Most built-in types are encoded in XML using the formats defined in XML Schema Part 2 specification. Any special restrictions or usages are discussed below. Some of the built-in types have an XML Schema defined for them using the syntax defined in XML Schema Part 2.
The prefix xs: is used to denote a symbol defined by the XML Schema specification.
A Boolean value is encoded as an xs:boolean value.
Integer values are encoded using one of the subtypes of the xs:decimal type. The mappings between the OPC UA integer types and XML schema data types are shown in Table 18.
Table 18 – XML Data Type Mappings for Integers
|
Name |
XML Type |
|
SByte |
xs:byte |
|
Byte |
xs:unsignedByte |
|
Int16 |
xs:short |
|
UInt16 |
xs:unsignedShort |
|
Int32 |
xs:int |
|
UInt32 |
xs:unsignedInt |
|
Int64 |
xs:long |
|
UInt64 |
xs:unsignedLong |
Floating point values are encoded using one of the XML floating point types. The mappings between the OPC UA floating point types and XML schema data types are shown in Table 19.
Table 19 – XML Data Type Mappings for Floating Points
|
Name |
XML Type |
|
Float |
xs:float |
|
Double |
xs:double |
The XML floating point type supports positive infinity (INF), negative infinity (-INF) and not-a-number (NaN).
A String value is encoded as an xs:string value.
A DateTime value is encoded as an xs:dateTime value.
All DateTime values shall be encoded as UTC times or with the time zone explicitly specified.
Correct:
2002-10-10T00:00:00+05:00
2002-10-09T19:00:00Z
Incorrect:
2002-10-09T19:00:00
It is recommended that all xs:dateTime values be represented in UTC format.
The earliest and latest date/time values that can be represented on a DevelopmentPlatform have special meaning and shall not be literally encoded in XML.
The earliest date/time value on a DevelopmentPlatform shall be encoded in XML as '0001-01-01T00:00:00Z'.
The latest date/time value on a DevelopmentPlatform shall be encoded in XML as '9999-12-31T23:59:59Z'
If a decoder encounters a xs:dateTime value that cannot be represented on the DevelopmentPlatform it should convert the value to either the earliest or latest date/time that can be represented on the DevelopmentPlatform. The XML decoder should not generate an error if it encounters an out of range date value.
The earliest date/time value on a DevelopmentPlatform is equivalent to a null date/time value.
A Guid is encoded using the string representation defined in 5.1.3.
The XML schema for a Guid is:
<xs:complexType name="Guid">
<xs:sequence>
<xs:element name="String" type="xs:string" minOccurs="0" />
</xs:sequence>
</xs:complexType>
A ByteString value is encoded as an xs:base64Binary value (see Base64).
The XML schema for a ByteString is:
<xs:element name="ByteString" type="xs:base64Binary" nillable="true"/>
An XmlElement value is encoded as an xs:complexType with the following XML schema:
<xs:complexType name="XmlElement">
<xs:sequence>
<xs:any minOccurs="0" maxOccurs="1" processContents="lax" />
</xs:sequence>
</xs:complexType>
XmlElements may only be used inside Variant or ExtensionObject values.
A NodeId value is encoded as an xs:string with the syntax:
ns=<namespaceindex>;<type>=<value>
The elements of the syntax are described in Table 20.
