Keep-alive
Keep-alive in OPC UA means two different mechanisms. The session keep-alive is a periodic cheap read the client makes to prove the session is alive and to detect a dead server. The subscription keep-alive is an empty NotificationMessage the server sends when a publishing cycle produced no data. Both exist because silence is ambiguous.
The session keep-alive is the client's job
Clients read Server, ServerStatus, CurrentTime - or the whole ServerStatus - on a timer of roughly a third of the revised session timeout, count consecutive failures, and trigger reconnection after two or three. Reference stacks expose this as a keep-alive interval plus a status event, and every reconnect handler is driven from it. It also yields a continuous clock-offset measurement, which matters because skew causes certificate errors.
The subscription keep-alive is the server's job
If a subscription produces no notifications for MaxKeepAliveCount publishing cycles, the server answers an outstanding Publish with a keep-alive message: sequence-number bookkeeping, no data. That tells the client the subscription still exists and the pipe is clear. It says nothing about whether the underlying process value is still being updated - that is a data-level question a heartbeat answers.
TCP keepalive is not a substitute
Operating-system TCP keepalive defaults to two hours on Linux and Windows, so a silently dropped connection - a NAT table expiry, a firewall state timeout, a pulled fibre - is invisible for far longer than any industrial application can tolerate. The application-level timers above are what actually detect it, and they are also what makes a half-open connection detectable at all.
Example
Three different silences, three different detectors
Session keep-alive read ServerStatus every 10 s detects: server or link gone
Subscription keep-alive server sends every N cycles detects: subscription deleted server-side
Data heartbeat client re-emits last value detects: value stopped updating
A subscription keep-alive arriving on time while a tag has not changed
for an hour proves the connection is healthy - and nothing about the tag.Defined in: Part 4 - Services, Publish and the subscription keep-alive; session keep-alive is client behaviour, not a service.
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.