Heartbeat can be configured to allow clients to detect when a broker has failed and connect to another broker or cluster member.Heartbeats are sent by the broker at a client specified, per-connection frequency. If the client does not receive a heartbeat or any other traffic for two heartbeat intervals, the connection will be made to fail.

What would happen when there is a no heartbeat within a predefined interval?

If there is no traffic for two heartbeat intervals, the client will fail the connection. The application will see the exact same response as when the connection is killed.

What happens if the broker is unable to send heartbeat?

As above, if there is no other traffic the client will eventually kill the connection.

Does the client retry?

You can control the heartbeat interval on the client through the heartbeat member of ConnectionSettings (it is measured in seconds). Some of the options on policies do vary for different clients.