Subscription
A subscription is the server-side container that groups MonitoredItems and delivers their notifications on one cyclic publishing interval, in response to Publish requests the client leaves outstanding. It owns PublishingInterval, LifetimeCount, MaxKeepAliveCount, MaxNotificationsPerPublish, Priority and PublishingEnabled. Everything about what and how often to sample belongs to the items, not here.
The parameter split is the whole design
Subscription-level parameters control delivery: how often the queues are drained, how many notifications one Publish response may carry, how long silence is tolerated, and which subscription wins when the server is saturated. Item-level parameters control acquisition: what to watch, how fast to sample it, what to filter out, how much to buffer. Confusing the two is the root cause of most subscription tuning failures.
One subscription per distinct publishing interval
A subscription has exactly one publishing interval, so every distinct rate you want forces another subscription - a client that wants fifty different rates ends up with fifty subscriptions, each with its own Publish pipeline, keep-alive traffic and lifetime bookkeeping. Standardise on a small set of rates such as 100, 1000 and 10000 ms, and use per-item sampling intervals for the finer differences.
Delivery, sequence numbers and gaps
Each NotificationMessage carries a sequence number. The client acknowledges numbers in its next Publish, and the server keeps unacknowledged messages available for Republish, which is how a client recovers a message lost across a reconnect. If the client stops publishing for LifetimeCount cycles the server deletes the subscription outright and later calls return BadSubscriptionIdInvalid.
Example
Where each parameter lives
Subscription (one per publishing rate) MonitoredItem (one per attribute)
PublishingInterval 1000 ms SamplingInterval 250 ms / 0 / -1
MaxKeepAliveCount 10 QueueSize 4
LifetimeCount 60 DiscardOldest true
MaxNotificationsPerPublish 0 = unlimited Filter DataChangeFilter
Priority 0..255 MonitoringMode Reporting
PublishingEnabled true ClientHandle 17
Fifty different publishing intervals = fifty subscriptions. Consolidate the rates.Defined in: Part 4 - Services, Subscription Service Set (CreateSubscription, ModifySubscription, SetPublishingMode, Publish, Republish, TransferSubscriptions, DeleteSubscriptions).
Last verified 7 September 2026 against OPC UA 1.05.07. OPC UA Connect is an educational resource and is not affiliated with the OPC Foundation.