# External stack output resolver

The external stack output resolver reads the parameter value from a stack output of a stack. The source stack does not have to belong to the same Takomo project as the target stack.

### Properties

Here are the properties of the external stack output resolver:

| Key          | Required | Type    | Description                                                                                                                            |
| ------------ | -------- | ------- | -------------------------------------------------------------------------------------------------------------------------------------- |
| resolver     | yes      | string  | Resolver name, this must be **external-stack-output**.                                                                                 |
| stack        | yes      | string  | Name of the source stack.                                                                                                              |
| output       | yes      | string  | Name of the stack output whose value is read.                                                                                          |
| region       | no       | string  | Region of the source stack. Region is optional. By default, the region of the target stack is used.                                    |
| commandRole  | no       | string  | IAM role used to access the stack output. Command role is optional. By default, credentials associated with the target stack are used. |
| confidential | no       | boolean | Conceal the resolved parameter value from logs, defaults to **false**                                                                  |
| immutable    | no       | boolean | Mark the parameter as immutable, defaults to **false**                                                                                 |

### Example

Say, we have two accounts: **123456789012** and **888888888888**.

The account **123456789012** has one stack: **src-bucket**. It is located in the **us-east-1** region and exposes the name of an application source bucket in a stack output named **SrcBucketName**. The **123456789012** account also has a read-only role that the **888888888888** account can assume.

The **888888888888** account has two stacks: **assets-bucket** and **build-infra**. The stacks are located in the **us-east-1** and **eu-west-1** regions, respectively. The **assets-bucket** stack exposes the name of an assets bucket in a stack output named **AssetsBucket**.

Only the **build-infra** stack is managed in our Takomo project. The two other stacks are configured elsewhere. The **build-infra** stack has two parameters: **SrcBucket** and **AssetsBucket**. To get the values for them, we use the **external-stack-output** resolver to read the two other stacks' outputs.

The directory structure looks like this:

```bash
.
├─ stacks
│  └─ build-infra.yml
└─ templates
   └─ build-infra.yml
```

The configuration of **build-infra** stack looks like this:

{% code title="stacks/build-infra.yml" %}

```yaml
regions: us-east-1
parameters:
  SrcBucket:
    resolver: external-stack-output
    stack: src-bucket
    output: SrcBucketName
    commandRole: arn:aws:iam::123456789012:role/read-only
  AssetsBucket:
    resolver: external-stack-output
    stack: assets-bucket
    output: AssetsBucketName
    region: eu-west-1
```

{% endcode %}

For the **SrcBucket** parameter, we need to specify the **commandRole** property because the source stack is located in a different account. We don't need to specify the **region** because both stacks are located in the same region.

For the **AssetsBucket** parameter, we must specify the **region** but not the **commandRole** because the stacks are located in the same account but different regions.