Documentation

The channel is like the event name

It's used in the postgres NOTIFY call:

NOTIFY channel [ , payload ]

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

Returned by a call to subscribe

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.

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.

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

PGListener.stop pgListener

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 (notification.notificationData)
        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.

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.

Stops the callback of a subscription from receiving further notifications

pgListener <- PGListener.init

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