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Fn::GetAZs

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The intrinsic function `Fn::GetAZs` returns an array that lists Availability Zones for a specified region in alphabetical order. Because customers have access to different Availability Zones, the intrinsic function `Fn::GetAZs` enables template authors to write templates that adapt to the calling user's access. That way you don't have to hard-code a full list of Availability Zones for a specified region.

• Important*

For the EC2-Classic platform, the `Fn::GetAZs` function returns all Availability Zones for a region. For the EC2\-VPC platform, the `Fn::GetAZs` function returns only Availability Zones that have a default subnet unless none of the Availability Zones has a default subnet; in that case, all Availability Zones are returned.

Similarly to the response from the `describe-availability-zones` AWS CLI command, the order of the results from the `Fn::GetAZs` function is not guaranteed and can change when new Availability Zones are added.

EC2-VPC

IAM permissions

The permissions that you need in order to use the `Fn::GetAZs` function depend on the platform in which you're launching Amazon EC2 instances. For both platforms, you need permissions to the Amazon EC2 `DescribeAvailabilityZones` and `DescribeAccountAttributes` actions. For EC2-VPC, you also need permissions to the Amazon EC2 `DescribeSubnets` action.

Declaration

JSON

{ "Fn::GetAZs" : "region" }

YAML

Syntax for the full function name:

Fn::GetAZs: region

Syntax for the short form:

!GetAZs region

Parameters

region

The name of the region for which you want to get the Availability Zones.

You can use the `AWS::Region` pseudo parameter to specify the region in which the stack is created. Specifying an empty string is equivalent to specifying `AWS::Region`.

Return value

The list of Availability Zones for the region.

Examples

Evaluate a Region

For these examples, AWS CloudFormation evaluates `Fn::GetAZs` to the following array—assuming that the user has created the stack in the `us-east-1` region:

`[ "us-east-1a", "us-east-1b", "us-east-1c", "us-east-1d" ]`

JSON

1. { "Fn::GetAZs" : "" }
2. { "Fn::GetAZs" : { "Ref" : "AWS::Region" } }
3. { "Fn::GetAZs" : "us-east-1" }

YAML

1. Fn::GetAZs: ""
2. Fn::GetAZs:
3.   Ref: "AWS::Region"
4. Fn::GetAZs: us-east-1

Specify a subnet's Availability Zone

The following example uses `Fn::GetAZs` to specify a subnet's Availability Zone:

JSON

"mySubnet" : {
  "Type" : "AWS::EC2::Subnet",
  "Properties" : {
    "VpcId" : { 
      "Ref" : "VPC"   
    },
    "CidrBlock" : "10.0.0.0/24",
    "AvailabilityZone" : {
      "Fn::Select" : [ 
        "0", 
        { 
          "Fn::GetAZs" : "" 
        } 
      ]
    }
  }
}

YAML

mySubnet: 
  Type: "AWS::EC2::Subnet"
  Properties: 
    VpcId: 
      !Ref VPC
    CidrBlock: 10.0.0.0/24
    AvailabilityZone: 
      Fn::Select: 
        - 0
        - Fn::GetAZs: ""

Nested functions with short form YAML

The following examples show valid patterns for using nested intrinsic functions using short form YAML. You can't nest short form functions consecutively, so a pattern like `!GetAZs !Ref` is invalid.

YAML

1. AvailabilityZone: !Select 
2.   - 0
3.   - !GetAZs 
4.     Ref: 'AWS::Region'

YAML

1. AvailabilityZone: !Select 
2.   - 0
3.   - Fn::GetAZs: !Ref 'AWS::Region'

Supported functions

You can use the `Ref` function in the `Fn::GetAZs` function.