onDisconnect class allows you to write or clear data when your client
disconnects from the Database server. These updates occur whether your
client disconnects cleanly or not, so you can rely on them to clean up data
even if a connection is dropped or a client crashes.
onDisconnect class is most commonly used to manage presence in
applications where it is useful to detect how many clients are connected and
when other clients disconnect. See
To avoid problems when a connection is dropped before the requests can be transferred to the Database server, these functions should be called before writing any data.
onDisconnect operations are only triggered once. If you want an
operation to occur each time a disconnect occurs, you'll need to re-establish
onDisconnect operations each time you reconnect.