Constraints

Constraints are server-side rules enforced in the Chronicle kernel before events are committed. They protect invariants that must hold under concurrency — uniqueness being the canonical case. Two built-in forms cover the common cases; complex rules delegate to a file.

Syntax

constraint <Name>
  unique <property> on <EventType>         ← unique property constraint

constraint <Name>
  unique event <EventType>                 ← unique event type constraint

constraint <Name>
  file <Path>                              ← custom C# implementation

Form	Meaning
unique <property> on <EventType>	No two event sources may commit the event type with the same value for the property.
unique event <EventType>	The event type may occur at most once per event source.
file <Path>	The rule is implemented in C# in the referenced file.

Examples

constraint UniqueInvoiceNumber
  unique invoiceNumber on InvoiceRegistered

constraint OneRegistrationPerInvoice
  unique event InvoiceRegistered

constraint InvoiceStatusTransition
  file Constraints/InvoiceStatusTransitionConstraint.cs

Guidance

• Constraints belong in the StateChange slice whose events they guard.
• Use a constraint — not command validation — for any rule that must hold under concurrent writers; validation happens before the append, constraints are enforced atomically at the event store.