WriterGroup
In OPC UA PubSub, a WriterGroup batches DataSetWriters into a single NetworkMessage and owns the timing: PublishingInterval, KeepAliveTime, priority, encoding and security group. It sits under a PubSubConnection, stamps a WriterGroupId into every message it sends, and is the level at which you enable, disable and secure publishing.
The publisher hierarchy in one line
PubSubConnection holds the transport and the PublisherId; a WriterGroup under it holds the timing and the encoding; DataSetWriters under the group each publish one PublishedDataSet. The subscriber side mirrors it exactly: PubSubConnection, then ReaderGroup, then DataSetReader with a SubscribedDataSet. Enable and disable cascade downward, so disabling a connection stops every group beneath it.
Publishing here is nothing like a subscription
The group PublishingInterval is a hard cyclic timer on the publisher. Nothing is requested, nothing is acknowledged and nothing is retransmitted, which is the opposite of a client-server Subscription where the server can only send in response to a queued Publish request. There is no back-pressure and no lifetime count: if no subscriber exists, the publisher keeps transmitting at full rate. KeepAliveTime governs keep-alive messages when a cycle produces no data change.
Batching, size limits and ordering
MaxNetworkMessageSize caps one NetworkMessage; if the enabled writers produce more than that, the group splits the payload across messages. NetworkMessageContentMask decides which optional headers appear, and DataSetOrdering states whether DataSetMessages arrive in a defined order. Batching several writers into one message cuts per-frame overhead considerably, at the cost of tying all of them to the same cadence.
Security and quality of service sit at group level
SecurityMode and SecurityGroupId are group properties, so the group is the unit of key distribution from a Security Key Service. On the datagram-v2 connection transport the group also exposes the QoS hooks that matter for TSN: QosCategory, which maps to the IPv4 or IPv6 DSCP bits, plus MessageRepeatCount and MessageRepeatDelay, where receivers deduplicate repeats on the sequence number rather than seeing them twice.
Example
The full publisher tree, showing which level owns which decision
PubSubConnection "Cell3" opc.udp://239.0.0.1:4840 PublisherId 17 (UInt16)
WriterGroup "Fast" WriterGroupId 1
PublishingInterval 100 ms <- timing lives here
KeepAliveTime 1000 ms
SecurityMode SignAndEncrypt <- keys live here
SecurityGroupId "sg-cell3"
MaxNetworkMessageSize 1400 <- keep under path MTU
DataSetWriter "Press" DataSetWriterId 1 <- content lives here
DataSetWriter "Counters" DataSetWriterId 2Defined in: Part 14 - PubSub, WriterGroup and connection transport data types.
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.