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JSON Event Format for CloudEvents - Version 1.0.3-wip

Abstract

The JSON Format for CloudEvents defines how events are expressed in JavaScript Object Notation (JSON) Data Interchange Format (RFC8259).

Table of Contents

  1. Introduction
  2. Attributes
  3. Envelope
  4. JSON Batch Format
  5. References

1. Introduction

CloudEvents is a standardized and protocol-agnostic definition of the structure and metadata description of events. This specification defines how the elements defined in the CloudEvents specification are to be represented in the JavaScript Object Notation (JSON) Data Interchange Format (RFC8259).

The Attributes section describes the naming conventions and data type mappings for CloudEvents attributes.

The Envelope section defines a JSON container for CloudEvents attributes and an associated media type.

1.1. Conformance

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC2119.

2. Attributes

This section defines how CloudEvents attributes are mapped to JSON. This specification does not explicitly map each attribute, but provides a generic mapping model that applies to all current and future CloudEvents attributes, including extensions.

For clarity, extension attributes are serialized using the same rules as core attributes. This includes their syntax and placement within the JSON object. In particular, extensions are placed as top-level JSON properties. Extensions MUST be serialized as a top-level JSON property. There were many reasons for this design decision and they are covered in more detail in the Primer.

2.1. Base Type System

The core CloudEvents specification defines a minimal abstract type system, which this mapping leans on.

2.2. Type System Mapping

The CloudEvents type system MUST be mapped to JSON types as follows, with exceptions noted below.

CloudEvents JSON
Boolean boolean
Integer number, only the integer component optionally prefixed with a minus sign is permitted
String string
Binary string, Base64-encoded binary
URI string following RFC 3986
URI-reference string following RFC 3986
Timestamp string following RFC 3339 (ISO 8601)

Unset attributes MAY be encoded to the JSON value of null. When decoding attributes and a null value is encountered, it MUST be treated as the equivalent of unset or omitted.

Extension specifications MAY define secondary mapping rules for the values of attributes they define, but MUST also include the previously defined primary mapping.

For instance, the attribute value might be a data structure defined in a standard outside of CloudEvents, with a formal JSON mapping, and there might be risk of translation errors or information loss when the original format is not preserved.

An extension specification that defines a secondary mapping rule for JSON, and any revision of such a specification, MUST also define explicit mapping rules for all other event formats that are part of the CloudEvents core at the time of the submission or revision.

If necessary, the CloudEvents type can be determined by inference using the rules from the mapping table, whereby the only potentially ambiguous JSON data type is string. The value is compatible with the respective CloudEvents type when the mapping rules are fulfilled.

2.3. Examples

The following table shows exemplary attribute mappings:

CloudEvents Type Exemplary JSON Value
type String "com.example.someevent"
specversion String "1.0"
source URI-reference "/mycontext"
subject String "larger-context"
subject String (null) null
id String "1234-1234-1234"
time Timestamp "2018-04-05T17:31:00Z"
time Timestamp (null) null
datacontenttype String "application/json"

2.4. JSONSchema Validation

The CloudEvents JSONSchema for the spec is located here and contains the definitions for validating events in JSON.

3. Envelope

Each CloudEvents event can be wholly represented as a JSON object.

Such a representation MUST use the media type application/cloudevents+json.

All REQUIRED and all not omitted OPTIONAL attributes in the given event MUST become members of the JSON object, with the respective JSON object member name matching the attribute name, and the member's type and value being mapped using the type system mapping.

OPTIONAL not omitted attributes MAY be represented as a null JSON value.

3.1. Handling of "data"

The JSON representation of the event "data" payload is determined by the runtime type of the data content and the value of the datacontenttype attribute.

3.1.1. Payload Serialization

Before taking action, a JSON serializer MUST first determine the runtime data type of the data content.

If the implementation determines that the type of data is Binary, the value MUST be represented as a JSON string expression containing the Base64 encoded binary value, and use the member name data_base64 to store it inside the JSON representation. If present, the datacontenttype MUST reflect the format of the original binary data. If a datacontenttype value is not provided, no assumptions can be made as to the format of the data and therefore the datacontenttype attribute MUST NOT be present in the resulting CloudEvent.

Note: Definition of data_base64 is a JSON-specific marshaling rule and not part of the formal CloudEvents context attributes definition. This means the rules governing CloudEvent attributes names do not apply to this JSON member.

If the type of data is not Binary, the implementation will next determine whether the value of the datacontenttype attribute declares the data to contain JSON-formatted content. Such a content type is defined as one having a media subtype equal to json or ending with a +json format extension. That is, a datacontenttype declares JSON-formatted content if its media type, when stripped of parameters, has the form */json or */*+json. If the datacontenttype is unspecified, processing SHOULD proceed as if the datacontenttype had been specified explicitly as application/json.

