# Service role for automatic scaling in Amazon EMR (Auto Scaling role)
<a name="emr-iam-role-automatic-scaling"></a>

The Auto Scaling role for Amazon EMR performs a similar function as the service role, but allows additional actions for dynamically scaling environments.
+ The default role name is `EMR_AutoScaling_DefaultRole`.
+ The default managed policy attached to `EMR_AutoScaling_DefaultRole` is `AmazonElasticMapReduceforAutoScalingRole`.

The contents of version 1 of `AmazonElasticMapReduceforAutoScalingRole` are shown below.

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#### [ JSON ]

****  

```
{
  "Version":"2012-10-17",		 	 	 
  "Statement": [
    {
      "Action": [
        "cloudwatch:DescribeAlarms",
        "elasticmapreduce:ListInstanceGroups",
        "elasticmapreduce:ModifyInstanceGroups"
      ],
      "Effect": "Allow",
      "Resource": [
        "*"
      ],
      "Sid": "AllowCLOUDWATCHDescribealarms"
    }
  ]
}
```

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Your service role should use the following trust policy.

**Important**  
The following trust policy includes the [https://docs.aws.amazon.com/IAM/latest/UserGuide/reference_policies_condition-keys.html#condition-keys-sourcearn](https://docs.aws.amazon.com/IAM/latest/UserGuide/reference_policies_condition-keys.html#condition-keys-sourcearn) and [https://docs.aws.amazon.com/IAM/latest/UserGuide/reference_policies_condition-keys.html#condition-keys-sourceaccount](https://docs.aws.amazon.com/IAM/latest/UserGuide/reference_policies_condition-keys.html#condition-keys-sourceaccount) global condition keys, which limit the permissions that you give Amazon EMR to particular resources in your account. Using them can protect you against [the confused deputy problem](https://docs.aws.amazon.com/IAM/latest/UserGuide/confused-deputy.html).

------
#### [ JSON ]

****  

```
{
  "Version":"2012-10-17",		 	 	 
  "Statement": [
    {
      "Effect": "Allow",
      "Action": [
        "sts:AssumeRole"
      ],
      "Resource": "arn:aws:iam::123456789012:role/ApplicationAutoScalingEMRRole",
      "Condition": {
        "StringEquals": {
          "aws:SourceAccount": "123456789012"
        },
        "ArnLike": {
          "aws:SourceArn": "arn:aws:application-autoscaling:*:123456789012:scalable-target/*"
        }
      },
      "Sid": "AllowSTSAssumerole"
    }
  ]
}
```

------