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For further +-- documentation, see http://haskell.org/cabal/users-guide/ + +name: load-balancing +version: 1.0.1.0 +synopsis: Client-side load balancing utilities. +-- description: +homepage: https://github.com/SumAll/haskell-load-balancing +license: Apache-2.0 +license-file: LICENSE +author: Rick Owens +maintainer: rowens@sumall.com +-- copyright: +-- category: +build-type: Simple +extra-source-files: README.md +cabal-version: >=1.10 + +library + exposed-modules: + Control.Concurrent.LoadDistribution + -- other-modules: + -- other-extensions: + build-depends: + PSQueue >= 1.1 && < 1.2, + base >= 4.8 && < 4.10, + containers >= 0.5.6.2 && < 0.6, + hslogger >= 1.2.1 && < 1.3, + stm >= 2.4.4.1 && < 2.5 + hs-source-dirs: src + default-language: Haskell2010 + ghc-options: -Wall diff --git a/src/Control/Concurrent/LoadDistribution.hs b/src/Control/Concurrent/LoadDistribution.hs new file mode 100644 --- /dev/null +++ b/src/Control/Concurrent/LoadDistribution.hs @@ -0,0 +1,144 @@ +{-# LANGUAGE NamedFieldPuns #-} +{- | + This module provides utilities for client-side load balancing. +-} +module Control.Concurrent.LoadDistribution ( + evenlyDistributed, + LoadBalanced, + withResource, + map, +) where + +import Prelude hiding (map) + +import Control.Concurrent.STM (atomically) +import Control.Concurrent.STM.TVar (TVar, newTVar, readTVar, writeTVar, + modifyTVar) +import Control.Exception (bracket) +import Data.PSQueue (PSQ, minView, adjust, Binding((:->)), alter, insert) +import Data.Set (Set, member) +import qualified Data.PSQueue as Q (empty) +import qualified Data.Set as Set (foldr, map) +import qualified System.Log.Logger as L (debugM) + +{- | + A load balancing technique that evenly load across multiple resources, where + "even" is figured based on the current number of simultaneous threads using + the resource. + + Some notes on implentation: The priority portion of the priority queue + is a tuple of the form (p :: Int, r :: Int). The first element, @p@, stands + for "processes", and indicates the number of simultaneous threads that are + using the specific resource. This value is incremented and decremented as + threads acquire and release the resource. The second element, @r@, stands for + "round robin". Its purpose is to act as a tie breaker between resources that + have the same @p@ value by giving priority to whichever resource has been + chosen fewer times. It is always incremented and never decremented. Without + the @r@ component, the deterministic nature of the priority queue will + heavily favor resources that come first in the sorted list of resources. +-} +evenlyDistributed + :: (Ord resource) + => IO (Set resource) + -- ^ An IO action that can be used to obtain a tagged list of + -- resources. This is an IO action because the set of resources + -- is allowed to change dynamically. + -> IO (LoadBalanced resource) +evenlyDistributed getResources = do + psQueueT <- atomically (newTVar Q.empty) + return LB {psQueueT, getResources} + + +{- | + An object that keeps track of load balancing across threads. +-} +data LoadBalanced resource = + LB { + psQueueT :: TVar (PSQ resource (Int, Int)), + getResources :: IO (Set resource) + } + + +{- | + Derives a new load balancer from an existing load balancer, allowing the + resource type to be mapped from one type to another. This requires IO + (as opposed to implementing this using @instance Functor@) because we + have to create a new shared load balancing state. +-} +map :: (Ord b) => (a -> b) -> LoadBalanced a -> IO (LoadBalanced b) +map f lb = do + psQueueT <- atomically (newTVar Q.empty) + return LB {psQueueT, getResources = Set.map f <$> getResources lb} + + +{- | + Execute some kind of IO Action with the least loaded resource. If, for + whatever reason, there are no available resources, then `Nothing` is passed + to the user-provided action function. +-} +withResource + :: (Show resource, Ord resource) + => LoadBalanced resource + -> (Maybe resource -> IO a) + -> IO a +withResource LB {psQueueT, getResources} action = do + resources <- getResources + invokeResource psQueueT resources action + + +{- | + Invoke the resource. +-} +invokeResource :: (Ord resource, Show resource) + => TVar (PSQ resource (Int, Int)) + -> Set resource + -> (Maybe resource -> IO a) + -> IO a +invokeResource psQueueT targets = + bracket checkout checkin + where + checkout = do + url <- atomically $ do + queue <- updateTargets <$> readTVar psQueueT + (newQueue, resource) <- getBest queue + writeTVar psQueueT newQueue + return resource + debugM ("Choosing: " ++ show url) + return url + where + getBest queue = + case minView queue of + Nothing -> return (queue, Nothing) + Just (resource :-> (p, r), newQueue) -> + if resource `member` targets + then return ( + insert resource (p + 1, r + 1) newQueue, + Just resource + ) + else getBest newQueue + + checkin Nothing = return () + checkin (Just resource) = atomically $ + modifyTVar + psQueueT + (adjust decrement resource) + where + decrement (p, r) = (p - 1, r) + + updateTargets queue = + Set.foldr + (alter insertMissing) + queue + targets + where + insertMissing Nothing = Just (0, 0) + insertMissing (Just p) = Just p + + +{- | + Shorthand logging. +-} +debugM :: String -> IO () +debugM = L.debugM "load-balancing" + +