If the datacontenttype declares the data to contain JSON-formatted content, a JSON serializer MUST translate the data value to a JSON value, and use the member name data to store it inside the JSON representation. The data value MUST be stored directly as a JSON value, rather than as an encoded JSON document represented as a string. An implementation MAY fail to serialize the event if it is unable to translate the runtime value to a JSON value.

Otherwise, if the datacontenttype does not declare JSON-formatted data content, a JSON serializer MUST store a string representation of the data value, properly encoded according to the datacontenttype, in the data member of the JSON representation. An implementation MAY fail to serialize the event if it is unable to represent the runtime value as a properly encoded string.

Out of this follows that the presence of the data and data_base64 members is mutually exclusive in a JSON serialized CloudEvent.

Furthermore, unlike attributes, for which value types are restricted by the type-system mapping, the data member JSON value is unrestricted, and MAY contain any valid JSON if the datacontenttype declares the data to be JSON-formatted. In particular, the data member MAY have a value of null, representing an explicit null payload as distinct from the absence of the data member.

3.1.2. Payload Deserialization

When a CloudEvents is deserialized from JSON, the presence of the data_base64 member clearly indicates that the value is a Base64 encoded binary data, which the deserializer MUST decode into a binary runtime data type. The deserializer MAY further interpret this binary data according to the datacontenttype. If the datacontenttype attribute is absent, the decoding MUST NOT make an assumption of JSON-formatted data (as described below for the data member).

When a data member is present, the decoding behavior is dependent on the value of the datacontenttype attribute. If the datacontenttype declares the data to contain JSON-formatted content (that is, its subtype is json or has a +json format extension), then the data member MUST be treated directly as a JSON value and decoded using an appropriate JSON type mapping for the runtime. Note: if the data member is a string, a JSON deserializer MUST interpret it directly as a JSON String value; it MUST NOT further deserialize the string as a JSON document.

If the datacontenttype does not declare JSON-formatted data content, then the data member SHOULD be treated as an encoded content string. An implementation MAY fail to deserialize the event if the data member is not a string, or if it is unable to interpret the data with the datacontenttype.

When a data member is present, if the datacontenttype attribute is absent, a JSON deserializer SHOULD proceed as if it were set to application/json, which declares the data to contain JSON-formatted content. Thus, it SHOULD treat the data member directly as a JSON value as specified above. Furthermore, if a JSON-formatted event with no datacontenttype attribute, is deserialized and then re-serialized using a different format or protocol binding, the datacontenttype in the re-serialized event SHOULD be set explicitly to the implied application/json content type to preserve the semantics of the event.

3.2. Examples

Example event with Binary-valued data:

{
    "specversion" : "1.0",
    "type" : "com.example.someevent",
    "source" : "/mycontext",
    "id" : "A234-1234-1234",
    "time" : "2018-04-05T17:31:00Z",
    "comexampleextension1" : "value",
    "comexampleothervalue" : 5,
    "datacontenttype" : "application/vnd.apache.thrift.binary",
    "data_base64" : "... base64 encoded string ..."
}

The above example re-encoded using HTTP Binary Content Mode:

ce-specversion: 1.0
ce-type: com.example.someevent
ce-source: /mycontext
ce-id: A234-1234-1234
ce-time: 2018-04-05T17:31:00Z
ce-comexampleextension1: value
ce-comexampleothervalue: 5
content-type: application/vnd.apache.thrift.binary

...raw binary bytes...

Example event with a serialized XML document as the String (i.e. non-Binary) valued data, and an XML (i.e. non-JSON-formatted) content type:

{
    "specversion" : "1.0",
    "type" : "com.example.someevent",
    "source" : "/mycontext",
    "id" : "B234-1234-1234",
    "time" : "2018-04-05T17:31:00Z",
    "comexampleextension1" : "value",
    "comexampleothervalue" : 5,
    "unsetextension": null,
    "datacontenttype" : "application/xml",
    "data" : "<much wow=\"xml\"/>"
}

The above example re-encoded using HTTP Binary Content Mode:

ce-specversion: 1.0
ce-type: com.example.someevent
ce-source: /mycontext
ce-id: B234-1234-1234
ce-time: 2018-04-05T17:31:00Z
ce-comexampleextension1: value
ce-comexampleothervalue: 5
content-type: application/xml

<much wow="xml"/>

Example event with JSON Object-valued data and a content type declaring JSON-formatted data:

{
    "specversion" : "1.0",
    "type" : "com.example.someevent",
    "source" : "/mycontext",
    "subject": null,
    "id" : "C234-1234-1234",
    "time" : "2018-04-05T17:31:00Z",
    "comexampleextension1" : "value",
    "comexampleothervalue" : 5,
    "datacontenttype" : "application/json",
    "data" : {
        "appinfoA" : "abc",
        "appinfoB" : 123,
        "appinfoC" : true
    }
}

The above example re-encoded using HTTP Binary Content Mode:

ce-specversion: 1.0
ce-type: com.example.someevent
ce-source: /mycontext
ce-id: C234-1234-1234
ce-time: 2018-04-05T17:31:00Z
ce-comexampleextension1: value
ce-comexampleothervalue: 5
content-type: application/json

