Address & Identity Types¶

In OPC UA, every node, every browse name, and every human-readable string is addressed with one of four built-in datatypes. The names and shapes come straight from the OPC UA specification — see OPC UA Part 3 — Address Space Model, §8 Standard DataTypes for the normative definitions.

o6\Python type
`o6.NodeId`	The unique handle of a node in a server's address space.
`o6.ExpandedNodeId`	A `NodeId` plus the optional namespace URI and server index — needed when the node lives outside the local server (e.g. in another server behind a server array).
`o6.QualifiedName`	The (namespace, name) pair used as a node's browse name — the locale-independent path component that address-space navigation walks along.

NodeId¶

The machine-readable identity of a node. Every operation on the address space (read, write, browse, call, monitor, …) ultimately targets a NodeId. Structurally a NodeId is just two fields:

NodeId {
    namespace_index     # which naming authority the identifier belongs to
    id                  # numeric, string, GUID
}

Parsing NodeId¶

o6.NodeId's can be constructed from several different expressions, i.e. from namespace index and id parameter:

o6.NodeId(84)                       # ns=0 shorthand, numeric
o6.NodeId(ns=1, i=5)                # numeric (ns, i)
o6.NodeId(ns=1, s="Temperature")    # string   (ns, s)
o6.NodeId(ns=1, b=b"\x00\x01")      # bytestring (ns, b) — rare
o6.NodeId(ns=1, g=uuid.UUID("…"))   # GUID (ns, g) — globally unique

Namespace index and id can also be parsed from a single string:

o6.NodeId("i=84")                   # ns=0 numeric shorthand
o6.NodeId("ns=1;s=Temperature")     # string id
o6.NodeId("ns=1;i=5")               # numeric id
o6.NodeId("ns=1;b=AQIDBA==")        # bytestring (base64)
o6.NodeId("ns=1;g=09087e75-…")      # GUID

Parsing from shortname or URI¶

o6\Python provides a complete abstraction from manually handling namespace indices. As an extension NodeIds may be constructied by supplying a shortname, a human-readable string, that uniquely identifies a nodeset, or an (ammended) URI.

o6.NodeId("ns=di;i=1")                                                  # ns by shortname
o6.NodeId("nsu=http://opcfoundation.org/UA/DI/;i=1")                    # ns by URI
o6.NodeId("nsu=http://opcfoundation.org/UA/DI/@::global!1.05.0;i=1")    # ns by URI + scope + version

Note

URI alone is not necessarily unique, because there may be cases in which the same nodeset in different version needs to be addressed. To disambiguate the URI string can be ammended with a scope and version number. See Namespaces for more information.

Getting OPC UA type information¶

Finally NodeId can also be constructed from DataTypes to easily obtain their id:

o6.NodeId(o6.Double)                                    # >>> o6.NodeId(i=11)
dbl = o6.Double(3.1415)
o6.NodeId(type(dbl))                                    # >>> o6.NodeId(i=11)
o6.NodeId(o6.ns.di.datatypes.DeviceHealthEnumeration)   # >>> o6.NodeId('ns=di;i=6244')

Inspecting a NodeId's namespace¶

In o6\Python we can obtain additional information from a NodeId instance through it's namespace index. The NodeId.ns property is therefore not just an integer but makes namespace metadata retrievable. Among other information e.g.:

n = o6.NodeId("ns=di;i=1")      # assume ns=2 is 'di'
n.ns.shortname                  # 'di'
n.ns.uri                        # 'http://opcfoundation.org/UA/DI/'
n.ns.version                    # '1.05.0'

ExpandedNodeId¶

Same identity as NodeId, but augmented with a namespace URI (nsu) and an optional server index (svr). You will see ExpandedNodeId almost everywhere a server hands you a reference — Browse results, method outputs, monitored-item notifications — because the spec uses the expanded form whenever there is any ambiguity about which namespace or server the target lives in.

Like for NodeId, an ExpandedNodeId allows us to retrieve additional information through its namespace index via the ns property, see here

eni = o6.ExpandedNodeId("svr=1;nsu=http://opcfoundation.org/UA/DI/;i=5")
eni.ns       # Namespace module when loaded; otherwise its numeric index
eni.id       # id
eni.nsu      # 'http://opcfoundation.org/UA/DI/' — namespace URI
eni.svr      # server index

QualifiedName¶

A QualifiedName is the cross-server, locale-independent, human-readable vocabulary used to describe nodes.

A QualifiedName carries the meaning of a node as a (namespace, name) pair. The same QualifiedName therefore identifies the same idea on every server, while the underlying NodeId is free to differ.

o6.QualifiedName(1, "Temperature")                                  # namespace index + name
o6.QualifiedName("1:Pump")                                          # ns:name shorthand
o6.QualifiedName("ns=di;DeviceSet")                                # ns by shortname
o6.QualifiedName("nsu=http://opcfoundation.org/UA/DI/;DeviceSet")   # ns by URI

A QualifiedName exposes two properties, like for NodeId and ExpandedNodeId we can retrieve additional information through its namespace index via the ns property, see here

qn = o6.QualifiedName(1, "Temperature")
qn.ns       # Namespace module, or a numeric index if it is not loaded
qn.name     # 'Temperature'

Like NodeId, the namespace can be supplied as an index, a shortname, or a URI. Unlike a NodeId, a QualifiedName is not unique — many nodes can share the same one. It identifies what a node is, not which one.