Application.hs

{-# LANGUAGE ConstraintKinds #-}
{- |
  This module contains the data types necessary for implementing the
  user application.
-}
module Network.Legion.Application (
  LegionConstraints,
  Persistence(..),
) where

import Data.Binary (Binary)
import Data.Conduit (Source)
import Data.Default.Class (Default)
import Network.Legion.ClusterState (ClusterPowerState)
import Network.Legion.Distribution (Peer)
import Network.Legion.Index (Indexable)
import Network.Legion.PartitionKey (PartitionKey)
import Network.Legion.PartitionState (PartitionPowerState)
import Network.Legion.PowerState (Event)

{- |
  This is a more convenient way to write the somewhat unwieldy set of
  constraints

  > (
  >   Event e o s,
  >   Indexable s,
  >   Binary e,
  >   Binary o,
  >   Binary s,
  >   Default s,
  >   Eq e,
  >   Show e,
  >   Show o,
  >   Show s
  > )

  The @ToJSON s@ requirement is strictly for servicing the admin web
  endpoints.
-}
type LegionConstraints e o s = (
    Event e o s,
    Indexable s,
    Binary e,
    Binary o,
    Binary s,
    Default s,
    Eq e,
    Show e,
    Show o,
    Show s
  )


{- |
  The type of a user-defined persistence strategy used to persist
  partition states. See 'Network.Legion.newMemoryPersistence' or
  'Network.Legion.diskPersistence' if you need to get started quickly.
-}
data Persistence e o s = Persistence {
      saveCluster :: Peer -> ClusterPowerState -> IO (),
         getState :: PartitionKey -> IO (Maybe (PartitionPowerState e o s)),
        saveState :: PartitionKey -> Maybe (PartitionPowerState e o s) -> IO (),
             list :: Source IO (PartitionKey, PartitionPowerState e o s)
                     {- ^
                       List all the keys known to the persistence layer. It is
                       important that the implementation do the right thing
                       with regard to `Data.Conduit.addCleanup`, because
                       there are cases where the conduit is terminated
                       without reading the entire list.
                     -}
  }