A stateless service is one that provides a response after your request, and then requires no further attention. To make a stateless service highly available, you need to provide redundant instances and load balance them. OpenStack services that are stateless include nova-api, nova-conductor, glance-api, keystone-api, neutron-api and nova-scheduler.
A stateful service is one where subsequent requests to the service depend on the results of the first request. Stateful services are more difficult to manage because a single action typically involves more than one request, so simply providing additional instances and load balancing will not solve the problem. For example, if the Horizon user interface reset itself every time you went to a new page, it wouldn’t be very useful. OpenStack services that are stateful include the OpenStack database and message queue.
Making stateful services highly available can depend on whether you choose an active/passive or active/active configuration.
