ECS PlacementStrategy setup with Terraform but not working as intended

Question

Hello, Fellows!

This is my first question on re:Post. I'm glad to see you all! I'm investigating an issue where the ECS Placement strategy is not working as intended.

We've set up the ECS tasks with 2 services in the cluster.

Below is the source code for the module we've configured:

```
  dynamic "ordered_placement_strategy" {
    for_each = var.scheduling_strategy == "REPLICA" ? [1] : []
    content {
      type  = "spread"
      field = "attribute:ecs.availability-zone"
    }
  }

dynamic "ordered_placement_strategy" {
    for_each = var.scheduling_strategy == "REPLICA" ? [1] : []
    content {
      type  = "spread"
      field = "instanceId"
    }
  }

```
Even though it seems to have been set up correctly, and it's all deployed as REPLICA, it's not working as intended.

```
  "PlacementStrategy": [
    {
      "type": "spread",
      "field": "attribute:ecs.availability-zone"
    },
    {
      "type": "spread",
      "field": "instanceId"
    }
  ],

```
After the cluster was set up, it appears that the tasks from service-b are placed on one instance (e.g., in AZ-B).

If you have any ideas, please let me know.

Accepted Answer

Hey Cy, the issue is not with the Task Placement but with the Task Constraint. You want the ECS to spread the tasks on different instances so that if an EC2 dies you have spread the tasks to other EC2s etc..
You need to setup a Constraint https://docs.aws.amazon.com/AmazonECS/latest/developerguide/task-placement-constraints.html

You will need to setup
**distinctInstance**
* Place each task on a different container instance. This task placement constraint can be specified when either running a task or creating a new service.

```
"placementConstraints": [
    {
        "type": "distinctInstance"
    }
]
```

In the aws_ecs_service resource in TF, you will need

```
placement_constraints {
    type       = "distinctInstance"
  }
```

**placement_constraints**- (Optional) Rules that are taken into consideration during task placement. Updates to this configuration will take effect next task deployment unless force_new_deployment is enabled

**Task placement strategies are a best effort. Amazon ECS still attempts to place tasks even when the most optimal placement option is unavailable. However, task placement constraints are binding, and they can prevent task placement.**

ECS PlacementStrategy setup with Terraform but not working as intended

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