Achieving warm standby

Updated: May 06, 2022

In warm standby, another instance of the process is already resident in memory, and may be partially initialized. When the primary process fails, the overlord or the standby notices that the primary process has failed, and informs the standby process that it should now assume the duties of the primary process. For this system to work, the newly started process should arrange to create another warm standby copy of itself, in case it meets with an untimely end.

Generally, a warm standby process would be something that might take a long time to initialize (perhaps precalculating some tables), but once called into action can switch over to active mode quickly.

The MTTR of a warm standby process is in between the MTTR of cold standby and hot standby. The implementation of a warm standby process is still relatively straightforward; it works just like a newly started process, except that after it reaches a certain point in its processing, it lies dormant, waiting for the primary process to fail. Then it wakes up, performs whatever further initialization it needs to, and runs.

The reason a warm standby process may need to perform further initialization only after it's been activated is that it may depend on being able to determine the current state of the system before it can service requests, but such determination cannot be made a priori; it can only be made when the standby is about to service requests.