{-# LANGUAGE CPP #-}
-- |
-- This module has functions to send commands LISTEN and NOTIFY to the database server.
-- It also has a function to wait for and handle notifications on a database connection.
--
-- For more information check the [PostgreSQL documentation](https://www.postgresql.org/docs/current/libpq-notify.html).
module Hasql.Notifications
( notifyPool,
notify,
listen,
unlisten,
waitForNotifications,
PgIdentifier,
toPgIdentifier,
fromPgIdentifier,
FatalError (..),
)
where
import Control.Concurrent (threadDelay, threadWaitRead)
import Control.Exception (Exception, throw)
import Control.Monad (forever, unless, void, when)
import Data.ByteString.Char8 (ByteString)
import Data.Functor.Contravariant (contramap)
import Data.Text (Text)
import qualified Data.Text as T
import qualified Data.Text.Encoding as T
import qualified Database.PostgreSQL.LibPQ as PQ
import Hasql.Connection (Connection, withLibPQConnection)
import qualified Hasql.Decoders as HD
import qualified Hasql.Encoders as HE
import Hasql.Pool (Pool, UsageError, use)
import Hasql.Session (run, sql, statement)
import qualified Hasql.Session as S
import qualified Hasql.Statement as HST
-- | A wrapped text that represents a properly escaped and quoted PostgreSQL identifier
newtype PgIdentifier = PgIdentifier Text deriving (Show)
-- | Uncatchable exceptions thrown and never caught.
newtype FatalError = FatalError {fatalErrorMessage :: String}
instance Exception FatalError
instance Show FatalError where
show = fatalErrorMessage
-- | Given a PgIdentifier returns the wrapped text
fromPgIdentifier :: PgIdentifier -> Text
fromPgIdentifier (PgIdentifier bs) = bs
-- | Given a text returns a properly quoted and escaped PgIdentifier
toPgIdentifier :: Text -> PgIdentifier
toPgIdentifier x =
PgIdentifier $ "\"" <> strictlyReplaceQuotes x <> "\""
where
strictlyReplaceQuotes :: Text -> Text
strictlyReplaceQuotes = T.replace "\"" ("\"\"" :: Text)
-- | Given a Hasql Pool, a channel and a message sends a notify command to the database
notifyPool ::
-- | Pool from which the connection will be used to issue a NOTIFY command.
Pool ->
-- | Channel where to send the notification
Text ->
-- | Payload to be sent with the notification
Text ->
IO (Either UsageError ())
notifyPool pool channel mesg =
use pool (statement (channel, mesg) callStatement)
where
callStatement = HST.Statement ("SELECT pg_notify" <> "($1, $2)") encoder HD.noResult False
encoder = contramap fst (HE.param $ HE.nonNullable HE.text) <> contramap snd (HE.param $ HE.nonNullable HE.text)
-- | Given a Hasql Connection, a channel and a message sends a notify command to the database
notify ::
-- | Connection to be used to send the NOTIFY command
Connection ->
-- | Channel where to send the notification
PgIdentifier ->
-- | Payload to be sent with the notification
Text ->
IO (Either S.SessionError ())
notify con channel mesg =
run (sql $ T.encodeUtf8 ("NOTIFY " <> fromPgIdentifier channel <> ", '" <> mesg <> "'")) con
-- |
-- Given a Hasql Connection and a channel sends a listen command to the database.
-- Once the connection sends the LISTEN command the server register its interest in the channel.
-- Hence it's important to keep track of which connection was used to open the listen command.
--
-- Example of listening and waiting for a notification:
--
-- @
-- import System.Exit (die)
--
-- import Hasql.Connection
-- import Hasql.Notifications
--
-- main :: IO ()
-- main = do
-- dbOrError <- acquire "postgres://localhost/db_name"
-- case dbOrError of
-- Right db -> do
-- let channelToListen = toPgIdentifier "sample-channel"
-- listen db channelToListen
-- waitForNotifications (\channel _ -> print $ "Just got notification on channel " <> channel) db
-- _ -> die "Could not open database connection"
-- @
listen ::
-- | Connection to be used to send the LISTEN command
Connection ->
-- | Channel this connection will be registered to listen to
PgIdentifier ->
IO ()
listen con channel =
void $ withLibPQConnection con execListen
where
execListen = executeOrPanic $ T.encodeUtf8 $ "LISTEN " <> fromPgIdentifier channel
-- | Given a Hasql Connection and a channel sends a unlisten command to the database
unlisten ::
-- | Connection currently registerd by a previous 'listen' call
Connection ->
-- | Channel this connection will be deregistered from
PgIdentifier ->
IO ()
unlisten con channel =
void $ withLibPQConnection con execUnlisten
where
execUnlisten = executeOrPanic $ T.encodeUtf8 $ "UNLISTEN " <> fromPgIdentifier channel
executeOrPanic :: ByteString -> PQ.Connection -> IO ()
executeOrPanic cmd pqCon = do
mResult <- PQ.exec pqCon cmd
case mResult of
Nothing -> do
mError <- PQ.errorMessage pqCon
panic $ maybe ("Error executing" <> show cmd) (T.unpack . T.decodeUtf8Lenient) mError
Just result -> do
status <- PQ.resultStatus result
when (status == PQ.FatalError) $ do
mError <- PQ.resultErrorMessage result
panic $ maybe ("Error executing" <> show cmd) (T.unpack . T.decodeUtf8Lenient) mError
-- |
-- Given a function that handles notifications and a Hasql connection it will listen
-- on the database connection and call the handler everytime a message arrives.
--
-- The message handler passed as first argument needs two parameters channel and payload.
-- See an example of handling notification on a separate thread:
--
-- @
-- import Control.Concurrent.Async (async)
-- import Control.Monad (void)
-- import System.Exit (die)
--
-- import Hasql.Connection
-- import Hasql.Notifications
--
-- notificationHandler :: ByteString -> ByteString -> IO()
-- notificationHandler channel payload =
-- void $ async do
-- print $ "Handle payload " <> payload <> " in its own thread"
--
-- main :: IO ()
-- main = do
-- dbOrError <- acquire "postgres://localhost/db_name"
-- case dbOrError of
-- Right db -> do
-- let channelToListen = toPgIdentifier "sample-channel"
-- listen db channelToListen
-- waitForNotifications notificationHandler db
-- _ -> die "Could not open database connection"
-- @
waitForNotifications ::
-- | Callback function to handle incoming notifications
(ByteString -> ByteString -> IO ()) ->
-- | Connection where we will listen to
Connection ->
IO ()
waitForNotifications sendNotification con =
withLibPQConnection con $ void . forever . pqFetch
where
pqFetch pqCon = do
mNotification <- PQ.notifies pqCon
case mNotification of
Nothing -> do
mfd <- PQ.socket pqCon
case mfd of
Nothing -> void $ threadDelay 1000000
Just fd -> do
void $ threadWaitRead fd
result <- PQ.consumeInput pqCon
unless result $ do
mError <- PQ.errorMessage pqCon
panic $ maybe "Error checking for PostgreSQL notifications" (T.unpack . T.decodeUtf8Lenient) mError
Just notification ->
sendNotification (PQ.notifyRelname notification) (PQ.notifyExtra notification)
panic :: String -> a
panic a = throw (FatalError a)