AWS ECS 503 Service Temporarily Unavailable error - amazon-ecs

Question

I'm following a tutorial How to Deploy a Dockerised Application on AWS ECS With Terraform and running into a 503 error trying to hit my App.
The App runs fine in a local Container (http://localhost:3000/contacts), but is unreachable via ECS deployment. When I check the AWS Console, I see health checks are failing, so there's no successful deployment of the App.
I've read through / watched a number of tutorials, and they all have the same configuration as in the tutorial mentioned above. I'm thinking something must have changed on the AWS side, but I can't figure it out.
I've also read a number of 503-related posts here, and tried various things such as opening different ports, and setting SG ingress wide open, but to no avail.
If anyone is interested in troubleshooting, and has a chance, here's a link to my code: https://github.com/CumulusCycles/tf-cloud-demo
Thanks for any insights anyone may have on this!
Cheers,
Rob

Answer 1

Your target group is configured to forward traffic to port 80 on the container. Your container is listening on port 3000. You need to modify your target group to forward traffic to the port your container is actually listening on:
resource "aws_lb_target_group" "target_group" {
name = "target-group"
port = 3000
protocol = "HTTP"
target_type = "ip"
vpc_id = "${aws_default_vpc.default_vpc.id}" # Referencing the default VPC
}
Your load balancer port is the only port external users will see. Your load balancer is listening on port 80 so people can hit it over HTTP without specifying a port. When the load balancer receives traffic on that port it forwards it to the target group. The target group receives traffic and then forwards it to an instance in the target group, on the configured port.
It does seem a bit redundant, but you need to specify the port(s) that your container listens on in the ECS task definition, and then configure that same port again in both the target group configuration, and the ECS service's load balancer configuration. You may even need to configure it again in the target group's health check configuration if the default health checks don't work for your application.
Note: If you look at the comments on that blog post you linked, you'll see several people saying the same thing about the target group port mapping.