*6.9.6 Dual-role operation*

To maximize resource usage, [24] introduces a technique which enables a host to operate as the primary host for some applications and to be the backup host for some other applications. In this architecture, clients send their requests to the backup host first, then the backup host transmits those requests to the primary host. After processing, the primary host sends a log to the backup and eventually replies to the clients. When a failure happens, the primary host becomes unavailable, and the backup host has to handle the requests sent to the failed host. However, this technique cannot guarantee a good service restoration by itself, because the backup site must share the resources between the requests that are directly sent to it and the redirected requests.
