Tablet Accessory User Manual

iIS Process Engine Components
34 iPlanet Integration Server • Process System Guide • August 2001
opens a channel to the backup unit, which takes over by recovering all the
persistent state information stored in the engine database. The new primary engine
unit starts process execution at the point where the old primary engine unit left it.
Client sessions with the engine are maintained throughout this failover transition.
Governor The governor is an application that assists in deciding when failover
should occur. The governor maintains an open communication channel with both
the primary and backup engine units. It uses these channels to determine if a
closing of the communication channel between primary and backup engine units is
due to engine unit failure or network failure. For example, if the communication
channel between the primary and backup engine units closes, but the governor’s
communication channel with the primary engine unit remains open, the governor
assumes the primary engine unit is still online. However, if neither the governor
nor backup engine unit can communicate with the primary engine unit, the
governor assumes the primary engine unit has failed, and places the backup engine
unit online, making it the new primary engine unit.
(If the primary engine unit has not failed, but cannot communicate with either the
governor or backup engine unit, the primary engine unit will drop into STANDBY
state, since it has no way of knowing the state of its partner.)
NOTE Failover (and recovery of state information) is not instantaneous. In
a production system, the state information stored in the engine
database can be quite extensive. It includes information on the state
of every process instance, the state of every activity and timer in
each process instance, the values of process attributes and their lock
states, the state of routers and triggers for each activity instance,
session activity lists, queue lists, and so on. Accordingly, the amount
of time for failover to occur is dependent on the amount of state
information in the database and the speed of database access.