implement service data migration for stabilise

src/Hash2Pub/DHTProtocol.hs

@@ -381,7 +381,7 @@ respondLeave nsSTM msgSet = do
         pure leaveResponse
     -- if awaiting an incoming service data migration, collect the lock without blocking this thread
     when (maybe False leaveDoMigration (payload  aRequestPart)) $ do
-        ownService <- atomically $ nodeService <$> ((readTVar nsSTM) >>= (readTVar . parentRealNode))
+        ownService <- atomically $ nodeService <$> (readTVar nsSTM >>= (readTVar . parentRealNode))
         void (forkIO $ waitForMigrationFrom ownService leaveSenderID)
     pure $ serialiseMessage sendMessageSize responseMsg
 
@@ -425,8 +425,7 @@ respondPing nsSTM msgSet = do
                                 }
     pure $ serialiseMessage sendMessageSize pingResponse
 
--- this modifies node state, so locking and IO seems to be necessary.
+
--- Still try to keep as much code as possible pure
 respondJoin :: Service s (RealNodeSTM s) => LocalNodeStateSTM s -> Set.Set FediChordMessage -> IO (Map.Map Integer BS.ByteString)
 respondJoin nsSTM msgSet = do
     -- atomically read and modify the node state according to the parsed request

src/Hash2Pub/FediChord.hs

@@ -465,9 +465,9 @@ checkCacheSliceInvariants ns
 -- | Periodically send @StabiliseRequest' s to the closest neighbour nodes, until
 -- one responds, and get their neighbours for maintaining the own neighbour lists.
 -- If necessary, request new neighbours.
-stabiliseThread :: LocalNodeStateSTM s -> IO ()
+stabiliseThread :: Service s (RealNodeSTM s) => LocalNodeStateSTM s -> IO ()
 stabiliseThread nsSTM = forever $ do
-    ns <- readTVarIO nsSTM
+    oldNs <- readTVarIO nsSTM
 
     putStrLn "stabilise run: begin"
 
@@ -478,8 +478,8 @@ stabiliseThread nsSTM = forever $ do
 
     -- don't contact all neighbours unless the previous one failed/ Left ed
 
-    predStabilise <- stabiliseClosestResponder ns predecessors 1 []
+    predStabilise <- stabiliseClosestResponder oldNs predecessors 1 []
-    succStabilise <- stabiliseClosestResponder ns predecessors 1 []
+    succStabilise <- stabiliseClosestResponder oldNs predecessors 1 []
 
     let
         (predDeletes, predNeighbours) = either (const ([], [])) id predStabilise
@@ -518,6 +518,28 @@ stabiliseThread nsSTM = forever $ do
             writeTVar nsSTM $ addSuccessors [nextEntry] latestNs
                                                                      )
 
+    newNs <- readTVarIO nsSTM
+
+    let
+        oldPredecessor = headDef (toRemoteNodeState oldNs) $ predecessors oldNs
+        newPredecessor = headMay $ predecessors newNs
+    -- manage need for service data migration:
+    maybe (pure ()) (\newPredecessor' ->
+        when (
+            isJust newPredecessor
+            && oldPredecessor /= newPredecessor'
+        -- case: predecessor has changed in some way => own responsibility has changed in some way
+        -- case 1: new predecessor is further away => broader responsibility, but new pred needs to push the data
+        -- If this is due to a node leaving without transfering its data, try getting it from a redundant copy
+        -- case 2: new predecessor is closer, it takes some of our data but somehow didn't join on us => push data to it
+            && isInOwnResponsibilitySlice newPredecessor' oldNs) $ do
+                ownService <- nodeService <$> (liftIO . readTVarIO $ parentRealNode newNs)
+                migrationResult <- migrateData ownService (getNid newNs) (getNid oldPredecessor) (getNid newPredecessor') (getDomain newPredecessor', fromIntegral $ getServicePort newPredecessor')
+                -- TODO: deal with migration failure, e.g retry
+                pure ()
+                    )
+                    newPredecessor
+
     putStrLn "stabilise run: end"
     -- TODO: make delay configurable
     threadDelay (60 * 10^6)