# Configuring pre-initialized instances for your Amazon ECS Auto Scaling group
<a name="using-warm-pool"></a>

Amazon ECS supports Amazon EC2 Auto Scaling warm pools. A warm pool is a group of pre-initialized Amazon EC2 instances ready to be placed into service. Whenever your application needs to scale out, Amazon EC2 Auto Scaling uses the pre-initialized instances from the warm pool rather than launching cold instances, allows for any final initialization process to run, and then places the instance into service.

To learn more about warm pools and how to add a warm pool to your Auto Scaling group, see [Warm pools for Amazon EC2 Auto Scaling](https://docs.aws.amazon.com/autoscaling/ec2/userguide/ec2-auto-scaling-warm-pools.html) *in the Amazon EC2 Auto Scaling User Guide*.

When you create or update a warm pool for an Auto Scaling group for Amazon ECS , you cannot set the option that returns instances to the warm pool on scale in (`ReuseOnScaleIn`). For more information, see [put-warm-pool](https://docs.aws.amazon.com/cli/latest/reference/autoscaling/put-warm-pool.html) in the *AWS Command Line Interface Reference*.

To use warm pools with your Amazon ECS cluster, set the `ECS_WARM_POOLS_CHECK` agent configuration variable to `true` in the **User data** field of your Amazon EC2 Auto Scaling group launch template. 

The following shows an example of how the agent configuration variable can be specified in the **User data** field of an Amazon EC2 launch template. Replace *MyCluster* with the name our your cluster.

```
#!/bin/bash
cat <<'EOF' >> /etc/ecs/ecs.config
ECS_CLUSTER=MyCluster
ECS_WARM_POOLS_CHECK=true
EOF
```

The `ECS_WARM_POOLS_CHECK` variable is only supported on agent versions `1.59.0` and later. For more information about the variable, see [Amazon ECS container agent configuration](ecs-agent-config.md).