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AMQP Transport Binding for CloudEvents

Abstract

The AMQP Transport Binding for CloudEvents defines how events are mapped to OASIS AMQP 1.0 (OASIS; ISO/IEC 19464:2014) messages.

Status of this document

This document is a working draft.

Table of Contents

  1. Introduction
  1. Use of CloudEvents Attributes
  1. AMQP 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 AMQP 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 AMQP

This specification does not prescribe rules constraining transfer or settlement of event messages with AMQP; it solely defines how CloudEvents are expressed as AMQP 1.0 messages.

AMQP-based messaging and eventing infrastructures often provide higher-level programming-level abstractions that do not expose all AMQP protocol elements, or map AMQP protocol elements or names to proprietary constructs. This specification uses AMQP terminology, and implementers can refer the the respective infrastructure's AMQP documentation to determine the mapping into a programming-level abstraction.

This specification assumes use of the default AMQP message format.

1.3. Content Modes

The specification defines two content modes for transferring events: structured and binary. Every compliant implementation SHOULD support both modes.

In the structured content mode, event metadata attributes and event data are placed into the AMQP message's application data section using an event format.

In the binary content mode, the value of the event data attribute is placed into the AMQP message's application data section as-is, with the contenttype attribute value declaring its media type; all other event attributes are mapped to the AMQP application-properties section.

1.4. Event Formats

Event formats, used with the stuctured content mode, define how an event is expressed in a particular data format. All implementations of this specification MUST support the JSON event format as well as the AMQP event format for the application-properties section, but MAY support any additional, including proprietary, formats.

1.5. Security

This specification does not introduce any new security features for AMQP, 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 AMQP 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 AMQP 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 AMQP.

3. AMQP Message Mapping

The content mode is chosen by the sender of the event, which is either the requesting or the responding party. Protocol interaction 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 modes by inspecting the content-type message property field. If the value 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 detects the CloudEvents media type, 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.

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. AMQP content-type

For the binary mode, the AMQP content-type property field value maps directly to the CloudEvents contenttype attribute.

3.1.2. Event Data Encoding

The data attribute byte-sequence is used as the AMQP application-data section.

3.1.3. Metadata Headers

All CloudEvents attributes with exception of contenttype and data MUST be individually mapped to and from the AMQP application-properties section, with exceptions noted below.

CloudEvents extensions that define their own attributes MAY define a diverging mapping to AMQP properties for those attributes, also in different message sections, especially if specific attributes or their names need to align with AMQP features or with other specifications that have explicit AMQP header bindings.

An extension specification that defines a diverging mapping rule for AMQP, 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 AMQP Application Property Names

Cloud Event attributes are prefixed with "cloudEvents:" for use in the application-properties section.

Examples:

* `time` maps to `cloudEvents:time`
* `id` maps to `cloudEvents:id`
* `specversion` maps to `cloudEvents:specversion`
3.1.3.2 AMQP Application Property Values

The value for each AMQP application property is constructed from the respective attribute's AMQP type representation, compliant with the AMQP event format specification.

3.1.4 Examples

This example shows the binary mode mapping of an event into the bare message sections of AMQP:

--------------- properties ------------------

to: myqueue
content-type: application/json; charset=utf-8

----------- application-properties -----------

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

------------- application-data ---------------

{
    ... 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.

3.2.1. AMQP Content-Type

The AMQP content-type property field is set to the media type of an event format.

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 is then rendered in accordance with the event format specification and the resulting data becomes the AMQP application data section.

3.2.3. Metadata Headers

Implementations MAY include the same AMQP application-properties as defined for the binary mode.

3.2.4 Examples

This example shows a JSON event format encoded event:

--------------- properties ------------------------------

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

----------- application-properties ----------------------

------------- application-data --------------------------

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

    ... further attributes omitted ...

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

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

4. References

  • 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 The JavaScript Object Notation (JSON) Data Interchange Format
  • OASIS-AMQP-1.0 OASIS Advanced Message Queuing Protocol (AMQP) Version 1.0