public interface ConnectionPoolListener {

    // Since a connection becomes active immediately after it is open, 
    // this method can be replaced with `connectionActive()`.
    @Deprecated
    void connectionOpen(SessionProtocol protocol,
                        InetSocketAddress remoteAddr,
                        InetSocketAddress localAddr,
                        AttributeMap attrs) throws Exception;

    // This method will be invoked when either 1) a connection is established or
    // 2) an idle connection becomes active.
    default void connectionActive(SessionProtocol protocol,
                                  InetSocketAddress remoteAddr,
                                  InetSocketAddress localAddr,
                                  AttributeMap attrs, boolean isNew) throws Exception {
        if (isNew) {
            connectionOpen(protocol, remoteAddr, localAddr, attrs);
        }
    }

    // This method will be invoked when `KeepAliveHandler` detects 
    // that the connection's inactivity exceeds the idle timeout.
    default void connectionIdle(SessionProtocol protocol,
                                InetSocketAddress remoteAddr,
                                InetSocketAddress localAddr,
                                AttributeMap attrs) throws Exception {}

    default void connectionFailed(SessionProtocol protocol,
                                  InetSocketAddress remoteAddr,
                                  InetSocketAddress localAddr,
                                  Throwable cause) throws Exception {}

    void connectionClosed(SessionProtocol protocol,
                          InetSocketAddress remoteAddr,
                          InetSocketAddress localAddr,
                          AttributeMap attrs) throws Exception;

}