feat: event ids, outbox identity migration

crates/domain/src/events.rs

@@ -150,14 +150,44 @@ pub trait AckHandle: Send + Sync {
     async fn nack(&self) -> Result<(), DomainError>;
 }
 
+/// Stable identity of one queued event; the key for de-duplicating deliveries.
+#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
+pub struct EventId(uuid::Uuid);
+
+impl EventId {
+    pub fn new() -> Self {
+        Self(uuid::Uuid::new_v4())
+    }
+
+    pub fn from_uuid(uuid: uuid::Uuid) -> Self {
+        Self(uuid)
+    }
+
+    pub fn value(&self) -> uuid::Uuid {
+        self.0
+    }
+}
+
+impl Default for EventId {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
 pub struct EventEnvelope {
     pub event: DomainEvent,
+    id: Option<EventId>,
     ack: Box<dyn AckHandle>,
 }
 
 impl EventEnvelope {
-    pub fn new(event: DomainEvent, ack: Box<dyn AckHandle>) -> Self {
-        Self { event, ack }
+    /// `id` is `None` only for events queued before identities existed.
+    pub fn new(id: Option<EventId>, event: DomainEvent, ack: Box<dyn AckHandle>) -> Self {
+        Self { event, id, ack }
+    }
+
+    pub fn id(&self) -> Option<&EventId> {
+        self.id.as_ref()
     }
 
     pub async fn ack(self) -> Result<(), DomainError> {