The Subscription Service Set is used by the Client to create and maintain Subscriptions. Subscriptions are entities that periodically publish NotificationMessages for the MonitoredItem assigned to them (see 6.10). The NotificationMessage contains a common header followed by a series of Notifications. The format of Notifications is specific to the type of item being monitored (i.e. Variables, Attributes, and EventNotifiers).
Once created, the existence of a Subscription is independent of the Client’s Session with the Server. This allows one Client to create a Subscription, and a second, possibly a redundant Client, to receive NotificationMessages from it.
To protect against non-use by Clients, Subscriptions have a configured lifetime that Clients periodically renew. If any Client fails to renew the lifetime, the lifetime expires and the Subscription is closed by the Server. When a Subscription is closed, all MonitoredItems assigned to the Subscription are deleted.
Subscriptions include features that support detection and recovery of lost Messages. Each NotificationMessage contains a sequence number that allows Clients to detect missed Messages. When there are no Notifications to send within the keep-alive time interval, the Server sends a keep-alive Message that contains the sequence number of the next NotificationMessage sent. If a Client fails to receive a Message after the keep-alive interval has expired, or if it determines that it has missed a Message, it can request the Server to resend one or more Messages.
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DOCPROPERTY Part2URL \* MERGEFORMAT http://www.opcfoundation.org/UA/Part2/
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DOCPROPERTY Part3URL \* MERGEFORMAT http://www.opcfoundation.org/UA/Part3/
OPC 10000-4, OPC Unified Architecture - Part 4: Services
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OPC 10000-5, OPC Unified Architecture - Part 5: Information Model
DOCPROPERTY Part5URL \* MERGEFORMAT http://www.opcfoundation.org/UA/Part5/
OPC 10000-6, OPC Unified Architecture - Part 6: Mappings
DOCPROPERTY Part6URL \* MERGEFORMAT http://www.opcfoundation.org/UA/Part6/
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DOCPROPERTY Part7URL \* MERGEFORMAT http://www.opcfoundation.org/UA/Part7/
OPC 10000-8, OPC Unified Architecture - Part 8: Data Access
DOCPROPERTY Part8URL \* MERGEFORMAT http://www.opcfoundation.org/UA/Part8/
OPC 10000-9, OPC Unified Architecture - Part 9: Alarms and Conditions
DOCPROPERTY Part9URL \* MERGEFORMAT http://www.opcfoundation.org/UA/Part9/
OPC 10000-10, OPC Unified Architecture - Part 10: Programs
DOCPROPERTY Part10URL \* MERGEFORMAT http://www.opcfoundation.org/UA/Part10/
OPC 10000-11, OPC Unified Architecture - Part 11: Historical Access
DOCPROPERTY Part11URL \* MERGEFORMAT http://www.opcfoundation.org/UA/Part11/
OPC 10000-12, OPC Unified Architecture - Part 12: Discovery and Global Services
DOCPROPERTY Part12URL \* MERGEFORMAT http://www.opcfoundation.org/UA/Part12/
OPC 10000-13, OPC Unified Architecture - Part 13: Aggregates
DOCPROPERTY Part13URL \* MERGEFORMAT http://www.opcfoundation.org/UA/Part13/
OPC 10000-14, OPC Unified Architecture - Part 14: PubSub
DOCPROPERTY Part14URL \* MERGEFORMAT http://www.opcfoundation.org/UA/Part14/
OPC 10000-15, OPC Unified Architecture - Part 15: Safety
DOCPROPERTY Part15URL \* MERGEFORMAT http://www.opcfoundation.org/UA/Part15/
OPC 10000-16, OPC Unified Architecture - Part 16: State Machines
DOCPROPERTY Part16URL \* MERGEFORMAT http://www.opcfoundation.org/UA/Part16/
OPC 10000-17, OPC Unified Architecture - Part 17: Alias Names
DOCPROPERTY Part17URL \* MERGEFORMAT http://www.opcfoundation.org/UA/Part17/
OPC 10000-18, OPC Unified Architecture - Part 18: User Authorization
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OPC 10000-19, OPC Unified Architecture - Part 19: Dictionary References
DOCPROPERTY Part19URL \* MERGEFORMAT http://www.opcfoundation.org/UA/Part19/
OPC 10000-20, OPC Unified Architecture - Part 20: File Transfer
DOCPROPERTY Part20URL \* MERGEFORMAT http://www.opcfoundation.org/UA/Part20/
OPC 10000-21, OPC Unified Architecture - Part 21: Device Onboarding
DOCPROPERTY Part21URL \* MERGEFORMAT http://www.opcfoundation.org/UA/Part21/
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