Highly available process support systems: implementing backup mechanisms

Availability in process support systems (PSS) can be achieved by using standby mechanisms that allow a backup server to take over in case a primary server fails. These mechanisms, resembling the process pair approach used in operating systems, require the primary to send information about state changes to the backup on a regular basis. In PSS where all relevant state information is stored in a database, there are two principal strategies for synchronizing a primary-backup pair. One is to use the replication mechanisms provided by the DBMS. Another is to implement a message mechanism to exchange state information between servers above the database level. For both approaches, several variants exist that allow to trade run-time performance for failover time. This paper discusses the possible strategies and evaluates their performance based on an implementation within the OPERA process support kernel.