Table 20 – Components of NodeId
|
Field |
Data Type |
Description |
||||||||
|
<namespaceindex> |
UInt16 |
The NamespaceIndex formatted as a base 10 number. If the index is 0 then the entire 'ns=0;' clause shall be omitted. |
||||||||
|
<type> |
Enumeration |
A flag that specifies the IdentifierType. The flag has the following values:
|
||||||||
|
<value> |
* |
The Identifier encoded as string. The Identifier is formatted using the XML data type mapping for the IdentifierType. Note that the Identifier may contain any non-null UTF-8 character including whitespace. |
Examples of NodeIds:
i=13
ns=10;i=-1
ns=10;s=Hello:World
g=09087e75-8e5e-499b-954f-f2a9603db28a
ns=1;b=M/RbKBsRVkePCePcx24oRA==
The XML schema for a NodeId is:
<xs:complexType name="NodeId">
<xs:sequence>
<xs:element name="Identifier" type="xs:string" minOccurs="0" />
</xs:sequence>
</xs:complexType>
An ExpandedNodeId value is encoded as an xs:string with the syntax:
svr=<serverindex>;ns=<namespaceindex>;<type>=<value>
or
svr=<serverindex>;nsu=<uri>;<type>=<value>
The possible fields are shown in Table 21.
Table 21 – Components of ExpandedNodeId
|
Field |
Data Type |
Description |
|
<serverindex> |
UInt32 |
The ServerIndex formatted as a base 10 number. If the ServerIndex is 0 then the entire 'svr=0;' clause shall be omitted. |
|
<namespaceindex> |
UInt16 |
The NamespaceIndex formatted as a base 10 number. If the NamespaceIndex is 0 then the entire 'ns=0;' clause shall be omitted. The NamespaceIndex shall not be present if the URI is present. |
|
<uri> |
String |
The NamespaceUri formatted as a string. Any reserved characters in the URI shall be replaced with a ‘%’ followed by its 8 bit ANSI value encoded as two hexadecimal digits (case insensitive). For example, the character ‘;’ would be replaced by ‘%3B’. The reserved characters are ‘;’ and ‘%’. If the NamespaceUri is null or empty, then 'nsu=;' clause shall be omitted. |
|
<type> |
Enumeration |
A flag that specifies the IdentifierType. This field is described in Table 20. |
|
<value> |
* |
The Identifier encoded as string. This field is described in Table 20. |
The XML schema for an ExpandedNodeId is:
<xs:complexType name="ExpandedNodeId">
<xs:sequence>
<xs:element name="Identifier" type="xs:string" minOccurs="0" />
</xs:sequence>
</xs:complexType>
A StatusCode is encoded as an xs:unsignedInt with the following XML schema:
<xs:complexType name="StatusCode">
<xs:sequence>
<xs:element name="Code" type="xs:unsignedInt" minOccurs="0" />
</xs:sequence>
</xs:complexType>
An DiagnosticInfo value is encoded as an xs:complexType with the following XML schema:
<xs:complexType name="DiagnosticInfo">
<xs:sequence>
<xs:element name="SymbolicId" type="xs:int" minOccurs="0" />
<xs:element name="NamespaceUri" type="xs:int" minOccurs="0" />
<xs:element name="Locale" type="xs:int" minOccurs="0/>
<xs:element name="LocalizedText" type="xs:int" minOccurs="0/>
<xs:element name="AdditionalInfo" type="xs:string" minOccurs="0"/>
<xs:element name="InnerStatusCode" type="tns:StatusCode"
minOccurs="0" />
<xs:element name="InnerDiagnosticInfo" type="tns:DiagnosticInfo"
minOccurs="0" />
</xs:sequence>
</xs:complexType>
DiagnosticInfo allows unlimited nesting which could result in stack overflow errors even if the message size is less than the maximum allowed. Decoders shall support at least 100 nesting levels. Decoders shall report an error if the number of nesting levels exceeds what it supports.
