The JSON DataEncoding was developed to allow OPC UA applications to interoperate with web and enterprise software that use this format. The OPC UA JSON DataEncoding defines standard JSON representations for all OPC UA Built-In types.
The JSON format is defined in RFC 7159. It is partially self-describing because each field has a name encoded in addition to the value, however, JSON has no mechanism to qualify names with namespaces.
The JSON format does not have a published standard for a schema that can be used to describe the contents of a JSON document. However, the schema mechanisms defined in this specification can be used to describe JSON documents. Specifically, the DataTypeDescription structure defined in OPC 10000-3 can define any JSON document that conforms to the rules described below.
Servers that support the JSON DataEncoding shall add DataTypeEncoding Nodes called “Default JSON” to all DataTypes which can be serialized with the JSON encoding. The NodeIds of these Nodes are defined by the information model which defines the DataType. These NodeIds are used in ExtensionObjects as described in 5.4.2.16.
There are two important use cases for the JSON encoding: Cloud applications which consume PubSub messages and JavaScript Clients (JSON is the preferred serialization format for JavaScript). For the Cloud application use case, the PubSub message needs to be self-contained which implies it cannot contain numeric references to an externally defined namespace table. Cloud applications also often rely on scripting languages to process the incoming messages so artefacts in the DataEncoding that exist to ensure fidelity during decoding are not necessary. For this reason, this DataEncoding defines a ‘non-reversible’ form which is designed to meet the needs of Cloud applications. Applications, such as JavaScript Clients, which use the DataEncoding for communication with other OPC UA applications use the normal or ‘reversible’ from. The differences, if any, between the reversible and non-reversible forms are described for each type.
Any value for a Built-In type that is NULL shall be encoded as the JSON literal ‘null’ if the value is an element of an array. If the NULL value is a field within a Structure or Union, the field shall not be encoded.
A Boolean value shall be encoded as the JSON literal ‘true’ or ‘false’.
Integer values other than Int64 and UInt64 shall be encoded as a JSON number.
Int64 and UInt64 values shall be formatted as a decimal number encoded as a JSON string
(See the XML encoding of 64-bit values described in 5.3.1.3).
Normal Float and Double values shall be encoded as a JSON number.
Special floating-point numbers such as positive infinity (INF), negative infinity (-INF) and not-a-number (NaN) shall be represented by the values “Infinity”, “-Infinity” and “NaN” encoded as a JSON string. See 5.2.2.3 for more information on the different types of special floating-point numbers.
String values shall be encoded as JSON strings.
Any characters which are not allowed in JSON strings are escaped using the rules defined in RFC 7159.
DateTime values shall be formatted as specified by ISO 8601:2004 and encoded as a JSON string.
DateTime values which exceed the minimum or maximum values supported on a platform shall be encoded as “0001-01-01T00:00:00Z” or “9999-12-31T23:59:59Z” respectively. During decoding, these values shall be converted to the minimum or maximum values supported on the platform.
DateTime values equal to “0001-01-01T00:00:00Z” are considered to be NULL values.
Guid values shall be formatted as described in 5.1.3 and encoded as a JSON string.
ByteString values shall be formatted as a Base64 text and encoded as a JSON string.
Any characters which are not allowed in JSON strings are escaped using the rules defined in RFC 7159.
XmlElement value shall be encoded as a String as described in 5.4.2.5.
NodeId values shall be encoded as a JSON object with the fields defined in Table 23.
The abstract NodeId structure is defined in OPC 10000-3 and has three fields Identifier, IdentifierType and NamespaceIndex. The representation these abstract fields are described in the table.
