implement k-choices join functions

large commit, contains a number of things:
- k-choices #2 cost calculation
- k-choices parameters
- adjusting ASN.1 network messages to contain all values required for
  cost calculation #71
- adjusting stats to contain required values
- k-choices node and vserver join functions
- placeholder/ dummy invocation of k-choices join

FediChord.asn1

@@ -111,6 +111,7 @@ LoadRequestPayload ::= SEQUENCE {
 LoadResponsePayload ::= SEQUENCE {
 	loadSum					REAL,
 	remainingLoadTarget		REAL,
+	totalCapacity			REAL,
 	lowerBound				NodeID
 	}
 

app/Main.hs

@@ -49,6 +49,9 @@ readConfig = do
           , confRequestTimeout = 5 * 10^6 `div` speedup
           , confRequestRetries = 3
           , confEnableKChoices = loadBalancingEnabled /= "off"
+          , confKChoicesOverload = 0.9
+          , confKChoicesUnderload = 0.1
+          , confKChoicesMaxVS = 8
           }
         sConf = ServiceConf
           { confSubscriptionExpiryTime = 24*3600 / fromIntegral speedup

src/Hash2Pub/ASN1Coding.hs

@@ -193,6 +193,7 @@ encodePayload payload'@LoadResponsePayload{} =
   [ Start Sequence
   , Real $ loadSum payload'
   , Real $ loadRemainingTarget payload'
+  , Real $ loadTotalCapacity payload'
   , IntVal . getNodeID $ loadSegmentLowerBound payload'
   , End Sequence
   ]
@@ -472,10 +473,12 @@ parseLoadResponsePayload :: ParseASN1 ActionPayload
 parseLoadResponsePayload = onNextContainer Sequence $ do
     loadSum' <- parseReal
     loadRemainingTarget' <- parseReal
+    loadTotalCapacity' <- parseReal
     loadSegmentLowerBound' <- fromInteger <$> parseInteger
     pure LoadResponsePayload
         { loadSum = loadSum'
         , loadRemainingTarget = loadRemainingTarget'
+        , loadTotalCapacity = loadTotalCapacity'
         , loadSegmentLowerBound = loadSegmentLowerBound'
         }
 

src/Hash2Pub/DHTProtocol.hs

@@ -93,6 +93,7 @@ import           Hash2Pub.FediChordTypes        (CacheEntry (..),
                                                  getKeyID, localCompare,
                                                  rMapFromList, rMapLookupPred,
                                                  rMapLookupSucc,
+                                                 remainingLoadTarget,
                                                  setPredecessors, setSuccessors)
 import           Hash2Pub.ProtocolTypes
 import           Hash2Pub.RingMap
@@ -292,7 +293,7 @@ handleIncomingRequest nsSTM sendQ msgSet sourceAddr = do
                 -- only when joined
                 Leave -> if isJoined ns then dropSpoofedIDs sourceIP nsSTM msgSet respondLeave else pure Nothing
                 Stabilise -> if isJoined ns then dropSpoofedIDs sourceIP nsSTM msgSet respondStabilise else pure Nothing
-                QueryLoad -> if isJoined ns then Just <$> respondLoadQuery nsSTM msgSet else pure Nothing
+                QueryLoad -> if isJoined ns then Just <$> respondQueryLoad nsSTM msgSet else pure Nothing
             )
       -- for single part request, response starts with part number 1. For multipart requests, response starts with part number n+1.
 
@@ -433,8 +434,8 @@ respondPing nsSTM msgSet = do
                                 }
     pure $ serialiseMessage sendMessageSize pingResponse
 
