add state notification support in raft core

raft/src/main/scala/zio/raft/Raft.scala

@@ -28,6 +28,7 @@ class Raft[S, A <: Command](
   peers: Peers,
   private[raft] val raftState: Ref[State[S]],
   commandsQueue: Queue[StreamItem[A, S]],
+  stateNotificationsQueue: Queue[StateNotification],
   stable: Stable,
   private[raft] val logStore: LogStore[A],
   snapshotStore: SnapshotStore,
@@ -39,6 +40,9 @@ class Raft[S, A <: Command](
   val batchSize = 100
   val numberOfServers = peers.size + 1
 
+  def stateNotifications: ZStream[Any, Nothing, StateNotification] =
+    ZStream.fromQueue(stateNotificationsQueue)
+
   private def stepDown(newTerm: Term, leaderId: Option[MemberId]) =
     for
       _ <- stable.newTerm(newTerm, None)
@@ -55,6 +59,15 @@ class Raft[S, A <: Command](
       _ <- ZIO.logDebug(
         s"memberId=${this.memberId} Following $leaderId $currentTerm"
       )
+
+      _ <- (currentState, leaderId) match
+        case (_: Leader[S], _) =>
+          stateNotificationsQueue.offer(StateNotification.SteppedDown(leaderId))
+        case (f: State.Follower[S], Some(leaderId)) if f.leaderId != Some(leaderId) =>
+          stateNotificationsQueue.offer(StateNotification.LeaderChanged(leaderId))
+        case (_: State.Candidate[S], Some(leaderId)) =>
+          stateNotificationsQueue.offer(StateNotification.LeaderChanged(leaderId))
+        case _ => ZIO.unit
     yield newTerm
 
   private def convertToFollower(leaderId: MemberId) =
@@ -66,9 +79,14 @@ class Raft[S, A <: Command](
 
       _ <- s match
         case s: State.Follower[S] if s.leaderId != Some(leaderId) =>
-          ZIO.logDebug(
-            s"memberId=${this.memberId} Following $leaderId $currentTerm"
-          )
+          for
+            _ <- ZIO.logDebug(
+              s"memberId=${this.memberId} Following $leaderId $currentTerm"
+            )
+
+            // Leader changed, so let's notify
+            _ <- stateNotificationsQueue.offer(StateNotification.LeaderChanged(leaderId))
+          yield ()
         case s: State.Follower[S] => ZIO.unit
         case s =>
           ZIO.logDebug(
@@ -514,6 +532,8 @@ class Raft[S, A <: Command](
             PendingCommands.empty
           )
         )
+
+        _ <- stateNotificationsQueue.offer(StateNotification.SteppedUp)
       yield ()
 
     for
@@ -1046,12 +1066,14 @@ object Raft:
         State.Follower[S](previousIndex, previousIndex, electionTimeout, None)
       )
       commandsQueue <- Queue.unbounded[StreamItem[A, S]] // TODO: should this be bounded for back-pressure?
+      stateNotificationsQueue <- Queue.unbounded[StateNotification]
 
       raft = new Raft[S, A](
         memberId,
         peers,
         state,
         commandsQueue,
+        stateNotificationsQueue,
         stable,
         logStore,
         snapshotStore,

raft/src/main/scala/zio/raft/Types.scala

@@ -126,3 +126,9 @@ object InstallSnapshotResult:
     done: Boolean
   ) extends InstallSnapshotResult[A]
   case class Failure[A <: Command](from: MemberId, term: Term, index: Index) extends InstallSnapshotResult[A]
+
+sealed trait StateNotification
+object StateNotification:
+  case object SteppedUp extends StateNotification
+  case class SteppedDown(leaderId: Option[MemberId]) extends StateNotification
+  case class LeaderChanged(leaderId: MemberId) extends StateNotification