A QualifiedName value is encoded as an xs:complexType with the following XML schema:
<xs:complexType name="QualifiedName">
<xs:sequence>
<xs:element name="NamespaceIndex" type="xs:int" minOccurs="0" />
<xs:element name="Name" type="xs:string" minOccurs="0" />
</xs:sequence>
</xs:complexType>
A LocalizedText value is encoded as an xs:complexType with the following XML schema:
<xs:complexType name="LocalizedText">
<xs:sequence>
<xs:element name="Locale" type="xs:string" minOccurs="0" />
<xs:element name="Text" type="xs:string" minOccurs="0" />
</xs:sequence>
</xs:complexType>
An ExtensionObject value is encoded as an xs:complexType with the following XML schema:
<xs:complexType name="ExtensionObject">
<xs:sequence>
<xs:element name="TypeId" type="tns:NodeId" minOccurs="0" />
<xs:element name="Body" minOccurs="0">
<xs:complexType>
<xs:sequence>
<xs:any minOccurs="0" processContents="lax"/>
</xs:sequence>
</xs:complexType>
</xs:element>
</xs:sequence>
</xs:complexType>
The body of the ExtensionObject contains a single element which is either a ByteString or XML encoded Structure. A decoder can distinguish between the two by inspecting the top-level element. An element with the name tns:ByteString contains an OPC UA Binary encoded body. Any other name shall contain an OPC UA XML encoded body. The TypeId specifies the syntax of a ByteString body which could be UTF-8 encoded JSON, UA Binary or some other format.
The TypeId is the NodeId for the DataTypeEncoding Object.
A Variant value is encoded as an xs:complexType with the following XML schema:
<xs:complexType name="Variant">
<xs:sequence>
<xs:element name="Value" minOccurs="0" nillable="true">
<xs:complexType>
<xs:sequence>
<xs:any minOccurs="0" processContents="lax"/>
</xs:sequence>
</xs:complexType>
</xs:element>
</xs:sequence>
</xs:complexType>
If the Variant represents a scalar value, then it shall contain a single child element with the name of the built-in type. For example, the single precision floating point value 3,141 5 would be encoded as:
<tns:Float>3.1415</tns:Float>
If the Variant represents a single dimensional array, then it shall contain a single child element with the prefix 'ListOf' and the name built-in type. For example, an Array of strings would be encoded as:
<tns:ListOfString>
<tns:String>Hello</tns:String>
<tns:String>World</tns:String>
</tns:ListOfString>
If the Variant represents a multidimensional Array, then it shall contain a child element with the name ‘Matrix’ with the two sub-elements shown in this example:
<tns:Matrix>
<tns:Dimensions>
<tns:Int32>2</tns:Int32>
<tns:Int32>2</tns:Int32>
</tns:Dimensions>
<tns:Elements>
<tns:String>A</tns:String>
<tns:String>B</tns:String>
<tns:String>C</tns:String>
<tns:String>D</tns:String>
</tns:Elements>
</tns:Matrix>
In this example, the array has the following elements:
[0,0] = "A"; [0,1] = "B"; [1,0] = "C"; [1,1] = "D"
The elements of a multi-dimensional Array are always flattened into a single dimensional Array where the higher rank dimensions are serialized first. This single dimensional Array is encoded as a child of the ‘Elements’ element. The ‘Dimensions’ element is an Array of Int32 values that specify the dimensions of the array starting with the lowest rank dimension. The multi-dimensional Array can be reconstructed by using the dimensions encoded. All dimensions shall be specified and shall be greater than zero. If the dimensions are inconsistent with the number of elements in the array, then the decoder shall stop and raise a Bad_DecodingError.
The complete set of built-in type names is found in Table 1.
A DataValue value is encoded as a xs:complexType with the following XML schema:
<xs:complexType name="DataValue">
<xs:sequence>
<xs:element name="Value" type="tns:Variant" minOccurs="0"
nillable="true" />
<xs:element name="StatusCode" type="tns:StatusCode"
minOccurs="0" />
<xs:element name="SourceTimestamp" type="xs:dateTime"
minOccurs="0" />
<xs:element name="SourcePicoseconds" type="xs:unsignedShort"
minOccurs="0"/>
<xs:element name="ServerTimestamp" type="xs:dateTime"
minOccurs="0" />
<xs:element name="ServerPicoseconds" type="xs:unsignedShort"
minOccurs="0"/>
</xs:sequence>
</xs:complexType>