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- Two Universal Controller instances (cluster nodes)
- Two High Availability#Universal Universal Message Service (OMS) network communications providers in an OMS cluster
- Four Universal Agent (Agent) machines
The components in blue are active and operating. The components in gray are available for operations but currently are inactive (passive).
See High Availability#High Availability Components for a detailed description of how each component type functions in a High Availability environment.
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At any given time under High Availability, one node operates in Active mode and the remaining nodes operate in Passive mode (see High Availability#Determining Determining Mode of a Cluster Node at Start-up).
An Active node performs all system processing functions; Passive nodes can perform limited processing functions.
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OMS provides for reliable message communication by persisting all OMS queued messages to persistent storage. The OMS Server maintains OMS queues in an OMS message database that resides on persistent storage.
See Universal Message Service (OMS) for detailed information on OMS.
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A Universal Controller HA configuration can use a single OMS server, that is not an HA cluster, with the understanding that a single OMS server would introduce a single point of failure. Using an OMS HA cluster is recommended.
See High Availability#High Availability Configuration for information on how these connections are made.
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A cluster node starts in High Availability#Passive Passive mode. It then determines if it should remain in Passive mode or switch to High Availability#Active Active mode.
The following flow chart describes how a cluster node determines its mode at start-up:
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When all cluster nodes have started, each one continuously monitors the heartbeats of the other running cluster nodes.
If a High Availability#Passive Passive cluster node determines that the High Availability#Active Active cluster node is no longer running, the Passive cluster node automatically takes over as the Active cluster node based upon the same criteria described above.
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Step 1 | The Active cluster node sends a heartbeat by updating a timestamp in the database. The heartbeat interval is 10 (seconds). |
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Step 2 | All Passive cluster nodes check the Active cluster node's timestamp to determine if it is current. (This check runs every 60 seconds.) |
Step 3 | If a Passive cluster node determines that the Active cluster node's timestamp is stale, failover occurs: the Passive cluster node changes the mode of the Active cluster node to High Availability#Offline Offline and takes over as the Active cluster node. If more than one cluster node is operating in Passive mode, the first cluster node eligible to become Active that determines that the Active cluster node is not running becomes the Active cluster node. A stale cluster node is one whose timestamp is older than 5 minutes. |
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Step 1 | Access the new Active cluster node in your browser. To determine which cluster node is now Active, check the Mode column on the Cluster Nodes list in the user interface (see High Availability#Viewing Viewing Cluster Node Status, below). |
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Step 2 | If you were adding, deleting, or updating records at the time of the failure, check the record you were working on. Any data you had not yet saved will be lost. |
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