# Use strategies to define Amazon ECS task placement
<a name="task-placement-strategies"></a>

For tasks that use the EC2 launch type, Amazon ECS must determine where to place the task based on the requirements specified in the task definition, such as CPU and memory. Similarly, when you scale down the task count, Amazon ECS must determine which tasks to terminate. You can apply task placement strategies and constraints to customize how Amazon ECS places and terminates tasks. 

The default task placement strategies depend on whether you run tasks manually (standalone tasks) or within a service. For tasks running as part of an Amazon ECS service, the task placement strategy is `spread` using the `attribute:ecs.availability-zone`. There isn't a default task placement constraint for tasks not in services. For more information, see [Schedule your containers on Amazon ECS](scheduling_tasks.md).

**Note**  
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. 

You can use task placement strategies and constraints together. For example, you can use a task placement strategy and a task placement constraint to distribute tasks across Availability Zones and bin pack tasks based on memory within each Availability Zone, but only for G2 instances.

When Amazon ECS places tasks, it uses the following process to select container instances:

1. Identify the container instances that satisfy the CPU, GPU, memory, and port requirements in the task definition.

1. Identify the container instances that satisfy the task placement constraints.

1. Identify the container instances that satisfy the task placement strategies.

1. Select the container instances for task placement.

You specify task placement strategies in the service definition, or task definition using the `placementStrategy` parameter.

```
"placementStrategy": [
    {
        "field": "The field to apply the placement strategy against",
        "type": "The placement strategy to use"
    }
]
```

You can specify the strategies when you run a task ([RunTask](https://docs.aws.amazon.com/AmazonECS/latest/APIReference/API_RunTask.html)), create a new service ( [CreateService](https://docs.aws.amazon.com/AmazonECS/latest/APIReference/API_CreateService.html)), or update an existing service ([UpdateService](https://docs.aws.amazon.com/AmazonECS/latest/APIReference/API_UpdateService.html)).

The following table describes the available types and fields.


| type | Valid field values | 
| --- | --- | 
| binpack Tasks are placed on container instances so as to leave the least amount of unused CPU or memory. This strategy minimizes the number of container instances in use. When this strategy is used and a scale-in action is taken, Amazon ECS terminates tasks. It does this based on the amount of resources that are left on the container instance after the task is terminated. The container instance that has the most available resources left after task termination has that task terminated. |  [\[See the AWS documentation website for more details\]](http://docs.aws.amazon.com/AmazonECS/latest/developerguide/task-placement-strategies.html)  | 
| random Tasks are placed randomly. | Not used | 
| spreadTasks are placed evenly based on the specified value. Service tasks are spread based on the tasks from that service. Standalone tasks are spread based on the tasks from the same task group. For more information about task groups, see [Group related Amazon ECS tasks](task-groups.md). When the `spread` strategy is used and a scale-in action is taken, Amazon ECS selects tasks to terminate that maintain a balance across Availability Zones. Within an Availability Zone, tasks are selected at random. |  [\[See the AWS documentation website for more details\]](http://docs.aws.amazon.com/AmazonECS/latest/developerguide/task-placement-strategies.html)  | 

The task placement strategies can be updated for existing services as well. For more information, see [How Amazon ECS places tasks on container instances](task-placement.md).

You can create a task placement strategy that uses multiple strategies by creating arrays of strategies in the order that you want them performed. For example, if you want to spread tasks across Availability Zones and then bin pack tasks based on memory within each Availability Zone, specify the Availability Zone strategy followed by the memory strategy. For example strategies, see [Example Amazon ECS task placement strategies](strategy-examples.md).

# Example Amazon ECS task placement strategies
<a name="strategy-examples"></a>

You can specify task placement strategies with the following actions: [CreateService](https://docs.aws.amazon.com/AmazonECS/latest/APIReference/API_CreateService.html), [UpdateService](https://docs.aws.amazon.com/AmazonECS/latest/APIReference/API_UpdateService.html), and [RunTask](https://docs.aws.amazon.com/AmazonECS/latest/APIReference/API_RunTask.html).

**Topics**
+ [

## Distribute tasks evenly across Availability Zones
](#even-az)
+ [

## Distribute tasks evenly across all instances
](#even-instance)
+ [

## Bin pack tasks based on memory
](#binpack)
+ [

## Place tasks randomly
](#random)
+ [

## Distribute tasks evenly across Availability Zones and then distributes tasks evenly across the instances within each Availability Zone
](#az-instance)
+ [

## Distribute tasks evenly across Availability Zones and then bin pack tasks based on memory within each Availability Zone
](#az-memory)
+ [

## Distribute tasks evenly across instances and then bin pack tasks based on memory
](#instance-memory)

## Distribute tasks evenly across Availability Zones
<a name="even-az"></a>

The following strategy distributes tasks evenly across Availability Zones.

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

## Distribute tasks evenly across all instances
<a name="even-instance"></a>

The following strategy distributes tasks evenly across all instances.

```
"placementStrategy": [
    {
        "field": "instanceId",
        "type": "spread"
    }
]
```

## Bin pack tasks based on memory
<a name="binpack"></a>

The following strategy bin packs tasks based on memory.

```
"placementStrategy": [
    {
        "field": "memory",
        "type": "binpack"
    }
]
```

## Place tasks randomly
<a name="random"></a>

The following strategy places tasks randomly.

```
"placementStrategy": [
    {
        "type": "random"
    }
]
```

## Distribute tasks evenly across Availability Zones and then distributes tasks evenly across the instances within each Availability Zone
<a name="az-instance"></a>

The following strategy distributes tasks evenly across Availability Zones and then distributes tasks evenly across the instances within each Availability Zone.

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

## Distribute tasks evenly across Availability Zones and then bin pack tasks based on memory within each Availability Zone
<a name="az-memory"></a>

The following strategy distributes tasks evenly across Availability Zones and then bin packs tasks based on memory within each Availability Zone.

```
"placementStrategy": [
    {
        "field": "attribute:ecs.availability-zone",
        "type": "spread"
    },
    {
        "field": "memory",
        "type": "binpack"
    }
]
```

## Distribute tasks evenly across instances and then bin pack tasks based on memory
<a name="instance-memory"></a>

The following strategy distributes tasks evenly across evenly across all instances and then bin packs tasks based on memory within each instance. 

```
"placementStrategy": [
    {
        "field": "instanceId",
        "type": "spread"
    },
    {
        "field": "memory",
        "type": "binpack"
    }
]
```