API for notifications (pub/sub) in PostgreSQL.

In order to use it, first you need to start the notification process. In your supervision tree:

{Postgrex.Notifications, name: MyApp.Notifications}

Then you can listen to certain channels:

{:ok, listen_ref} = Postgrex.Notifications.listen(MyApp.Notifications, "channel")

Now every time a message is broadcast on said channel, for example via PostgreSQL command line:

NOTIFY "channel", "Oh hai!";

You will receive a message in the format:

{:notification, notification_pid, listen_ref, channel, message}

Async connect, auto-reconnects and missed notifications

By default, the notification system establishes a connection to the database on initialization, you can configure the connection to happen asynchronously. You can also configure the connection to automatically reconnect.

Note however that when the notification system is waiting for a connection, any notifications that occur during the disconnection period are not queued and cannot be recovered. Similarly, any listen command will be queued until the connection is up.

There is a race condition between starting to listen and notifications being issued "at the same time", as explained in the PostgreSQL documentation. If your application needs to keep a consistent representation of data, follow the three-step approach of first subscribing, then obtaining the current state of data, then handling the incoming notifications.

Beware that the same race condition applies to auto-reconnects. A simple way of dealing with this issue is not using the auto-reconnect feature directly, but monitoring and re-starting the Notifications process, then subscribing to channel messages over again, using the same three-step approach.

A note on casing

While PostgreSQL seems to behave as case-insensitive, it actually has a very peculiar behaviour on casing. When you write:

SELECT * FROM POSTS

PostgreSQL actually converts POSTS into the lowercase posts. That's why both SELECT * FROM POSTS and SELECT * FROM posts feel equivalent. However, if you wrap the table name in quotes, then the casing in quotes will be preserved.

These same rules apply to PostgreSQL notification channels. More importantly, whenever Postgrex.Notifications listens to a channel, it wraps the channel name in quotes. Therefore, if you listen to a channel named "fooBar" and you send a notification without quotes in the channel name, such as:

NOTIFY fooBar, "Oh hai!";

The notification will not be received by Postgrex.Notifications because the notification will be effectively sent to "foobar" and not "fooBar". Therefore, you must guarantee one of the two following properties:

1. If you can wrap the channel name in quotes when sending a notification, then make sure the channel name has the exact same casing when listening and sending notifications

2. If you cannot wrap the channel name in quotes when sending a notification, then make sure to give the lowercased channel name when listening