-respondLoadQuery :: Service s (RealNodeSTM s) => LocalNodeStateSTM s -> Set.Set FediChordMessage -> IO (Map.Map Integer BS.ByteString)
+respondQueryLoad :: Service s (RealNodeSTM s) => LocalNodeStateSTM s -> Set.Set FediChordMessage -> IO (Map.Map Integer BS.ByteString)
-respondLoadQuery nsSTM msgSet = do
+respondQueryLoad nsSTM msgSet = do
     nsSnap <- readTVarIO nsSTM
     -- this message cannot be split reasonably, so just
     -- consider the first payload
@@ -444,14 +445,18 @@ respondLoadQuery nsSTM msgSet = do
     responsePl <- maybe (pure Nothing) (\pl ->
         case pl of
           LoadRequestPayload{} -> do
-            serv <- nodeService <$> readTVarIO (parentRealNode nsSnap)
+            parentNode <- readTVarIO (parentRealNode nsSnap)
+            let
+                serv = nodeService parentNode
+                conf = nodeConfig parentNode
             lStats <- getServiceLoadStats serv
             let
                 directSucc = getNid . head . predecessors $ nsSnap
                 handledTagSum = sum . takeRMapSuccessorsFromTo directSucc (loadSegmentUpperBound pl) $ loadPerTag lStats
             pure $ Just LoadResponsePayload
                 { loadSum = handledTagSum
-                , loadRemainingTarget = remainingLoadTarget lStats
+                , loadRemainingTarget = remainingLoadTarget conf lStats
+                , loadTotalCapacity = totalCapacity lStats
                 , loadSegmentLowerBound = directSucc
                 }
           _ -> pure Nothing
@@ -808,7 +813,9 @@ requestQueryLoad ns upperIdBound target = do
         { segmentLowerKeyBound = loadSegmentLowerBound loadResPl
         , segmentUpperKeyBound = upperIdBound
         , segmentLoad = loadSum loadResPl
-        , segmentOwnerLoadTarget = loadRemainingTarget loadResPl
+        , segmentOwnerRemainingLoadTarget = loadRemainingTarget loadResPl
+        , segmentOwnerCapacity = loadTotalCapacity loadResPl
+        , segmentCurrentOwner = target
         }
 
 

src/Hash2Pub/FediChord.hs

@@ -63,6 +63,7 @@ import           Control.Exception
 import           Control.Monad                 (forM_, forever)
 import           Control.Monad.Except
 import           Crypto.Hash
+import           Data.Bifunctor                (first)
 import qualified Data.ByteArray                as BA
 import qualified Data.ByteString               as BS
 import qualified Data.ByteString.UTF8          as BSU
@@ -71,9 +72,11 @@ import           Data.Foldable                 (foldr')
 import           Data.Functor.Identity
 import           Data.HashMap.Strict           (HashMap)
 import qualified Data.HashMap.Strict           as HMap
+import           Data.HashSet                  (HashSet)
+import qualified Data.HashSet                  as HSet
 import           Data.IP                       (IPv6, fromHostAddress6,
                                                 toHostAddress6)
-import           Data.List                     ((\\))
+import           Data.List                     (sortBy, sortOn, (\\))
 import qualified Data.Map.Strict               as Map
 import           Data.Maybe                    (catMaybes, fromJust, fromMaybe,
                                                 isJust, isNothing, mapMaybe)
@@ -123,7 +126,9 @@ fediChordInit initConf serviceRunner = do
     fediThreadsAsync <- if confEnableKChoices initConf
        then
        -- TODO: k-choices way of joining
-       async (fediMainThreads serverSock realNodeSTM)
+       -- placeholder
+       runExceptT (kChoicesNodeJoin realNodeSTM ("foo", fromIntegral 3))
+       >> async (fediMainThreads serverSock realNodeSTM)
        else do
        -- without k-choices, just initialise a single vserver
             firstVS <- nodeStateInit realNodeSTM 0
@@ -148,6 +153,23 @@ fediChordInit initConf serviceRunner = do
                 joinedState
     pure (fediThreadsAsync, realNodeSTM)
 
+
+-- | Create a new vserver and join it through a provided remote node.
+-- TODO: Many fediChord* functions already cover parts of this, refactor these to use
+-- this function.
+fediChordJoinNewVs :: (MonadError String m, MonadIO m, Service s (RealNodeSTM s))
+                     => RealNodeSTM s   -- ^ parent real node
+                     -> Integer         -- ^ vserver ID
+                     -> RemoteNodeState -- ^ target node to join on
+                     -> m (NodeID, LocalNodeStateSTM s)     -- ^ on success: (vserver ID, TVar of vserver)
+fediChordJoinNewVs nodeSTM vsId target = do
+    newVs <- liftIO $ nodeStateInit nodeSTM vsId
+    newVsSTM <- liftIO $ newTVarIO newVs
+    liftIO . putStrLn $ "Trying to join on " <> show (getNid target)
+    liftIO $ putStrLn "send a Join"
+    _ <- liftIO . requestJoin target $ newVsSTM
+    pure (getNid newVs, newVsSTM)
+
 -- | initialises the 'NodeState' for this local node.
 -- Separated from 'fediChordInit' to be usable in tests.
 nodeStateInit :: Service s (RealNodeSTM s) => RealNodeSTM s -> Integer -> IO (LocalNodeState s)
@@ -178,6 +200,114 @@ nodeStateInit realNodeSTM vsID' = do
                                            }
     pure initialState
 
