For better efficiency and more precision in monitoring the state of the servers, it is recommended to run a watchdog as a separate component of any HotStandby (HSB) configuration.
However, if only the two HSB servers are available, run the watchdog on the same machine as the secondary server. If the primary server fails, then the watchdog is able to switch the secondary server to become the new primary server.
There are some disadvantages to putting a watchdog in the same machine as the secondary server. The disadvantages include:
▪ If only the communication link between the watchdog and the primary server is down, this configuration can result in a false switchover between the primary server and the secondary server.
▪ The communication link becomes a "single point of failure", that is, a single failure that might disable the entire system. (In most HSB configurations, the entire system is not disabled unless there are at least two failures.)
▪ If there is a network failure and the secondary server cannot communicate with the primary server, the users and applications can still access the primary server and theoretically could continue operating with the primary server. However, if the primary server stops accepting transactions (switches to PRIMARY UNCERTAIN state), the watchdog cannot notify the primary server to continue operating, for example by switching to PRIMARY ALONE state.