A clear breakdown of AWS Spot Requests vs Spot Instances, & how they work together.

 

A clear breakdown of AWS Spot Requests vs Spot Instances, and how they work together:

 What Are Spot Instances?

• Spot Instances are unused EC2 capacity offered at a discount (up to 90% cheaper than On-Demand).
• They can be terminated by AWS at any time if the capacity is needed back or your max bid is exceeded.
• Great for fault-tolerant or flexible workloads (e.g., batch jobs, CI/CD, big data, stateless microservices).

 What Is a Spot Request?

A Spot Request is how you ask AWS to give you a Spot Instance.

There are two types:

Type	Description
One-time	AWS launches the Spot Instance once. After it runs (or is terminated), it’s done.
Persistent	AWS will keep trying to launch a new instance if the existing one is terminated (e.g., due to capacity issues or manual termination).

 Relationship Between the Two

• You submit a Spot Request, and if capacity and price allow, a Spot Instance is launched.
• If you terminate the instance but leave the request active (persistent), AWS may launch another.
• To prevent that, you need to cancel the request before terminating the instance.

 Lifecycle Flow (Typical)

1. You submit a Spot Request via Console, CLI, SDK, or IaC (like Terraform).
2. AWS evaluates if there’s capacity and if your max price is acceptable.
3. If yes, AWS launches a Spot Instance.
4. If the request is persistent, AWS will monitor and attempt re-launches if needed.

In AWS, Spot Instances are tied to Spot Requests — and here's why the request must be canceled before terminating the instance:

 Spot Request Auto-Replacement

When you launch a Spot Instance via a persistent spot request, AWS assumes you want that instance type at that price continuously. So if you just terminate the instance without canceling the request:

• AWS will automatically launch a replacement instance (because the request is still active).
• That means your cost continues, and you might lose control over what's running.

 Canceling the Spot Request First

When you cancel the spot request first, you tell AWS:

• "I don't want this anymore — stop trying to fulfill this request."
• Then when you terminate the instance, AWS won't try to replace it.

 One-time vs. Persistent Spot Requests

• Persistent spot request: Keeps trying to fulfill the request even after termination.
• One-time spot request: If fulfilled and the instance is terminated, AWS won't try to replace it. But still best practice to cancel it for clarity and cost control. 

twtech Best Practice:

1. First, cancel the spot request.
2. Second, terminate the instance.
3. Always cancel persistent requests before terminating instances to avoid auto-replacement.
4.  Clean up old or failed spot requests to avoid confusion and extra billing.
5.  Use Auto Scaling Groups with Mixed Instances or EC2 Fleet for more control and resiliency.