ECS Task Groups

Question

I'm trying to set some anti-affinity for my tasks, and looking at the AWS docs and this nifty article, task groups is the way to go:

Trouble is, I can't seem to find my task groups. According to the docs, if my tasks were launched by the service scheduler, the task group name is the same as the service:

However, if I query for instances that match that particular task group, I'm returned with 0 instances:

```
aws ecs list-container-instances --cluster dev --filter "task:group == workflow"
{
    "containerInstanceArns": []
}
```

Even though I'm quite certain I have that service, and that there are multiple instances of that task running:

```
aws ecs list-services --cluster dev
{
    "serviceArns": [
        "arn:aws:ecs:eu-west-1:1234567890123:service/dev/workflow",
        ...
    ]
}
```

What am I missing?

Answer

Hi  
The only thing I think you're missing is that the default task group for an ECS service is in fact **service:**.  
So for your example, you would need to provide the filter as follows:

```
aws ecs list-container-instances \
      --cluster dev \
      --filter "task:group == service:workflow"
```

On top of that, I would also double check that you do indeed have tasks running in that service. Otherwise, you'll see the above give the same empty list as an output.  
  
Once you get a hang of the query languge, anti-affinity can be found using the **not** keyword as follows:

```
aws ecs list-container-instances \
      --cluster dev \
      --filter "not(task:group == service:workflow)"
```

Additionally, the task group for a single task can be overidden when running the task via the RunTask API. In the case where no task group set with RunTask, the default is **family:**.  
  
Hope this helps. Let me know if you have any further questions.

Answer

Thanks @fortejas-amazon, that was exactly it. Re-reading the documentation it makes perfect sense now.  
  
>When you launch a task using the RunTask or StartTask action, you can specify the name of the task group for the task. If you don't specify a task group for the task, the default name is the family name of the task definition (for example, family:my-task-definition).  
  
>For tasks launched by the service scheduler, the task group name is the name of the service (for example, service:my-service-name).   
  
I was confused by the examples which show this:

```
"placementConstraints": [
    {
        "expression": "task:group == databases",
        "type": "memberOf"
    }
]
```

Which I understand now, would be for explicitly named task groups.

ECS Task Groups

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