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- Introduction to OPC UA
- ======================
- OPC UA, a collection of services
- --------------------------------
- In OPC UA, all communication is based on service calls, each consisting of a request and a response
- message. Be careful to note the difference between services and methods. Services are pre-defined in
- the standard and cannot be changed. But you can use the *Call* service to invoke user-defined
- methods on the server.
- For completeness, the following tables contain all services defined in the standard. Do not bother
- with their details yet. We will introduce the different services later in the text. In open62541,
- each service is implemented in a single function. See the \ref services section for details.
- **Establishing communication**
- +-----------------------------+-----------------------------+------------------------------+
- | Discovery Service Set | SecureChannel Service Set | Session Service Set |
- +=============================+=============================+==============================+
- | FindServers | OpenSecureChannel | CreateSession |
- +-----------------------------+-----------------------------+------------------------------+
- | GetEndpoints | CloseSecureChannel | ActivateSession |
- +-----------------------------+-----------------------------+------------------------------+
- | RegisterServer | | CloseSession |
- +-----------------------------+-----------------------------+------------------------------+
- | | | Cancel |
- +-----------------------------+-----------------------------+------------------------------+
- **Interaction with the information model**
- +-----------------------------+-------------------------------+------------------------------+------------------------------+----------------------+
- | Attribute Service Set | View Service Set | Method Service Set | NodeManagement Service Set | Query Service Set |
- +=============================+===============================+==============================+==============================+======================+
- | Read | Browse | Call | AddNodes | QueryFirst |
- +-----------------------------+-------------------------------+------------------------------+------------------------------+----------------------+
- | HistoryRead | BrowseNext | | AddReferences | QueryNext |
- +-----------------------------+-------------------------------+------------------------------+------------------------------+----------------------+
- | Write | TranslateBrowsePathsToNodeids | | DeleteNodes | |
- +-----------------------------+-------------------------------+------------------------------+------------------------------+----------------------+
- | HistoryUpdate | RegisterNodes | | DeleteReferences | |
- +-----------------------------+-------------------------------+------------------------------+------------------------------+----------------------+
- | | UnregisterNodes | | | |
- +-----------------------------+-------------------------------+------------------------------+------------------------------+----------------------+
- **Notifications**
- +-----------------------------+-------------------------------+
- | MonitoredItem Service Set | Subscription Service Set |
- +=============================+===============================+
- | CreateMonitoredItems | CreateSubscription |
- +-----------------------------+-------------------------------+
- | ModifyMonitoreditems | ModifySubscription |
- +-----------------------------+-------------------------------+
- | SetMonitoringMode | SetPublishingMode |
- +-----------------------------+-------------------------------+
- | SetTriggering | Publish |
- +-----------------------------+-------------------------------+
- | DeleteMonitoredItems | Republish |
- +-----------------------------+-------------------------------+
- | | TransferSubscription |
- +-----------------------------+-------------------------------+
- | | DeleteSubscription |
- +-----------------------------+-------------------------------+
- OPC UA, a web of nodes
- ----------------------
- The information model in each OPC UA server is a web of interconnected nodes.
- There are eight different types of nodes. Depending on its type, every node
- contains different attributes. Some attributes, such as the *NodeId* (unique
- identifier) and the *BrowseName*, are contained in all node types.
- +-----------------------+-------------------+
- | ReferenceTypeNode | MethodNode |
- +-----------------------+-------------------+
- | DataTypeNode | ObjectTypeNode |
- +-----------------------+-------------------+
- | VariableTypeNode | ObjectNode |
- +-----------------------+-------------------+
- | VariableNode | ViewNode |
- +-----------------------+-------------------+
-
- Nodes are interconnected by directed reference-triples of the form ``(nodeid,
- referencetype, target-nodeid)``. Therefore an OPC UA information model is
- easiest imagined as a *web of nodes*. Reference types can be
- - standard- or user-defined and
- - either non-hierarchical (e.g., indicating the type of a variable-node) or
- hierarchical (e.g., indicating a parent-child relationship).
- OPC UA, a protocol
- ------------------
- The OPC UA protocol (both binary and XML-based) is based on 25 *built-in*
- datatypes. In open62541, these are defined in ua_types.h.
- The builtin datatypes are combined to more complex structures. When the structure contains an array,
- then the size of the array is stored in an Int32 value just before the array itself. A size of -1
- indicates an undefined array. Positive sizes (and zero) have the usual semantics.
- Most importantly, every service has a request and a response message defined as such a data
- structure. The entire OPC UA protocol revolves around the exchange of these request and response
- messages. Their exact definitions can be looked up here:
- https://opcfoundation.org/UA/schemas/Opc.Ua.Types.bsd.xml. In open62541, we autogenerate the
- C-structs to handle the standard-defined structures automatically. See ua_types_generated.h for
- comparison.
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