Resource Orchestration Service:How to query a value from a map in a template?

ROS templates support querying values from maps stored as key-value pairs. Depending on whether the map is defined in the Mappings section and how many levels it has, use the intrinsic functions described in this topic.

Choose the right function based on your map type:

Query a value from a single-level map

Fn::Select

Fn::Select takes a key and a map object, and returns the matching value. For more information, see Fn::Select.

• Query value_a from a JSON object parameter — given key_a, returns value_a:

  ROSTemplateFormatVersion: '2015-09-01'
  Parameters:
    InputMap:
      Type: Json
  Outputs:
    value_a:
      Value:
        Fn::Select:
        - key_a
        - InputMap

• Query a value from an inline map defined directly in the template:

  ROSTemplateFormatVersion: '2015-09-01'
  Resources:
    Mock:
      Type: MockResource
      Properties:
        Prop1:
          Fn::Select:
          - key
          - key1: value1
            key2: value2

Fn::GetJsonValue

When the map is passed as a JSON string rather than a JSON object, use Fn::GetJsonValue to query the value at a first-level key. For more information, see Fn::GetJsonValue.

Given key_a, returns value_a:

ROSTemplateFormatVersion: '2015-09-01'
Parameters:
  InputMapJsonString:
    Type: Json
Outputs:
  value_a:
    Value:
      Fn::GetJsonValue:
      - key_a
      - InputMapJsonString

Query a value from a two-level map

Fn::FindInMap

Use Fn::FindInMap to look up a value by top-level key and second-level key. For more information, see Mappings and Fn::FindInMap.

The following example selects an ECS image ID based on region and architecture. Given regionParam=hangzhou and architecture '32', returns m-25l0rcfjo:

ROSTemplateFormatVersion: '2015-09-01'
Parameters:
  regionParam:
    Description: the region where you want to create the Elastic Compute Service (ECS) instance
    Type: String
    AllowedValues:
      - hangzhou
      - beijing
Mappings:
  RegionMap:
    hangzhou:
      '32': m-25l0rcfjo
      '64': m-25l0rcfj1
    beijing:
      '32': m-25l0rcfj2
      '64': m-25l0rcfj3
Resources:
  WebServer:
    Type: ALIYUN::ECS::Instance
    Properties:
      ImageId:
        Fn::FindInMap:
          - RegionMap
          - Ref: regionParam
          - '32'

Fn::Select

Nest Fn::Select twice to drill into a two-level map. The inner call retrieves the sub-map at key_1, and the outer call retrieves the value at key_1_1. For more information, see Fn::Select.

Returns value_1_1:

Fn::Select:
- key_1_1:
- Fn::Select:
  - key_1
  - key_1:
      key_1_1: value_1_1
      key_1_2: value_1_2
    key_2:
      key_2_1: value_2_1
      key_2_2: value_2_2

Query a value from a multi-level map

Fn::Select

• For maps with more than two levels in the Mappings section, combine Fn::FindInMap and Fn::Select: use Fn::FindInMap to retrieve a sub-map at the two-level boundary, then use Fn::Select to access the deeper key.

  Fn::Select:
    - key
    - Fn::FindInMap:
        - MapName
        - TopLevelKey
        - SecondLevelKey

• For multi-level maps outside the Mappings section, nest Fn::Select multiple times using the same pattern as the two-level example. For more information, see Fn::Select.

Fn::Jq

Use Fn::Jq when you need complex filtering across nested structures — for example, extracting specific fields from an array of objects. For more information, see Fn::Jq.

The following example extracts name and type from each object in the parameters array:

Fn::Jq:
  - All
  - '.parameters[] | {"param_name": .name, "param_type":.type}'
  - changeSet:
      items: []
      kind: git
    id: 2013-12-27_00-09-37
    parameters:
      - name: PKG_TAG_NAME
        value: trunk
      - name: GIT_COMMIT
        value: master
      - name: TRIGGERED_JOB
        value: trunk-buildall

Returns:

- param_name: PKG_TAG_NAME
- param_name: GIT_COMMIT
- param_name: TRIGGERED_JOB