Table 23 – JSON Object Definition for a NodeId
|
Name |
Description |
|
IdType |
The IdentifierType encoded as a JSON number. Allowed values are: 0 - UInt32 Identifier encoded as a JSON number. 1 - A String Identifier encoded as a JSON string. 2 - A Guid Identifier encoded as described in 5.4.2.7. 3 - A ByteString Identifier encoded as described in 5.4.2.8. This field is omitted for UInt32 identifiers. |
|
Id |
The Identifier. The value of the id field specifies the encoding of this field. |
|
Namespace |
The NamespaceIndex for the NodeId. The field is encoded as a JSON number for the reversible encoding. The field is omitted if the NamespaceIndex equals 0. For the non-reversible encoding, the field is the NamespaceUri associated with the NamespaceIndex, encoded as a JSON string. A NamespaceIndex of 1 is always encoded as a JSON number. |
ExpandedNodeId values shall be encoded as a JSON object with the fields defined in Table 24.
The abstract ExpandedNodeId structure is defined in OPC 10000-3 and has five fields Identifier, IdentifierType, NamespaceIndex, NamespaceUri and ServerIndex. The representation of these abstract fields are described in the table.
Table 24 – JSON Object Definition for an ExpandedNodeId
|
Name |
Description |
|
IdType |
The IdentifierType encoded as a JSON number. Allowed values are: 0 - UInt32 Identifier encoded as a JSON number. 1 - A String Identifier encoded as a JSON string. 2 - A Guid Identifier encoded as described in 5.4.2.7. 3 - A ByteString Identifier encoded as described in 5.4.2.8. This field is omitted for UInt32 identifiers. |
|
Id |
The Identifier. The value of the ‘t’ field specifies the encoding of this field. |
|
Namespace |
The NamespaceIndex or the NamespaceUri for the ExpandedNodeId. If the NamespaceUri is not specified, the NamespaceIndex is encoded with these rules: The field is encoded as a JSON number for the reversible encoding. The field is omitted if the NamespaceIndex equals 0. For the non-reversible encoding the field is the NamespaceUri associated with the NamespaceIndex encoded as a JSON string. A NamespaceIndex of 1 is always encoded as a JSON number. If the NamespaceUri is specified it is encoded as a JSON string in this field. |
|
ServerUri |
The ServerIndex for the ExpandedNodeId. This field is encoded as a JSON number for the reversible encoding. This field is omitted if the ServerIndex equals 0. For the non-reversible encoding, this field is the ServerUri associated with the ServerIndex portion of the ExpandedNodeId, encoded as a JSON string. |
StatusCode values shall be encoded as a JSON number for the reversible encoding.
For the non-reversible form, StatusCode values shall be encoded as a JSON object with the fields defined in Table 25.
Table 25 – JSON Object Definition for a StatusCode
|
Name |
Description |
|
Code |
The numeric code encoded as a JSON number. The Code is omitted if the numeric code is 0 (Good). |
|
Symbol |
The string literal associated with the numeric code encoded as JSON string. e.g. 0x80AB0000 has the associated literal “BadInvalidArgument”. The Symbol is omitted if the numeric code is 0 (Good). |
A StatusCode of Good (0) is treated like a NULL and not encoded. If it is an element of an JSON array it is encoded as the JSON literal ‘null’.
DiagnosticInfo values shall be encoded as a JSON object with the fields shown in Table 26.
Table 26 – JSON Object Definition for a DiagnosticInfo
|
Name |
Data Type |
Description |
|
SymbolicId |
Int32 |
A symbolic name for the status code. |
|
NamespaceUri |
Int32 |
A namespace that qualifies the symbolic id. |
|
Locale |
Int32 |
The locale used for the localized text. |
|
LocalizedText |
Int32 |
A human readable summary of the status code. |
|
AdditionalInfo |
String |
Detailed application specific diagnostic information. |
|
InnerStatusCode |
StatusCode |
A status code provided by an underlying system. |
|
InnerDiagnosticInfo |
DiagnosticInfo |
Diagnostic info associated with the inner status code. |
Each field is encoded using the rules defined for the built-in type specfied in the Data Type column.
The SymbolicId, NamespaceUri, Locale and LocalizedText fields are encoded as JSON numbers which reference the StringTable contained in the ResponseHeader.
