eventTag

Gives an event case a numeric tag in place of its default name-based tag.

Every event an EventStore appends is stored with a tag that identifies its kind, and reads can be filtered by tag. By default the tag is the case name, so renaming a case changes the tag that new events are stored with. Annotating the case with @eventTag(n) makes the tag the string form of n instead, for example "1", which stays the same when the case is renamed.

Put the annotation on each case class of a sealed event type, with a different number for each case. The numbers are not checked for uniqueness. It is also honoured on a single, non-sealed event type.

sealed trait UserEvent

@eventTag(1)
case class Created(userId: String, name: String) extends UserEvent

@eventTag(2)
case class Deleted(userId: String) extends UserEvent

The tag is the event's identity in the store's tag column and in EventEnvelope. It does not change how the event's payload is encoded.