Konakart 6.0.0.0 in Liferay portal 6.0.6.
05:58:34,926 ERROR [SerializableSessionAttributeListener:39] com.konakart.al.KKAppEng is not serializable and will prevent this session from being replicated
The KonaKart store front application saves non serializable objects in the session so the sessions cannot be serialized and shared between application servers. We tell our customers to use sticky sessions for the front end. Even if it were serializable, we save quite a lot of data on the session for performance reasons and so we wouldn't recommend serializing it because that would degrade performance considerably.
You can still cluster Liferay as long as you use the sticky session approach. Also note that the KonaKart engines are stateless (it is only the store front application that saves state in the session) so if you develop any of your own portlets using the API then you are free to manage the session as you like.