{
  "appinfoA" : "abc",
  "appinfoB" : 123,
  "appinfoC" : true
}

Example event with JSON Number-valued data and a content type declaring JSON-formatted data:

{
    "specversion" : "1.0",
    "type" : "com.example.someevent",
    "source" : "/mycontext",
    "subject": null,
    "id" : "C234-1234-1234",
    "time" : "2018-04-05T17:31:00Z",
    "comexampleextension1" : "value",
    "comexampleothervalue" : 5,
    "datacontenttype" : "application/json",
    "data" : 1.5
}

The above example re-encoded using HTTP Binary Content Mode:

ce-specversion: 1.0
ce-type: com.example.someevent
ce-source: /mycontext
ce-id: C234-1234-1234
ce-time: 2018-04-05T17:31:00Z
ce-comexampleextension1: value
ce-comexampleothervalue: 5
content-type: application/json

1.5

Example event with a literal JSON string as the non-Binary-valued data and no datacontenttype. The data is implicitly treated as if the datacontenttype were set to application/json:

{
    "specversion" : "1.0",
    "type" : "com.example.someevent",
    "source" : "/mycontext",
    "subject": null,
    "id" : "D234-1234-1234",
    "time" : "2018-04-05T17:31:00Z",
    "comexampleextension1" : "value",
    "comexampleothervalue" : 5,
    "data" : "I'm just a string"
}

The above example re-encoded using HTTP Binary Content Mode. Note that the Content Type is explicitly set to the application/json value that was implicit in JSON format. Note also that the content is quoted to indicate that it is a literal JSON string. If the quotes were missing, this would have been an invalid event because the content could not be decoded as application/json:

ce-specversion: 1.0
ce-type: com.example.someevent
ce-source: /mycontext
ce-id: D234-1234-1234
ce-time: 2018-04-05T17:31:00Z
ce-comexampleextension1: value
ce-comexampleothervalue: 5
content-type: application/json

"I'm just a string"

Example event with a Binary-valued data_base64 but no datacontenttype. Even though the data happens to be a valid JSON document when interpreted as text, no content type is inferred.

{
    "specversion" : "1.0",
    "type" : "com.example.someevent",
    "source" : "/mycontext",
    "id" : "D234-1234-1234",
    "data_base64" : "eyAieHl6IjogMTIzIH0="
}

The above example re-encoded using HTTP Binary Content Mode. Note that there is no content-type header present.

ce-specversion: 1.0
ce-type: com.example.someevent
ce-source: /mycontext
ce-id: D234-1234-1234

{ "xyz": 123 }

4. JSON Batch Format

In the JSON Batch Format several CloudEvents are batched into a single JSON document. The document is a JSON array filled with CloudEvents in the JSON Event format.

Although the JSON Batch Format builds ontop of the JSON Format, it is considered as a separate format: a valid implementation of the JSON Format doesn't need to support it. The JSON Batch Format MUST NOT be used when only support for the JSON Format is indicated.

4.1. Mapping CloudEvents

This section defines how a batch of CloudEvents is mapped to JSON.

The outermost JSON element is a JSON Array, which contains as elements CloudEvents rendered in accordance with the JSON event format specification.

4.2. Envelope

A JSON Batch of CloudEvents MUST use the media type application/cloudevents-batch+json.

4.3. Examples

An example containing two CloudEvents: The first with Binary-valued data, the second with JSON data.

[
  {
      "specversion" : "1.0",
      "type" : "com.example.someevent",
      "source" : "/mycontext/4",
      "id" : "B234-1234-1234",
      "time" : "2018-04-05T17:31:00Z",
      "comexampleextension1" : "value",
      "comexampleothervalue" : 5,
      "datacontenttype" : "application/vnd.apache.thrift.binary",
      "data_base64" : "... base64 encoded string ..."
  },
  {
      "specversion" : "1.0",
      "type" : "com.example.someotherevent",
      "source" : "/mycontext/9",
      "id" : "C234-1234-1234",
      "time" : "2018-04-05T17:31:05Z",
      "comexampleextension1" : "value",
      "comexampleothervalue" : 5,
      "datacontenttype" : "application/json",
      "data" : {
          "appinfoA" : "abc",
          "appinfoB" : 123,
          "appinfoC" : true
      }
  }
]

An example of an empty batch of CloudEvents (typically used in a response, but also valid in a request):

[]

5. References

  • RFC2046 Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types
  • RFC2119 Key words for use in RFCs to Indicate Requirement Levels
  • RFC4627 The application/json Media Type for JavaScript Object Notation (JSON)
  • RFC4648 The Base16, Base32, and Base64 Data Encodings
  • RFC6839 Additional Media Type Structured Syntax Suffixes
  • RFC8259 The JavaScript Object Notation (JSON) Data Interchange Format