Cannot add ECS service to attach service to load balancer

Question

I am trying to deploy 2 containers on ECS. I currently have the ECS container 1 attached to the load balancer for path "/". I want to attach container 2 to "/container2" path on the load balancer but I'm not able to do so. I have created some listeners rules for the "/container2" path in the load balancer and attached a target group to that rule. Here is where I am having the issue.

I tried to do it in the following ways:
* I created a target group and tried to register the service to it by clicking the "Register targets" button and choosing the instance option but the service name does not show up in the target group options. (Here I tried to use the IP address option but the IP address changed)
* I tried to  configure the ECS service to attach a load balancer but the service for container 2 does not have the section to assign a load balancer. I can see the load balancer section in the service for container 1.

Can someone please help me? Thanks.

Debapriyo · Accepted Answer

Could you clarify if you're referring to two containers within the same task definition or two separate ECS tasks?

When you click on "Register target," it adds a target to the ELB manually, bypassing the ECS service. If that target becomes unhealthy later, it won’t be automatically replaced due to the manual registration, so it's not advisable to do this.

You can only associate a load balancer with an ECS service during its creation. To update the service afterward, you’ll need to use the CLI command `update-service` (see the documentation here: https://awscli.amazonaws.com/v2/documentation/api/latest/reference/ecs/update-service.html).

I recommend using the CLI to update the service or creating a new service with the updated task definition (which includes both containers) and then associating the load balancer from scratch. This way, the load balancer will automatically add the necessary listener rules and targets.

Note: Using the IP address option, you might have noticed that the IP address changed each time. This is expected behavior because whenever a task is recycled, it launches with a new IP. If ECS manages the registration, it will automatically update the load balancer with the new IP address. However, since you registered the target manually, ECS had no way to make any updates to the configuration.

Gary Mclean · Answer

You are trying to register EC2 instances when clicking register on the target group. You can’t register ECS services this way.

When you create an ECS service this is the point you register the service with a target group and load balancer.

re:Post Agent · Answer

It seems you're encountering some challenges with attaching your second ECS container to the load balancer. Let's address these issues step by step:

1. For ECS services using a load balancer, you need to specify the load balancer configuration when creating or updating the service, not by manually registering targets in the target group.

2. The absence of the load balancer section in the service for container 2 could be due to how the service was initially created. You may need to update the service to include load balancer configuration.

3. When using ECS with a load balancer, you should use the 'ip' as the target type for your target group, not 'instance', especially if your tasks are using the 'awsvpc' network mode.

4. For path-based routing, you can use an Application Load Balancer (ALB) and configure multiple target groups with different path patterns.

Here's what you can do:

1. Ensure your load balancer is an Application Load Balancer for path-based routing.

2. Create a new target group for the "/container2" path with target type 'ip'.

3. Update your ECS service for container 2 to include the load balancer configuration. You'll need to specify:
- The container name (as it appears in the task definition)
- The container port
- The target group ARN for the "/container2" path

4. In the load balancer listener rules, ensure you have a rule that forwards traffic for the "/container2" path to the correct target group.

5. If you're using the AWS CLI or SDK, you can update the service with the load balancer configuration. If using the console, you might need to create a new service with the correct configuration.

Remember, you can't attach more than five target groups to a single ECS service. Also, ensure that your load balancer subnet configuration includes all Availability Zones where your container instances reside.

If you continue to face issues, double-check your IAM permissions, VPC configurations, and ensure that your ECS tasks are in the same VPC as your load balancer.  
**Sources**  
[Troubleshooting service load balancers in Amazon ECS - Amazon Elastic Container Service](https://docs.aws.amazon.com/AmazonECS/latest/developerguide/troubleshoot-service-load-balancers.html)  
[LoadBalancer - Amazon Elastic Container Service](https://docs.aws.amazon.com/AmazonECS/latest/APIReference/API_LoadBalancer.html)  
[Use a Network Load Balancer for Amazon ECS - Amazon Elastic Container Service](https://docs.aws.amazon.com/AmazonECS/latest/developerguide/nlb.html)  
[Fully Managed Container Orchestration – Amazon Elastic Container Service (Amazon ECS) Features – Amazon Web Services](https://aws.amazon.com/ecs/features/)

Cannot add ECS service to attach service to load balancer

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