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JSON is a format that uses human readable text. It is defined in RFC 7159.

The JSON based message mapping allows OPC UA Applications to interoperate with web and enterprise software that use this format.

Each JSON NetworkMessage contains one or more JSON DataSetMessages. The JSON NetworkMessage is a JSON object with the fields defined in Table 91.

Table 91 – JSON NetworkMessage Definition

Name	Type	Description
MessageId	String	A globally unique identifier for the message. This value is mandatory.
MessageType	String	This value shall be “ua-data”. This value is mandatory.
PublisherId	String	A unique identifier for the Publisher. It identifies the source of the message. This value is optional. The presence of the value depends on the setting in the JsonNetworkMessageContentMask. The source is the PublisherId on a PubSubConnection (see 6.2.6.1).
DataSetClassId	String	The DataSetClassId associated with the DataSets in the NetworkMessage. This value is optional. The presence of the value depends on the setting in the JsonNetworkMessageContentMask. If specified, all DataSetMessages in the NetworkMessage shall have the same DataSetClassId. The source is the DataSetClassId on the PublishedDataSet (see 6.2.2.2) associated with the DataSetWriters that produced the DataSetMessages.
Messages	*	A JSON array of JSON DataSetMessages (see 7.2.3.3). This value is mandatory.

All fields with a concrete DataType defined are encoded using reversible OPC UA JSON Data Encoding defined in OPC 10000-6.

The fields in the JSON NetworkMessage are controlled by the NetworkMessageContentMask of the JSON NetworkMessage mapping(see 6.3.2.1.1).

If the NetworkMessageHeader bit of the NetworkMessageContentMask is not set, the NetworkMessage is the contents of the Messages field (e.g. a JSON array of DataSetMessages).

If the DataSetMessageHeader bit of the NetworkMessageContentMask is not set, the content of the Messages field is an array of content from the Payload field for each DataSetMessage (see 7.2.3.3).

If the SingleDataSetMessage bit of the NetworkMessageContentMask is set, the content of the Messages field is a JSON object containing a single DataSetMessage.

If the NetworkMessageHeader and the DataSetMessageHeader bits are not set and SingleDataSetMessage bit is set, the NetworkMessage is a JSON object containing the set of name/value pairs defined for a single DataSet.

If the JSON encoded NetworkMessage size exceeds the Broker limits the message is dropped and a PubSubTransportLimitsExceeded Event is reported.

A DataSetMessage is produced by a DataSetWriter and contains list of name/value pairs which are specified by the PublishedDataSet associated with the DataSetWriter. The contents of the DataSetMessage are formally described by a DataSetMetData Objects. A DataSetMessage is a JSON object with the fields defined in Table 92.

DataSetWriters may periodically provide keep-alive messages which are DataSetMessages without any Payload field.

Table 92 – JSON DataSetMessage Definition

Name	Type	Description
DataSetWriterId	String	An identifier for DataSetWriter which created the DataSetMessage. This value is mandatory. It is unique within the scope of a Publisher.
SequenceNumber	UInt32	A strictly monotonically increasing sequence number assigned to the DataSetMessage by the DataSetWriter. This value is optional. The presence of the value depends on the setting in the JsonDataSetMessageContentMask.
MetaDataVersion	ConfigurationVersionDataType	The version of the DataSetMetaData which describes the contents of the Payload. This value is optional. The presence of the value depends on the setting in the JsonDataSetMessageContentMask.
Timestamp	DateTime	A timestamp which applies to all values contained in the DataSetMessage. This value is optional. The presence of the value depends on the setting in the JsonDataSetMessageContentMask.
Status	StatusCode	A status code which applies to all values contained in the DataSetMessage. This value is optional. The presence of the value depends on the setting in the JsonDataSetMessageContentMask.
Payload	Object	A JSON object containing the name-value pairs specified by the PublishedDataSet. The format of the value depends on the DataType of the field and the flags specified by the DataSetMessageContentMask.

All fields with a concrete DataType are encoded using reversible OPC UA JSON Data Encoding defined in OPC 10000-6.

The fields in the DataSetMessage are specified by the DataSetFieldContentMask in the DataSetWriter parameters.

DataSetFieldContentMask specifies the format of the field values in the Payload according to the following rules:

• If the DataSetFieldContentMask results in a RawData representation, the field value is a Variant encoded using the non-reversible OPC UA JSON Data Encoding defined in OPC 10000-6.
• If the DataSetFieldContentMask results in a DataValue representation, the field value is a DataValue encoded using the non-reversible OPC UA JSON Data Encoding defined in OPC 10000-6.
• If the DataSetFieldContentMask results in a Variant representation, the field value is encoded as a Variant encoded using the reversible OPC UA JSON Data Encoding defined in OPC 10000-6.

The JSON message mapping defines only one optional discovery message for the exchange of the DataSetMetaData. The main purpose is the exchange of additional information not contained in the DataSetMessages like Properties for the DataSet fields.

DataSetMetaData describe the content a DataSet published by a DataSetWriter. More specifically, it specifies the names and data types of the values that shall appear in the Payload of a DataSetMessage.

When the DataSetMetaData of a DataSet changes the, DataSetWriter may be configured to publish the updated value through the mechanism defined by the transport protocol mapping.

The DataSetWriterId and Version fields in a DataSetMessage are used to correlate a DataSetMessage with a DataSetMetaData.

A DataSetMetaData is a JSON object with the fields defined in Table 93.

Table 93 – JSON DataSetMetaData Definition

Name	Type	Description
MessageId	String	A globally unique identifier for the message. This value is mandatory.
MessageType	String	This value shall be “ua-metadata”. This value is mandatory.
PublisherId	String	A unique identifier for the Publisher. It identifies the source of the message. This value is mandatory.
DataSetWriterId	UInt16	An identifier for DataSetWriter which published the DataSetMetaData. This value is mandatory. It is unique within the scope of a Publisher.
MetaData	DataSetMetaDataType	The metadata as defined in 6.2.2.1.2. This value is mandatory.

All fields with a concrete DataType are encoded using reversible OPC UA JSON Data Encoding defined in OPC 10000-6.