How to understand available EC2 instances count in specific AZ

Question

My application runs on only several m5 instance in a region. But multiple times, I have observed due to unavailability in perticular AZ , we get instance unavailable message. How does this scalability works at AWS end, for provisioning and maintaining the required load?

Adeleke Adebowale J · Accepted Answer

To add to the above :-  designing your application with multi-AZ redundancy, utilizing flexible instance types, and considering capacity reservations where possible, you can improve your chances of maintaining high availability even in peak demand periods.

re:Post Agent · Answer

Understanding the available EC2 instance count in a specific Availability Zone (AZ) can be challenging, as AWS does not publicly disclose the exact quantity of instances available in each AZ. However, there are ways to check instance type availability and manage your resources effectively.

To check the availability of specific EC2 instance types by AZ, you can use the AWS CLI command:

aws ec2 describe-instance-type-offerings --region [your-region] --location-type availability-zone --filters "Name=instance-type,Values=[instance-type]" --output text

This command will show you if a particular instance type is available in a specific AZ, but it won't provide the exact count of available instances.

The scalability and provisioning of EC2 instances at AWS work dynamically. AWS continuously monitors and adjusts capacity to meet customer demand. However, there can be temporary capacity constraints in specific AZs, which is why you might occasionally encounter "instance unavailable" messages.

To mitigate this issue and improve your application's resilience, consider the following strategies:

1. Spread your instances across multiple AZs within a region. This reduces the risk of capacity issues in a single AZ affecting your entire application.

2. Use Auto Scaling groups that span multiple AZs. This allows AWS to automatically launch instances in AZs with available capacity.

3. Be flexible with instance types. Consider using a mix of instance types or families that can meet your performance requirements. This gives you more options when capacity is constrained for a specific instance type.

4. Implement a fallback mechanism in your application to try launching instances in different AZs if the primary AZ is unavailable.

5. For critical workloads, you may want to consider using Reserved Instances or Savings Plans to ensure capacity availability.

Remember that EC2 instances are tied to the AZ in which they are launched, and each region supports a subset of available instance types. AWS sets default quotas on resources per region and monitors your usage, automatically raising quotas based on your EC2 use over time.

By implementing these strategies and understanding how AWS manages capacity, you can improve your application's resilience to temporary capacity constraints in specific AZs.  
**Sources**  
[Amazon EC2 instance types by Region - Amazon EC2](https://docs.aws.amazon.com/ec2/latest/instancetypes/ec2-instance-regions.html)  
[How check the availability of specific types of ec2 by availability zone? | AWS re:Post](https://repost.aws/questions/QUu5kVNXlTSdC-mEg7Vj3kDA/how-check-the-availability-of-specific-types-of-ec2-by-availability-zone)  
[Manage your Amazon EC2 resources - Amazon Elastic Compute Cloud](https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/resources.html)

How to understand available EC2 instances count in specific AZ

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