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diff --git a/src/Cache.hs b/src/Cache.hs
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+module Cache where
+
+import Control.Concurrent.STM
+import Control.Monad (forM_)
+import Data.ByteString (ByteString)
+import Data.ByteString qualified as BS
+import Data.Map.Strict (Map)
+import Data.Map.Strict qualified as Map
+
+import ZNC
+
+
+-- value, previous (more recently accessed), next (less recently accessed)
+data LRUNode a = LRUNode a (TVar (Maybe (LRUNode a))) (TVar (Maybe (LRUNode a)))
+
+-- head (most recent), last (least recent)
+data LRUChain a = LRUChain (TVar (Maybe (LRUNode a))) (TVar (Maybe (LRUNode a)))
+
+lruchainNew :: IO (LRUChain a)
+lruchainNew = LRUChain <$> newTVarIO Nothing <*> newTVarIO Nothing
+
+lruchainAdd :: LRUChain a -> a -> STM (LRUNode a)
+lruchainAdd (LRUChain hd tl) x = do
+  mhdnode <- readTVar hd
+  case mhdnode of
+    Nothing -> do  -- chain empty
+      node <- LRUNode x <$> newTVar Nothing <*> newTVar Nothing
+      writeTVar hd (Just node)
+      writeTVar tl (Just node)
+      return node
+    Just hdnode -> do
+      node <- LRUNode x <$> newTVar Nothing <*> newTVar (Just hdnode)
+      writeTVar hd (Just node)
+      return node
+
+lruchainBump :: LRUChain a -> LRUNode a -> STM ()
+lruchainBump (LRUChain hd _) node@(LRUNode _ prevvar nextvar) = do
+  mprev <- readTVar prevvar
+  case mprev of
+    Nothing -> return ()  -- already most recent
+    Just (LRUNode _ _ prevnextvar) -> do
+      -- remove node from chain where it is now
+      mnext <- readTVar nextvar
+      writeTVar prevnextvar mnext
+      case mnext of
+        Nothing -> return ()
+        Just (LRUNode _ nextprevvar _) -> writeTVar nextprevvar mprev
+      -- add node to chain at head
+      writeTVar nextvar =<< readTVar hd
+      writeTVar hd (Just node)
+
+lruchainEvict :: LRUChain a -> STM (Maybe a)
+lruchainEvict (LRUChain _ tl) = do
+  mtlnode <- readTVar tl
+  case mtlnode of
+    Nothing -> return Nothing
+    Just (LRUNode x prevvar _) -> do
+      mprev <- readTVar prevvar
+      writeTVar tl mprev
+      case mprev of
+        Nothing -> return ()
+        Just (LRUNode _ _ prevnextvar) -> writeTVar prevnextvar Nothing
+      return (Just x)
+
+-- maxsize, elements in cache, linked list
+data LRU a = LRU Int (TVar (Map a (LRUNode a))) (LRUChain a)
+
+lruNew :: Ord a => Int -> IO (LRU a)
+lruNew maxsize = LRU maxsize <$> newTVarIO mempty <*> lruchainNew
+
+-- | If a value got evicted, it is returned
+lruBump :: Ord a => LRU a -> a -> IO (Maybe a)
+lruBump (LRU maxsize mapvar chain) value = atomically $ do
+  mp <- readTVar mapvar
+  case Map.lookup value mp of
+    Nothing -> do
+      node <- lruchainAdd chain value
+      writeTVar mapvar $! Map.insert value node mp
+      if Map.size mp >= maxsize
+        then do
+          mret <- lruchainEvict chain
+          forM_ mret $ \ret -> writeTVar mapvar $! Map.delete ret mp
+          return mret
+        else return Nothing
+    Just node -> do
+      lruchainBump chain node
+      return Nothing