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MQTT Transport Binding for CloudEvents

Abstract

The MQTT Transport Binding for CloudEvents defines how events are mapped to MQTT 3.1.1 (OASIS; ISO/IEC 20922:2016) and MQTT 5.0 (OASIS) messages.

Status of this document

This document is a working draft.

Table of Contents

  1. Introduction
  1. Use of CloudEvents Attributes
  1. MQTT PUBLISH Message Mapping
  1. References

1. Introduction

CloudEvents is a standardized and transport-neutral definition of the structure and metadata description of events. This specification defines how the elements defined in the CloudEvents specification are to be used in MQTT PUBLISH ([3.1.1][3-publish], [5.0][5-publish]) messages.

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.

1.2. Relation to MQTT

This specification does not prescribe rules constraining transfer or settlement of event messages with MQTT; it solely defines how CloudEvents are expressed as MQTT PUBLISH messages ([3.1.1][3-publish], [5.0][5-publish]).

1.3. Content Modes

The specification defines two content modes for transferring events: structured and binary.

The binary mode only applies to MQTT 5.0, because of MQTT 3.1.1's lack of support for custom metadata.

In the structured content mode, event metadata attributes and event data are placed into the MQTT PUBLISH message payload section using an event format.

In the binary content mode, the value of the event data attribute is placed into the MQTT PUBLISH message's payload section as-is, with the contenttype attribute value declaring its media type; all other event attributes are mapped to the MQTT PUBLISH message's [properties section][5-publish-properties].

1.4. Event Formats

Event formats, used with the structured content mode, define how an event is expressed in a particular data format. All implementations of this specification MUST support the JSON event format.

MQTT 5.0 implementations MAY support any additional, including proprietary, formats.

1.5. Security

This specification does not introduce any new security features for MQTT, or mandate specific existing features to be used.

2. Use of CloudEvents Attributes

This specification does not further define any of the CloudEvents event attributes.

Two of the event attributes, contenttype and data are handled specially and mapped onto MQTT constructs, all other attributes are transferred as metadata without further interpretation.

This mapping is intentionally robust against changes, including the addition and removal of event attributes, and also accommodates vendor extensions to the event metadata. Any mention of event attributes other than contenttype and data is exemplary.

2.1. contenttype Attribute

The contenttype attribute is assumed to contain a RFC2046 compliant media-type expression.

2.2. data Attribute

The data attribute is assumed to contain opaque application data that is encoded as declared by the contenttype attribute.

An application is free to hold the information in any in-memory representation of its choosing, but as the value is transposed into MQTT as defined in this specification, the assumption is that the data attribute value is made available as a sequence of bytes.

For instance, if the declared contenttype is application/json;charset=utf-8, the expectation is that the data attribute value is made available as UTF-8 encoded JSON text for use in MQTT.

3. MQTT PUBLISH Message Mapping

With MQTT 5.0, the content mode is chosen by the sender of the event. Protocol usage patterns that might allow solicitation of events using a particular content mode might be defined by an application, but are not defined here.

The receiver of the event can distinguish between the two content modes by inspecting theContent Type property of the MQTT PUBLISH message. If the value of the Content Type property is prefixed with the CloudEvents media type application/cloudevents, indicating the use of a known event format, the receiver uses structured mode, otherwise it defaults to binary mode.

If a receiver finds a CloudEvents media type as per the above rule, but with an event format that it cannot handle, for instance application/cloudevents+avro, it MAY still treat the event as binary and forward it to another party as-is.

With MQTT 3.1.1, the content mode is always structured and the message payload MUST use the JSON event format.

3.1. Binary Content Mode

The binary content mode accommodates any shape of event data, and allows for efficient transfer and without transcoding effort.

3.1.1. MQTT PUBLISH Content Type

For the binary mode, the MQTT PUBLISH message's Content Type property MUST be mapped directly to the CloudEvents contenttype attribute.

