IHP Api Reference
Copyright(c) digitally induced GmbH 2021
Safe HaskellNone



This module is solving the problem, that previously IHP was using one database connection per running LISTEN ..; statement. A PGListener provides one central object to listen on postgres channels, without manually dealing with connection management.



type Channel = ByteString Source #

The channel is like the event name

It's used in the postgres NOTIFY call:

NOTIFY channel [ , payload ]

type Callback = Notification -> IO () Source #

An event callback receives the postgres notification object and can do IO

data Subscription Source #

Returned by a call to subscribe




data PGListener Source #

The main datatype of the service. Keeps tracks of all channels we're watching on, as well as all open subscriptions

Use init to create a new object and stop to deallocate it.

init :: ModelContext -> IO PGListener Source #

Creates a new PGListener object

let modelContext = ..
pgListener <- PGListener.init modelContext

This will start a new async listening for postgres notifications. This will take one connection from the database pool and keep it blocked until stop is called.

stop :: PGListener -> IO () Source #

Stops the database listener async and puts the database connection used back into the database pool

PGListener.stop pgListener

subscribe :: Channel -> Callback -> PGListener -> IO Subscription Source #

After you subscribed to a channel, the provided callback will be called whenever there's a new notification on the channel.

pgListener <- PGListener.init

let callback notification = do
        let payload :: Text = cs (get #notificationData notification)
        putStrLn ("Received notification: " <> payload)

subscription <- pgListener |> PGListener.subscribe "my_channel" callback

The callback function will now be called whenever NOTIFY "my_channel", "my payload" is executed on the postgres server.

When the subscription is not used anymore, call unsubscribe to stop the callback from being called anymore.

subscribeJSON :: FromJSON jsonValue => Channel -> (jsonValue -> IO ()) -> PGListener -> IO Subscription Source #

Like subscribe but decodes the notification payload from JSON and passes the decoded data structure to the callback

When JSON parsing fails, this will ignore the notification.

pgListener <- PGListener.init

let callback (jsonObject :: Aeson.Value) = do
        putStrLn ("Received notification: " <> tshow jsonObject)

subscription <- pgListener |> PGListener.subscribeJSON "my_json_channel" callback

The callback function will now be called whenever NOTIFY "my_json_channel", "{"hello":"world"}" is executed on the postgres server.

unsubscribe :: Subscription -> PGListener -> IO () Source #

Stops the callback of a subscription from receiving further notifications

pgListener <- PGListener.init

subscription <- pgListener |> PGListener.subscribe "my_channel" callback
pgListener |> PGListener.unsubscribe subscription