Laws of Event Sourcing

• All Events Are Immutable and Past Tense

• Applying a Failure Event Must Always Return the Previous State

• All Data Required for a Projection Must Be on the Events

• Work Is a Side Effect

• All Projections Must Stem from Events

• Never Manage More than One Flow per Process Manager

• Process Managers Must Not Read from Projections

• Event Schemas Are Immutable

• Different Projectors Cannot Share Projections

• Never Test Internal State