AppEngine redundancy states

Redundant pairs of AppEngines can have one of the following states at a time:

• Active: The state of an AppEngine when it has communication with its partner object, its partner is in Standby-Not Ready, Standby-Sync’ing with Active, or Standby-Ready state. A Standby AppEngine transitions into this state when a failover condition is detected. In this state, an AppEngine schedules and runs deployed objects, sends checkpoint data and sends subscriber list updates to the Standby AppEngine.

• Active - Standby not Available: The state of an Active AppEngine when it determines it cannot achieve communications with its partner object. This could mean that checkpoint, subscription and alarm state changes are not successfully transmitted to the Standby object because the partner AppEngine is not deployed, number of heartbeats missed from standby objects exceeds the configured max consecutive number of heartbeats missed, or notification is received that the Standby AppEngine shutdown or is not running. If an AppEngine is in this state, it 1) continues normal execution of hosted objects, 2) cannot be manually switched to Standby state, and 3) while continuing to attempt communicate with the Standby, does not attempt to send data to the Standby object.

• Determining Failover Status: The initial state of a redundancy-enabled AppEngine when it is first started. It has not determined yet whether it is the Active or Standby AppEngine. Communication between the two AppEngines is attempted first over the RMC and then over the primary network to make this determination. If communication cannot be made after a certain time-out period, an AppEngine assumes the Active role if it has all of the code modules and checkpoint file data to do so. Continued attempts are made at communicating with its partner.

• Standby - Missed Heartbeats: The state of an AppEngine when 1) the heartbeat pings are not received from its Active partner through the RMC, but the number of missed beats haven't reached the maximum consecutive number of heartbeats missed, or 2) the heartbeats from active engine have been missed through the primary channel. When in this state, the Standby object attempts to determine whether or not the Active object failed. If a manual failover is started by using the ForceFailoverCmd attribute, it is processed only if the heartbeats were missed over the primary network and not missed over the RMC.

• Standby - Not Ready: The state of an AppEngine when one of several conditions occurs: 1) its lost communications with its partner object or it maintains communications with its partner but missed checkpoint updates or alarm state changes from the Active AppEngine, 2) new objects are deployed to the Active AppEngine and necessary files are not installed on the Standby AppEngine yet, or 3) the Standby AppEngine lost communications over the RMC before it completed synchronizing data. Typically, the AppEngine’s partner is in one of the following states: Active-Standby not Available, or Active.

• Standby - Ready: The state of an AppEngine when it completed synchronizing code modules and checkpoint data with the Active AppEngine. In this state, the AppEngine monitors for Active AppEngine failure by verifying heartbeat pings received from the Active engine and checks that all files required for execution are in sync with the Active engine. It receives the following from the Active AppEngine: checkpoint change data, subscription-related notifications, alarm state changes, and history blocks.

• Standby - Sync'ng with Active: The state of an AppEngine when it is synchronizing code modules with the Active object. If code modules exist on the Standby computer that do not exist on the Active node, they are uninstalled, and likewise, any code modules that exist on the Active node but not on the Standby node are installed. After all code modules are synchronized, the AppEngine transitions to Standby-Sync’d Code state.

• Standby - Syncing Code: The state of a Standby AppEngine that successfully synchronized all code modules with the Active object.

• Standby - Syncing Data: The state of a Standby AppEngine when all object-related data, including checkpoint and subscriber information, are synchronized with the Active object. An object in this state typically transitions to Standby-Ready state.

• Switching to Active: A transitional state when a Standby AppEngine is commanded to become Active.

• Switching to Standby: A transitional state when an Active AppEngine is commanded to become Standby. When the active engine is switched to standby, the engine will be restarted. This transition state can be switched off by the user changing the attribute of the engine.

• Failed: The state of a redundant partner when its process crashes or is terminated by the user. The AppEngine process can be restarted using Operations Control Management Console/PlatformManager.

• Unknown: The state of a redundant partner when a communication loss occurs between AppEngines or when the partner AppEngine is stopped, shutdown, or undeployed.

For examples on redundant configuration, see the AVEVA Software Global Customer Support website.