A media company's CloudWatch alarm is in ALARM, and the action names an SNS topic in a different Region from the alarm. Subscribers on a topic in the alarm's own Region receive tests, but this action never delivers. What should the action use?
Select an answer to reveal the explanation.
Short Explanation
The alarm and the topic have to stand in the same room. You pointed the action at a topic in another Region, and an alarm action can only publish to a topic in the alarm's own Region. Stretching the evaluation period does not change which Region an action may name.
Full Explanation
A CloudWatch alarm action that publishes to SNS must name a topic in the same Region as the alarm. The alarm cannot deliver to a topic ARN from another Region, so subscribers on that foreign topic never receive the state change even when the alarm is in ALARM. Email and HTTPS subscribers do not themselves have to live in the Region. They subscribe to the local topic, and SNS delivers outward from there. A topic left in the other Region is the configuration that is failing, not a default cross-Region action. CloudTrail records API calls. It is not the transport for an alarm notification. The evaluation period changes how long the metric must breach before the state changes. It does not relax the Region check on the action ARN. Caveat: if operators in the other Region must be paged, subscribe them to the local topic or have a subscriber relay onward. Do not point the alarm at the foreign topic. Operational check: describe the alarm, confirm the action ARN's Region matches the alarm, set the state to ALARM in a test, and confirm the local subscription received it.