state, or gracefully shut down within those 120 seconds.
Warning: Spot Instances can terminate in the middle of processing. I learned this the hard way running a video encoding job that lost 6 hours of progress because I didn’t implement checkpointing. Always design for interruption.
Spot pricing varies by instance type and availability zone. You can see historical pricing in the EC2 console. Some instance types in certain zones almost never get interrupted, while others fluctuate wildly. Check the interruption frequency rating AWS provides for each instance type.
Spot Instances work brilliantly for stateless, fault-tolerant workloads: batch processing, big data analytics, containerized applications with auto-scaling, CI/CD pipelines, web crawlers, and rendering farms. Anything that can checkpoint progress or restart without data loss is a good candidate.
You can also use Spot Fleets, which request multiple instance types across multiple availability zones. This diversification reduces your interruption risk. If one instance type becomes expensive or unavailable, the fleet automatically launches different types to maintain your target capacity.
Gotcha: Spot Instances don’t work with all AWS services. You can’t use them with RDS, for example. But you can use them with ECS, EKS, EMR, and Batch. Always check service compatibility before planning your architecture.
When to Use Each
Choose Reserved Instances when you need guaranteed capacity and can forecast demand. Your production database that runs 24/7? Reserved Instance. Your application servers that need to be available for customers? Reserved Instances for the baseline capacity.
Choose Spot Instances when you can tolerate interruptions and your workload is flexible. Your nightly ETL job? Spot Instances. Your Jenkins build agents? Spot Instances. Your Kubernetes worker nodes that can drain gracefully? Spot Instances.
Many teams use both. They run baseline capacity on Reserved Instances and handle burst traffic or batch processing with Spot Instances. This hybrid approach balances cost savings with reliability.
One more consideration: commitment level. Reserved Instances require planning and lock you in. If your business is unpredictable or you’re in a growth phase where requirements change rapidly, that commitment becomes risky. Spot Instances give you massive savings without long-term obligations, but require engineering effort to handle interruptions properly.
Conclusion
Reserved Instances trade commitment for cost savings and capacity guarantees, making them perfect for predictable, always-on workloads. Spot Instances offer deeper discounts by using spare capacity, but can be interrupted with minimal notice, requiring your applications to be fault-tolerant. The best cost optimization strategy often combines both: use Reserved Instances for your baseline capacity and Spot Instances for flexible, interruptible workloads. Choose based on your workload characteristics, risk tolerance, and ability to engineer for interruption handling.