QualifiedName values shall be encoded as a JSON object with the fields shown in Table 27.
The abstract QualifiedName structure is defined in OPC 10000-3 and has two fields Name and NamespaceIndex. The NamespaceIndex is represented by the Uri field in the JSON object.
Table 27 – JSON Object Definition for a QualifiedName
|
Name |
Description |
|
Name |
The Name component of the QualifiedName. |
|
Uri |
The NamespaceIndex component of the QualifiedName encoded as a JSON number. The Uri field is omitted if the NamespaceIndex equals 0. For the non-reversible form, the NamespaceUri associated with the NamespaceIndex portion of the QualifiedName is encoded as JSON string unless the NamespaceIndex is 1 or if NamespaceUri is unknown. In these cases, the NamespaceIndex is encoded as a JSON number. |
LocalizedText values shall be encoded as a JSON object with the fields shown in Table 28.
The abstract LocalizedText structure is defined in OPC 10000-3 and has two fields Text and Locale.
Table 28 – JSON Object Definition for a LocalizedText
|
Name |
Description |
|
Locale |
The Locale portion of LocalizedText values shall be encoded as a JSON string |
|
Text |
The Text portion of LocalizedText values shall be encoded as a JSON string. |
For the non-reversible form, LocalizedText value shall be encoded as a JSON string containing the Text component.
ExtensionObject values shall be encoded as a JSON object with the fields shown in Table 29.
Table 29 – JSON Object Definition for a ExtensionObject
|
Name |
Description |
|
TypeId |
The NodeId of a DataTypeEncoding Node formatted using the rules in 5.4.2.10. |
|
Encoding |
The format of the Body field encoded as a JSON number. This value is 0 if the body is Structure encoded as a JSON object (see 5.4.6). This value is 1 if the body is a ByteString value encoded as a JSON string (see 5.4.2.8). This value is 2 if the body is a XmlElement value encoded as a JSON string (see 5.4.2.9). This field is omitted if the value is 0. |
|
Body |
Body of the ExtensionObject. The type of this field is specified by the Encoding field. If the Body is empty, the ExtensionObject is NULL and is omitted or encoded as a JSON null. |
For the non-reversible form, ExtensionObject values shall be encoded as a JSON object containing only the value of the Body field. The TypeId and Encoding fields are dropped.
Variant values shall be encoded as a JSON object with the fields shown in Table 30.
Table 30 – JSON Object Definition for a Variant
|
Name |
Description |
|
Type |
The Built-in type for the value contained in the Body (see Table 1) encoded as JSON number. If type is 0 (NULL) the Variant contains a NULL value and the containing JSON object shall be omitted or replaced by the JSON literal ‘null’ (when an element of a JSON array). |
|
Body |
If the value is a scalar it is encoded using the rules for type specified for the Type. If the value is a one-dimensional array it is encoded as JSON array (see 5.4.5). Multi-dimensional arrays are encoded as a one dimensional JSON array which is reconstructed using the value of the Dimensions field (see 5.2.2.16). |
|
Dimensions |
The dimensions of the array encoded as an JSON array of JSON numbers. The Dimensions are omitted for scalar and one-dimensional array values. |
For the non-reversible form, Variant values shall be encoded as a JSON object containing only the value of the Body field. The Type and Dimensions fields are dropped. Multi-dimensional arrays are encoded as a multi dimensional JSON array as described in 5.4.5.
DataValue values shall be encoded as a JSON object with the fields shown in Table 31.
Table 31 – JSON Object Definition for a DataValue
|
Name |
Data Type |
Description |
|
Value |
Variant |
The value. |
|
Status |
StatusCode |
The status associated with the value. |
|
SourceTimestamp |
DateTime |
The source timestamp associated with the value. |
|
SourcePicoSeconds |
UInt16 |
The number of 10 picosecond intervals for the SourceTimestamp. |
|
ServerTimestamp |
DateTime |
The Server timestamp associated with the value. |
|
ServerPicoSeconds |
UInt16 |
The number of 10 picosecond intervals for the ServerTimestamp. |
If a field has a null or default value it is omitted. Each field is encoded using the rules defined for the built-in type specfied in the Data Type column.
