Why can't my ECS service register available EC2 instances with my ELB?

Question

I\'ve got an EC2 launch configuration that builds the ECS optimized AMI. I\'ve got an auto scaling group that ensures that I\'ve got at least two available instances at all

Answer 1

There where several layers of problems in our case. I will list them out so it might give you some idea of the issues to pursue.

My gaol was to have 1 ECS in 1 host. But ECS forces you to have 2 subnets under your VPC and each have 1 instance of docker host. I was trying to just have 1 docker host in 1 availability zone and could not get it to work.

Then the other issue was that the only one of the subnets had an attached internet facing gateway to it. So one of them was not accessible from public.

The end result was DNS was serving 2 IPs for my ELB. And one of the IPs would work and the other did not. So I was seeing random 404s when accessing the NLB using the public DNS.