Compensate for recorded events

A compensating event records a correction as a new fact. The original event stays exactly as it was, and the new event carries the original’s sequence number so observers can tell what it corrects. Use it for ordinary domain corrections. Use revision or redaction only for administrative repair.

Declare the compensating type

Declare which event type a type compensates with events.WithCompensationFor. The target must be registered in the same store.

original, err := events.Define[OrderPlaced]()
if err != nil {
    return err
}
cancelled, err := events.Define[OrderPlacementRetracted](events.WithCompensationFor(original))
if err != nil {
    return err
}

Append the compensation

result, err := store.EventLog().Compensate(ctx, position, OrderPlacementRetracted{Reason: "duplicate"})
if err != nil {
    return err // refused before dispatch, or the outcome is unknown
}
if err := result.Err(); err != nil {
    return err // for example eventsequences.CompensatedEventNotFound
}

Compensate works like an append, except the event source, stream and subject come from the compensated event. WithRoute, WithScope, WithSubject and WithNamedTags are therefore refused with ErrInvalidConfiguration; WithCorrelation, WithTags and WithOccurred apply.

• A type that is not declared with WithCompensationFor is refused before dispatch.
• The kernel rejects a missing target (CompensatedEventNotFound) and a type that does not compensate the target’s type (EventTypeDoesNotCompensateEventType). Both come back as a Rejected result with a nil error; result.Err() promotes them.
• A failure after dispatch is an OutcomeUnknownError. Do not retry blindly: a retry can append a second compensation.
• The call is not retried, does not emit local append notifications and does not join a unit of work.
• It needs Chronicle 19.125.0 or later.

Read the link

events.Context.CompensationFor holds the position of the corrected event, or nil for an ordinary event. The wire format uses a plain number where zero means none, so a compensation of the event at position zero reads as nil.