What is Capacity Rebalancing in AWS?

Capacity Rebalancing automatically shifts your workloads to healthier Spot Instances before the ones you’re using get interrupted. This gives your applications a better chance to avoid downtime and keep working smoothly.

If you’ve ever worried about Spot Instances disappearing at the worst possible moment, Capacity Rebalancing is AWS’s way of giving you an early heads-up—and a tool to act fast.

Purpose of Capacity Rebalancing

Spot Instances are one of the best ways to cut down your EC2 costs—often by 70% to 90% compared to On-Demand pricing. But here’s the trade-off:

Spot Instances can be interrupted when AWS needs the capacity back.

If that happens, AWS gives you a two-minute warning before the instance shuts down. In production, two minutes is barely enough time to say, “Oh no,” let alone take action.

This is exactly why Capacity Rebalancing exists. It helps you:

• Proactively replace at-risk Spot Instances.
• Keep applications running during interruptions.
• Save money on Spot while reducing risk.

How Does It Work?

Here’s a real-world breakdown:

1. AWS Monitors Capacity

AWS is constantly checking the availability of the Spot capacity you’re using. If it sees that your instance’s capacity is getting tight (and likely to be interrupted soon), AWS triggers a rebalance recommendation.

2. You Get a Rebalance Warning

Your Auto Scaling Group or EC2 Fleet receives this early signal, usually several minutes before the official two-minute interruption notice.

3. Instances Are Replaced

Capacity Rebalancing kicks in, launching new Spot Instances in healthier pools. As soon as they’re ready, your Auto Scaling Group can shift workloads over and safely terminate the at-risk instances.

This makes the transition much smoother, without panicking over a sudden interruption.

Why Is It a Game Changer When Using Spot Instances?

From experience managing large-scale Spot workloads, I can tell you that the biggest headache with Spot Instances is the unpredictable interruptions.

Before Capacity Rebalancing, it was common to lose instances out of nowhere, scramble to replace them, and cross your fingers hoping no customer noticed the slowdown.

Now, with Capacity Rebalancing in place, you get:

• More time to react
• Better workload stability
• Lower chances of downtime

For anyone running production workloads on Spot (yes, it’s possible with the right setup!), Capacity Rebalancing is a game-changer.

How to Enable Capacity Rebalancing

Good news: it’s incredibly easy to turn on.

If you’re using Auto Scaling Groups (ASG) or EC2 Fleet, just add this setting:

jsonCopyEdit"CapacityRebalance": true

For example, here’s how you enable it in an Auto Scaling Group using the AWS CLI:

aws autoscaling update-auto-scaling-group \
  --auto-scaling-group-name my-asg \
  --capacity-rebalance
Copy Code

And that’s it. Once enabled, AWS handles the rest.

What’s the Difference Between Rebalance Recommendations and Interruption Notices?

Feature	Rebalance Recommendation	Interruption Notice
Timing	Several minutes ahead	2 minutes before termination
Purpose	Early warning to replace	Last call to shut down safely
How to use it	Trigger new instances early	Save work, sync data

In short:

• Rebalance Recommendations = Early action
• Interruption Notices = Emergency response

When Should You Use Capacity Rebalancing?

Capacity Rebalancing is essential if you:

• Run critical workloads on Spot Instances.
• Need high uptime and low risk of interruption.
• Want to save money with Spot but hate surprises.
• Use Auto Scaling Groups or EC2 Fleet.

If you’re using Spot Instances for things like web servers, batch jobs, or microservices, Capacity Rebalancing helps you get the best of both worlds: big savings with lower disruption.

Key Takeaways

• Capacity Rebalancing helps you avoid Spot Instance interruptions by moving workloads to healthier capacity pools.
• It works by detecting risk before a shutdown happens and replacing instances automatically.
• It’s easy to enable and works with Auto Scaling Groups and EC2 Fleet.
• For production workloads on Spot, it’s a must-have.

See Also

• AWS Docs
• Managing Spot Instance Interruptions
• Best Practices for EC2 Auto Scaling

FAQ