# Notifications and Listeners

"Observer" base class for notifications. More...

"Observer" base class for notifications.

To listen on a notification issued using the NOTIFY command, derive your own class from notify_listener and define its function call operator to perform whatever action you wish to take when the given notification arrives. Then create an object of that class and pass it to your connection. DO NOT use raw SQL to listen for notifications, or your attempts to listen won't be resumed when a connection fails--and you'll have no way to notice.

Notifications never arrive inside a backend transaction. Therefore, unless you may be using a nontransaction when a notification arrives, you are free to open a transaction of your own inside your listener's function invocation operator.

Notifications you are listening for may arrive anywhere within libpqxx code, but be aware that PostgreSQL defers notifications occurring inside transactions. (This was done for excellent reasons; just think about what happens if the transaction where you happen to handle an incoming notification is later rolled back for other reasons). So if you're keeping a transaction open, don't expect any of your listeners on the same connection to be notified.

Multiple listeners on the same connection may listen on a notification of the same name. An incoming notification is processed by invoking all listeners (zero or more) of the same name.