+
+kChoicesNodeJoin :: (MonadError String m, MonadIO m, Service s (RealNodeSTM s))
+                 => RealNodeSTM s
+                 -> (String, PortNumber)   -- ^ domain and port of a bootstrapping node
+                 -> m ()
+kChoicesNodeJoin nodeSTM bootstrapNode = do
+    node <- liftIO $ readTVarIO nodeSTM
+    -- use vserver 0 as origin of bootstrapping messages
+    vs0 <- liftIO $ nodeStateInit nodeSTM 0
+    vs0STM <- liftIO $ newTVarIO vs0
+
+    ownLoadStats <- liftIO . getServiceLoadStats . nodeService $ node
+    let
+        conf = nodeConfig node
+        -- T_a of k-choices
+        -- compute load target
+        joinLoadTarget = totalCapacity ownLoadStats * (confKChoicesUnderload conf + confKChoicesOverload conf) / 2
+        initialJoins = confKChoicesMaxVS conf `div` 2
+    -- edge case: however small the target is, at least join 1 vs
+    -- kCoicesVsJoin until target is met
+    joinedVss <- vsJoins vs0STM (totalCapacity ownLoadStats) (vservers node) joinLoadTarget (fromIntegral initialJoins) nodeSTM
+    liftIO . atomically . modifyTVar' nodeSTM $ \node' -> node'
+        { vservers = HMap.union (vservers node') joinedVss }
+    pure ()
+
+  where
+      vsJoins :: (MonadError String m, MonadIO m, Service s (RealNodeSTM s))
+              => LocalNodeStateSTM s -> Double -> VSMap s -> Double -> Int -> RealNodeSTM s -> m (VSMap s)
+      vsJoins _ _ vsmap _ 0 _ = pure vsmap
+      vsJoins queryVsSTM capacity vsmap remainingTargetLoad remainingJoins nodeSTM'
+        | remainingTargetLoad <= 0 = pure vsmap
+        | otherwise = (do
+
+            (acquiredLoad, (newNid, newVs)) <-  kChoicesVsJoin queryVsSTM bootstrapNode capacity vsmap nodeSTM' remainingTargetLoad
+            -- on successful vserver join add the new VS to node and recurse
+            vsJoins queryVsSTM capacity (HMap.insert newNid newVs vsmap) (remainingTargetLoad - acquiredLoad) (pred remainingJoins) nodeSTM'
+                      )
+                      -- TODO: decide on whether and how to catch errors
+                      -- error cause 1: not a single queried node has responded -> indicates permanent failure
+                      -- error cause 2: only a certain join failed, just ignore that join target for now, but problem: it will be the chosen
+                      -- target even at the next attempt again
+                      -- `catchError` (const .
+
+kChoicesVsJoin :: (MonadError String m, MonadIO m, Service s (RealNodeSTM s))
+               => LocalNodeStateSTM s   -- ^ vserver to be used for querying
+               -> (String, PortNumber)   -- ^ domain and port of a bootstrapping node
+               -> Double                -- ^ own capacity
+               -> VSMap s               -- ^ currently active VServers
+               -> RealNodeSTM s         -- ^ parent node is needed for initialising a new vserver
+               -> Double                -- ^ remaining load target
+               -> m (Double, (NodeID, LocalNodeStateSTM s))   -- ^ on success return tuple of acquired load and newly acquired vserver
+kChoicesVsJoin queryVsSTM bootstrapNode capacity activeVss nodeSTM remainingTarget = do
+    conf <- nodeConfig <$> liftIO (readTVarIO nodeSTM)
+    -- generate all possible vs IDs
+    let
+        activeVsSet = HMap.keysSet activeVss
+        -- tuples of node IDs and vserver IDs, because vserver IDs are needed for
+        -- LocalNodeState creation
+        nonJoinedIDs = filter (not . flip HSet.member activeVsSet . fst) [ (genNodeID (confIP conf) (confDomain conf) (fromInteger v), v) | v <- [0..confKChoicesMaxVS conf]]
+    queryVs <- liftIO $ readTVarIO queryVsSTM
+
+    -- query load of all possible segments
+    -- simplification: treat each load lookup failure as a general unavailability of that segment
+    -- TODO: retries for transient failures
+    segmentLoads <- fmap catMaybes . forM nonJoinedIDs $ (\(vsNid, vsId) -> (do
+        lookupResp <- liftIO $ bootstrapQueryId queryVsSTM bootstrapNode vsNid
+        currentlyResponsible <- liftEither lookupResp
+        segment <- requestQueryLoad queryVs vsNid currentlyResponsible
+        pure $ Just (segment, vsId, currentlyResponsible)
+        -- store segment stats and vserver ID together, so it's clear
+        -- which vs needs to be joined to acquire this segment
+                     ) `catchError` const (pure Nothing)
+                                                     )
+
+
+    -- cost calculation and sort by cost
+                                        -- edge case: no possible ID has responded
+    (mincost, vsId, toJoinOn) <- maybe (throwError "received no load information") pure
+                                  . headMay
+                                  . sortOn (\(cost, _, _) -> cost)
+                                  . fmap (\(segment, vsId, toJoinOn) -> (kChoicesJoinCost remainingTarget capacity segment, vsId, toJoinOn))
+                                  $ segmentLoads
+    -- join at min cost
+    joinedNode <- fediChordJoinNewVs nodeSTM vsId toJoinOn
+    pure (mincost, joinedNode)
+
+    -- Possible optimisation:
+    -- Instead of sampling all join candidates again at each invocation, querying
+    -- all segment loads before the first join and trying to re-use these
+    -- load information can save round trips.
+    -- possible edge case: detect when joining a subsegment of one already owned
+    -- joining into own segments => When first joining into segment S1 and then
+    -- later joining into the subsegment S2, the
+    -- resulting load l(S1+S2) = l(S1) != l(S1) + l(S2)
+    -- => need to re-query the load of both S1 and S2
+    -- possible edge case 2: taking multiple segments from the same owner
+    -- changes the remainingLoadTarget at each vsJoin. This target change
+    -- needs to be accounted for starting from the 2nd vsJoin.
+
+kChoicesJoinCost :: Double  -- ^ own remaining load target
+                 -> Double  -- ^ own capacity
+                 -> SegmentLoadStats
+                 -> Double
+kChoicesJoinCost remainingOwnLoad ownCap segment =
+    abs (segmentOwnerRemainingLoadTarget segment + segmentLoad segment) / segmentOwnerCapacity segment
+  + abs (remainingOwnLoad - segmentLoad segment) / ownCap
+  - abs (segmentOwnerRemainingLoadTarget segment) / segmentOwnerCapacity segment
+
 -- | Join a new node into the DHT, using a provided bootstrap node as initial cache seed
 -- for resolving the new node's position.
 fediChordBootstrapJoin :: Service s (RealNodeSTM s)
@@ -277,8 +407,7 @@ bootstrapQueryId nsSTM (bootstrapHost, bootstrapPort) targetID = do
                            Just (FORWARD resultset) -> foldr' (addCacheEntryPure now) cacheAcc resultset
                               )
                            initCache resp
-               currentlyResponsible <- runExceptT $ queryIdLookupLoop bootstrapCache ns 50 $ getNid ns
+               runExceptT $ queryIdLookupLoop bootstrapCache ns 50 $ getNid ns
-               pure currentlyResponsible
 
