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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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- Perform a workflow instance insert task operation.
- Perform a bulk import or list import.
- Run the LDAP Refresh server operation.
- Update a task instance.
- Update or delete an enabled trigger.
- Update an enabled Data Backup/Purge.
- Update the Task Execution Limit field in Agent records.
- Update the Task Execution Limit field and Distribution field in Agent Cluster records.
- Update the user Time Zone.
- List Composite Trigger component events.
However, Passive nodes do let you perform a limited number of processing functions, such as:
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If an Agent fails, Universal Broker restarts it. The Agent then attempts to determine what tasks or functions were in process at the time of failure.
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Warm Start Processing is a term used to refer to a process UAG goes through upon startup by which all task instances that were active at the time of the last shutdown (intentional or otherwise) are reviewed and proper action is taken based on state and platform.
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In order to support such a determination, Agent task processing includes the following steps:
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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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To view a list of all cluster nodes, from the Agents & Connections navigation pane select System > Cluster Nodes. The Cluster Nodes list identifies all registered cluster nodes. The Mode column on the list identifies the current mode (status) of all cluster nodes.
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A cluster node becomes registered the first time it starts. From then on, it always appears in the Cluster Nodes list, regardless of its current mode. |
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All cluster nodes in a High Availability environment must point to the same database by making sure the following entries in their uc.properties files are the same.
For example:
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OMS HA cluster configuration is described in the OMS Reference Guide.
The Universal Controller OMS Server definitions specify an OMS HA cluster as an ordered, comma-separated list of OMS Server addresses, one for each member of the OMS HA cluster.
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If you want to configure an Agent to be able to access an OMS HA cluster, you must configure the Universal Automation Center Agent (UAG) OMS_SERVERS configuration option.
Configuring Notifications Based on Component Status
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