Use ConnectionPool type synonym, same as Yesod's scaffold.

serversession-backend-persistent/serversession-backend-persistent.cabal

@@ -24,7 +24,6 @@ library
     , path-pieces
     , persistent                == 2.1.*
     , persistent-template       == 2.1.*
-    , resource-pool
     , text
     , time
 

serversession-backend-persistent/src/Web/ServerSession/Backend/Persistent/Internal/Impl.hs

@@ -12,7 +12,6 @@ module Web.ServerSession.Backend.Persistent.Internal.Impl
   ) where
 
 import Control.Monad (void)
-import Data.Pool (Pool)
 import Data.Time (UTCTime)
 import Data.Typeable (Typeable)
 import Database.Persist (PersistEntity(..))
@@ -68,7 +67,7 @@ fromPersistentSession PersistentSession {..} =
 -- | SQL session storage backend using @persistent@.
 newtype SqlStorage =
   SqlStorage
-    { connPool :: Pool P.SqlBackend
+    { connPool :: P.ConnectionPool
       -- ^ Pool of DB connections.  You may use the same pool as
       -- your application.
     } deriving (Typeable)