 
 -- | join a node to the DHT using the global node cache
@@ -296,6 +425,7 @@ fediChordVserverJoin nsSTM = do
     joinResult <- liftIO $ requestJoin currentlyResponsible nsSTM
     liftEither joinResult
 
+
 fediChordVserverLeave :: (MonadError String m, MonadIO m, Service s (RealNodeSTM s)) => LocalNodeState s -> m ()
 fediChordVserverLeave ns = do
     -- TODO: deal with failure of all successors, e.g. by invoking a stabilise

src/Hash2Pub/FediChordTypes.hs

@@ -18,8 +18,10 @@ module Hash2Pub.FediChordTypes
   , RemoteNodeState (..)
   , RealNode (..)
   , RealNodeSTM
+  , VSMap
   , LoadStats (..)
   , emptyLoadStats
+  , remainingLoadTarget
   , SegmentLoadStats (..)
   , setSuccessors
   , setPredecessors
@@ -153,7 +155,7 @@ a `localCompare` b
 -- Also contains shared data and config values.
 -- TODO: more data structures for k-choices bookkeeping
 data RealNode s = RealNode
-    { vservers              :: HashMap NodeID (LocalNodeStateSTM s)
+    { vservers              :: VSMap s
     -- ^ map of all active VServer node IDs to their node state
     , nodeConfig            :: FediChordConf
     -- ^ holds the initial configuration read at program start
@@ -168,6 +170,8 @@ data RealNode s = RealNode
     , nodeService           :: s (RealNodeSTM s)
     }
 
