The tutorial describes how to achieve HA in OpenstackSwitching application. Basically, ONOS supports HA by building ONOS cluster. You can check how to build the ONOS cluster from the ONOS Tutorial. If you have no knowledge of ONOS clustering, we strongly recommend to go through the wiki page before you start this tutorial.
In addition to the ONOS clustering, we need to set up a proxy server for the REST API HA as below.