3.1.2. Event Data Encoding

The data attribute byte-sequence MUST be used as the payload of the MQTT PUBLISH message.

3.1.3. Metadata Headers

All CloudEvents attributes with exception of contenttype and data MUST be individually mapped to and from the User Property fields in the MQTT PUBLISH message, with exceptions noted below.

CloudEvents extensions that define their own attributes MAY define a diverging mapping to MQTT user properties or features for those attributes, especially if specific attributes need to align with MQTT features, or with other specifications that have explicit MQTT header bindings.

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

3.1.3.1 User Property Names

CloudEvents attribute names MUST be used unchanged in each mapped User Property in the MQTT PUBLISH message.

3.1.3.2 User Property Values

The value for each MQTT PUBLISH User Property MUST be constructed from the respective CloudEvents attribute's JSON type representation, compliant with the JSON event format specification.

3.1.4 Examples

This example shows the binary mode mapping of an event into the MQTT 5.0 PUBLISH message. The CloudEvents contenttype attribute is mapped to the MQTT PUBLISH Content Type field; all other CloudEvents attributes are mapped to MQTT PUBLISH User Property fields. The Topic name is chosen by the MQTT client and not derived from the CloudEvents event data.

Mind that Content Type here does refer to the event data content carried in the payload.

------------------ PUBLISH -------------------

Topic Name: mytopic
Content Type: application/json; charset=utf-8

------------- User Properties ----------------

specversion: "0.2"
type: "com.example.someevent"
time: "2018-04-05T03:56:24Z"
id: "1234-1234-1234"
source: "/mycontext/subcontext"
       .... further attributes ...

------------------ payload -------------------

{
    ... application data ...
}

-----------------------------------------------

3.2. Structured Content Mode

The structured content mode keeps event metadata and data together in the payload, allowing simple forwarding of the same event across multiple routing hops, and across multiple transports. This is the only supported mode for MQTT 3.1.1

3.2.1. MQTT Content Type

For MQTT 5.0, the MQTT PUBLISH message's Content Type property MUST be set to the media type of an event format. For MQTT 3.1.1, the media type of the JSON event format is always implied:

Example for the JSON format:

content-type: application/cloudevents+json; charset=utf-8

3.2.2. Event Data Encoding

The chosen event format defines how all attributes, including the data attribute, are represented.

The event metadata and data MUST then be rendered in accordance with the event format specification and the resulting data becomes the MQTT PUBLISH payload.

3.2.3. Metadata Headers

For MQTT 5.0, implementations MAY include the same MQTT PUBLISH User Properties as defined for the binary mode.

3.2.4. Examples

The first example shows a JSON event format encoded event with MQTT 5.0

------------------ PUBLISH -------------------

Topic Name: mytopic
Content Type: application/cloudevents+json; charset=utf-8

------------------ payload -------------------

{
    "specversion" : "0.2",
    "type" : "com.example.someevent",

    ... further attributes omitted ...

    "data" : {
        ... application data ...
    }
}

-----------------------------------------------

For MQTT 3.1.1, the example looks nearly identical, but Content Type is absent because not yet supported in that version of the MQTT specification and therefore application/cloudevents+json is implied:

------------------ PUBLISH -------------------

Topic Name: mytopic

------------------ payload -------------------

{
    "specversion" : "0.2",
    "type" : "com.example.someevent",

    ... further attributes omitted ...

    "data" : {
        ... application data ...
    }
}

-----------------------------------------------

4. References

  • MQTT 3.1.1 MQTT Version 3.1.1
  • MQTT 5.0 MQTT Version 5.0
  • RFC2046 Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types
  • RFC2119 Key words for use in RFCs to Indicate Requirement Levels
  • RFC3629 UTF-8, a transformation format of ISO 10646
  • RFC4627 The application/json Media Type for JavaScript Object Notation (JSON)
  • [RFC7159][RFC7159] The JavaScript Object Notation (JSON) Data Interchange Format