PubSub

PubSub is a different communication model from client-server. A publisher sends sets of values as network messages; subscribers pick the messages they’re interested in. There’s no session between them, so many subscribers can receive the same message at once. Part 14 defines PubSub.

Building blocks

Publisher                                                  Subscriber
+--------------------------------------+                   +--------------------------------+
| PubSubConnection (PublisherId)       |                   | PubSubConnection               |
|  WriterGroup (WriterGroupId)         |  NetworkMessage   |  ReaderGroup                   |
|   DataSetWriter (DataSetWriterId) ---+-----------------> |   DataSetReader                |
|     PublishedDataSet (fields)        |   UDP or broker   |    filter: PublisherId,        |
+--------------------------------------+                   |    WriterGroupId, WriterId     |
                                                           +--------------------------------+

Component	Role
PublishedDataSet	The fields to publish and where their values come from
DataSetWriter	Encodes a DataSet into DataSetMessages; identified by its DataSetWriterId
WriterGroup	Bundles the messages of several DataSetWriters into one NetworkMessage and sets the publishing interval; identified by its WriterGroupId
PubSubConnection	Holds the transport address and the PublisherId
DataSetReader	On the subscriber side, selects messages by PublisherId, WriterGroupId and DataSetWriterId and decodes their fields
DataSetMetaData	Field names and types, and the configuration version (major and minor)

Messages

A NetworkMessage consists of a header and one or more DataSetMessages:

• NetworkMessage header: PublisherId, group header (WriterGroupId, GroupVersion, NetworkMessageNumber, SequenceNumber), the list of DataSetWriterIds it carries and a security header.
• DataSetMessage: sequence number, status, configuration version, timestamp and fields. A key frame carries all fields; a delta frame carries only changed fields; there are also event and keep-alive messages.

There are two message mappings:

Mapping	Format	Characteristics
UADP	Binary	Compact and fast; header fields are switched on and off with bit flags.
JSON	Text	Easy to read and to connect to IT systems.

Transports

Transport	Address	Characteristics
UDP	opc.udp://host:port, default port 4840	No broker; unicast or multicast.
MQTT	A broker address and a topic	The broker forwards messages to subscribers; QoS sets the delivery guarantee.
AMQP	A broker address and a queue	Broker-based transport.

Security and the SKS

PubSub messages are signed, or signed and encrypted, with symmetric keys. Publishers and subscribers must share the keys of the same security group. A Security Key Service (SKS) distributes those keys: publishers and subscribers call the GetSecurityKeys method on the SKS server to get the current and upcoming keys, and switch to new keys when the key lifetime ends. The message security header carries the security token ID and a nonce.

With a broker transport, you can also protect the broker connection itself with TLS. That protection is separate from message security.

PubSub in the PDComm simulator

The PubSub workspace of the PDComm simulator works with these settings:

Item	Value
Message mapping	UADP
Transport	UDP unicast, MQTT 5 (QoS 1)
Role	Publisher and subscriber (loopback in one process), publisher, subscriber
DataSetMessage	Key frames whose fields are Variant values
Identifiers	PublisherId (UInt16, default 77), WriterGroupId (default 1), DataSetWriterId (default 17)
Received messages	Up to 256 kept; take them with pubsub_received

The simulator exchanges UADP messages with security mode None, that is, without signatures or encryption.

Try it in the UDP PubSub tutorial and the MQTT PubSub tutorial.

Next steps

That’s all the background you need. Make your first connection with the quickstart.