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 7.9). 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.