+
+type VSMap s = HashMap NodeID (LocalNodeStateSTM s)
 type RealNodeSTM s = TVar (RealNode s)
 
 -- | represents a node and all its important state
@@ -438,6 +442,12 @@ data FediChordConf = FediChordConf
     -- ^ how often re-sending a timed-out request can be retried
     , confEnableKChoices            :: Bool
     -- ^ whether to enable k-choices load balancing
+    , confKChoicesOverload          :: Double
+    -- ^ fraction of capacity above which a node considers itself overloaded
+    , confKChoicesUnderload         :: Double
+    -- ^ fraction of capacity below which a node considers itself underloaded
+    , confKChoicesMaxVS             :: Integer
+    -- ^ upper limit of vserver index κ
     }
     deriving (Show, Eq)
 
@@ -448,11 +458,18 @@ data LoadStats = LoadStats
     -- ^ map of loads for each handled tag
     , totalCapacity      :: Double
     -- ^ total designated capacity of the service
-    , remainingLoadTarget :: Double
+    , compensatedLoadSum :: Double
-    -- ^ current mismatch between actual load and target load/capacity
+    -- ^ effective load reevant for load balancing after compensating for
     }
     deriving (Show, Eq)
 
+-- | calculates the mismatch from the target load by taking into account the
+-- underload and overload limits
+remainingLoadTarget :: FediChordConf -> LoadStats -> Double
+remainingLoadTarget conf lstats = targetLoad - compensatedLoadSum lstats
+    where
+        targetLoad = totalCapacity lstats * (confKChoicesUnderload conf + confKChoicesOverload conf) / 2
+
 data SegmentLoadStats = SegmentLoadStats
     { segmentLowerKeyBound            :: NodeID
     -- ^ segment start key
@@ -460,8 +477,12 @@ data SegmentLoadStats = SegmentLoadStats
     -- ^ segment end key
     , segmentLoad                     :: Double
     -- ^ sum of load of all keys in the segment
-    , segmentOwnerLoadTarget :: Double
+    , segmentOwnerRemainingLoadTarget :: Double
-    -- ^ remaining load target of the current segment handler
+    -- ^ remaining load target of the current segment handler:
+    , segmentOwnerCapacity            :: Double
+    -- ^ total capacity of the current segment handler node, used for normalisation
+    , segmentCurrentOwner             :: RemoteNodeState
+    -- ^ the current owner of the segment that needs to be joined on
     }
 
 -- TODO: figure out a better way of initialising
@@ -469,7 +490,7 @@ emptyLoadStats :: LoadStats
 emptyLoadStats = LoadStats
     { loadPerTag = emptyRMap
     , totalCapacity = 0
-    , remainingLoadTarget = 0
+    , compensatedLoadSum = 0
     }
 
 -- ====== Service Types ============

src/Hash2Pub/PostService.hs

@@ -890,12 +890,13 @@ evaluateLoadStats currentStats currentSubscribers = do
         )
         (0, emptyRMap)
         $ rMapToListWithKeys currentSubscribers
-    -- TODO: use underload and overload limits instead of capacity
     let remainingLoadTarget' = totalCapacity' - loadSum - postFetchRate currentStats
     pure LoadStats
         { loadPerTag = loadPerTag'
         , totalCapacity = totalCapacity'
-        , remainingLoadTarget = remainingLoadTarget'
+        -- load caused by post fetches cannot be influenced by re-balancing nodes,
+        -- but still reduces the totally available capacity
+        , compensatedLoadSum = loadSum + postFetchRate currentStats
         }
 
 

src/Hash2Pub/ProtocolTypes.hs

@@ -82,6 +82,7 @@ data ActionPayload = QueryIDRequestPayload
     | LoadResponsePayload
     { loadSum               :: Double
     , loadRemainingTarget   :: Double
+    , loadTotalCapacity     :: Double
     , loadSegmentLowerBound :: NodeID
     }
     deriving (Show, Eq)