In Event Storming, a Policy (often called an Event Storming Policy) is a rule or condition that defines what should happen in response to a Domain Event.
In Event Storming, policies are shown as purple sticky notes placed between Events (orange notes) and Commands (blue notes). They represent the business logic that drives system behavior.
Example:
"Whenever a customer places an order, check their credit limit."
Policies are triggered by events—these could be user actions or system changes. They often involve decision-making.
Example: Whenever Credit Checked has been done, one of these two policies will be triggered:
The typical flow in Event Storming: Event → Policy → Command
Should you specify exact values (e.g., the minimum credit amount) in the policy?
Alberto Brandolini, the creator of Event Storming, calls policies “lie detectors.” Adding words like Always or Whenever and asking domain experts— “Do we ALWAYS check the credit limit when an order is placed?”—often reveals exceptions and edge cases that might otherwise be missed.
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