Cloudwatch dashboard pattern variables and alarm annotation interaction

Question

Hello there, today I discovered weird interaction between [dashboard variables](https://docs.aws.amazon.com/AmazonCloudWatch/latest/monitoring/cloudwatch_dashboard_variables_pattern.html) and widget's [alarm annotation](https://docs.aws.amazon.com/AmazonCloudWatch/latest/APIReference/CloudWatch-Dashboard-Body-Structure.html#CloudWatch-Dashboard-Properties-Annotation-Format).

Mind you - following mishap **happens only when saving dashboard**, rendering itself works just fine.

Example JSON (`{{ Region }}` is templated the same way as `{{ AccountId }}`:
```json
"variables": [
        {
            "type": "pattern",
            "pattern": "{{ AccountId }}",
            ...
            "defaultValue": "0123456789",
            "values": [
                {
                    "value": "0123456789",
                    "label": "production"
                }
            ]
      }
],
"widgets": [
        {
            "type": "metric",
            ...
            "properties": {
                "title": "...",
                "annotations": {
                    "alarms": [
                        "arn:aws:cloudwatch:{{ Region }}:{{ AccountId }}:alarm:SomeAlarmName"
                    ]
                },
                "view": "timeSeries",
                "stacked": false
            }
        }
]
```

When clicking `Save` button / calling `PutDashboard` endpoint I'll get
```json
{
"dataPath": "/widgets/0/properties",
"message": "The metric widget should have specified a region and a data source or an alarm annotation"
},
{
"dataPath": "/widgets/0/properties",
"message": "Should have required property 'region'"
},
```

With enough trying, I discovered that if either Region or AccountId in alarm's ARN are placeholder values, saving will fail. Is this expected behavior of pattern variables?

Workaround is using patterns like `us-east-1` which would be replaced during dashboard rendering, while still being valid value during dashboard saving.

Thanks for any insight
V.

re:Post Agent · Answer

This behavior you're experiencing is indeed a limitation of how pattern variables work with alarm annotations in CloudWatch dashboards.

When you use pattern variables like `{{ AccountId }}` or `{{ Region }}` in alarm ARNs within annotations, the dashboard save operation (PutDashboard API call) attempts to validate the ARN structure before applying the variable substitution. Since the placeholders aren't valid ARN components during this validation phase, you get the errors about missing region and data source.

Your workaround is correct - using actual valid values (like "us-east-1") that will be replaced during rendering is the appropriate approach. This allows the dashboard to pass validation during saving while still enabling the dynamic behavior you want during viewing.

For many use cases involving Region switching, you can use property variables instead of pattern variables. However, as you've discovered, when working with ARNs that include Region names or account IDs, pattern variables are necessary - but they must contain syntactically valid values at save time.

The pattern variable is working as designed, but the validation process during saving doesn't recognize the placeholders as valid temporary values, which is why you're experiencing this limitation.  
**Sources**  
[Creating flexible CloudWatch dashboards with dashboard variables - Amazon CloudWatch](https://docs.aws.amazon.com/AmazonCloudWatch/latest/monitoring/cloudwatch_dashboard_variables.html)  
[Tutorial: Creating a dashboard that uses a regular expression pattern to switch between AWS Regions - Amazon CloudWatch](https://docs.aws.amazon.com/AmazonCloudWatch/latest/monitoring/cloudwatch_dashboard_variables_pattern.html)

Cloudwatch dashboard pattern variables and alarm annotation interaction

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