API for checking EC2 instance limit

Question

I use extensively EC2 instances for testing distributed system. Unfortunately, sometimes I hit the limit of running instances which cause the whole deployment to fail. I catch the appropriate exception, but I would prefer to avoid the whole deployment rather than failing after launching several instances. To avoid that kind of situation I would like to have a preflight check:

number_of_running_instances + number_of_required_instances <= instance_limit

I could easily check number_of_running_instances and number_of_required_instances, but I couldn't find an API for checking instance_limit. Moreover, I would prefer not to assume that the limit is default because some of the AWS accounts at our company applied for the higher limit.

Is there any workaround to solve that issue?

Answer 1

No, there is no way to do this. But there really isn't a need to do it because catching the exception (and gracefully handling it) is a better design. When launching an entire fleet any number of issues can arise: API calls mysteriously disappear in the void, AWS doesn't have the capacity to launch a certain instance type in the requested AZ, instances and EBS volumes can get stuck during creation, etc.

With the amount of unknown problems that can crop up when deploying AWS resources, you should plan on problems happening rather regularly. Your code needs to anticipate that and workaround it.

It's worth noting that you're using a CloudFormation template to do your provisioning, much of the pain associated with this is mitigated. Rollbacks of all resources are automatically executed on error.