Decimal values shall be encoded as a JSON object with the fields in Table 32.
Table 32 – JSON Object Definition for a Decimal
|
Name |
Description |
|
Scale |
A JSON number with the scale applied to the Value. |
|
Value |
A JSON string with the Value encoded as a base-10 signed integer. (See the XML encoding of Integer values described in 5.3.1.3). |
See 5.1.7 for a description of the Scale and Value fields.
Enumeration values shall be encoded as a JSON number for the reversible encoding.
For the non-reversible form, Enumeration values are encoded the literal with the value appended as a JSON string.
The format of the string literal is:
<name>_<value>
Where the name is the enumeration literal and the value is the numeric value.
If the literal is not known to the encoder, the numeric value is encoded as a JSON string.
One dimensional Arrays shall be encoded as JSON arrays.
If an element is NULL, the element shall be encoded as the JSON literal ‘null’.
Otherwise, the element is encoded according to the rules defined for the type.
Multi-dimensional Arrays are encoded as nested JSON arrays. The outer array is the first dimension and the innermost array is the last dimension. For example, the following matrix
|
0 2 3 |
|
1 3 4 |
is encoded in JSON as
[ [0, 2, 3], [1, 3, 4] ]
Structures shall be encoded as JSON objects.
If the value of a field is NULL it shall be omitted from the encoding.
For example, instances of the structures:
struct Type2
{
Int32 A;
Int32 B;
Char* C;
};
struct Type1
{
Int32 X;
Int32 NoOfY;
Type2* Y;
Int32 Z;
};
Are represented in JSON as:
{
"X":1234,
"Y":[ { "A":1, "B":2, "C":"Hello" }, { "A":3, "B":4 } ],
"Z":5678
}
Where “C” is omitted from the second Type2 instance because it has a NULL value.
Structures with optional fields shall be encoded as JSON objects as shown in Table 33.
Table 33 – JSON Object Definition for a Structures with Optional Fields
|
Name |
Description |
|
EncodingMask |
A bit mask indicating what fields are encoded in the structure (see 5.2.7) This mask is encoded as a JSON number. The bits are sequentially assigned to optional fields in the order that they are defined. |
|
<FieldName> |
The fields structure encoded according to the rules defined for their DataType. |
For the non-reversible form, Structures with optional fields are encoded like Structures.
If a Structure with optional fields is subtyped, the subtypes extend the EncodingMask (e) defined for the the parent.
The following is an example of a structure with optional fields using C++ syntax:
struct TypeA
{
Int32 X;
Int32* O1;
SByte Y;
Int32* O2;
};
O1 and O2 are optional fields where a NULL indicates that the field is not present.
Assume that O1 is not specified and the value of O2 is 0.
The reversible encoding would be:
{ "EncodingMask": 2 "X": 1, "Y": 2 }
Where decoders would assign the default value of 0 to O2 since the mask bit is set, however, the field was omitted (this is the behaviour defined for the Int32 DataType). Decoders would mark O1 as ‘not specified’.
Unions shall be encoded as JSON objects as shown in Table 34 for the reversible encoding.
Table 34 – JSON Object Definition for a Union
|
Name |
Description |
|
SwitchField |
The identifier for the field in the Union which is encoded as a JSON number. |
|
Value |
The value of the field encoded using the rules that apply to the data type. |
For the non-reversible form, Unions values are encoded using the rule for the current value.
For example, instances of the union:
struct Union1
{
Byte Selector;
{
Int32 A;
Double B;
Char* C;
}
Value;
};
would be represented in reversible form as:
{ "SwitchField":2, "Value":3.1415 }
In non- reversible form, it is represented as:
3.1415
Messages are encoded ExtensionObjects (see 5.4.2.16).