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AMQP Protocol Binding for CloudEvents - Version 1.0.3-wip

Abstract

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

Table of Contents

  1. Introduction

  1. Use of CloudEvents Attributes

  2. AMQP Message Mapping

  1. 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 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 to 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 CloudEvents specification defines three content modes for transferring events: structured, binary and batch. The AMQP protocol binding does not currently support the batch content mode. Every compliant implementation SHOULD support both structured and binary 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 as defined in the CloudEvents spec.

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

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 that support the structured content mode MUST support the JSON event format.

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.

One event attribute, datacontenttype is handled specially in binary content mode and mapped onto the AMQP content-type message property. 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 datacontenttype is exemplary.

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.

When the content-type message property is not prefixed with the CloudEvents media type, being able to know when the message ought to be attempted to be parsed as a CloudEvent can be a challenge. While this specification can not mandate that senders do not include any of the CloudEvents message properties when the message is not a CloudEvent, it would be reasonable for a receiver to assume that if the message has all of the mandatory CloudEvents attributes as message properties then it's probably a CloudEvent. However, as with all CloudEvent messages, if it does not adhere to all of the normative language of this specification then it is not a valid CloudEvent.

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 datacontenttype attribute.

3.1.2. Event Data Encoding

Event data is assumed to contain opaque application data that is encoded as declared by the datacontenttype attribute.

An application is free to hold the information in any in-memory representation of its choosing, but as it is transposed into AMQP as defined in this specification, the assumption is that the event data is made available as a sequence of bytes. The byte sequence is used as the AMQP application-data section.

Example:

If the declared datacontenttype is application/json;charset=utf-8, the expectation is that the event data is made available as UTF-8 encoded JSON text for use in AMQP.

3.1.3. Metadata Headers

All CloudEvents attributes with exception of datacontenttype MUST be individually mapped to and from the AMQP application-properties section.

CloudEvents extensions that define their own attributes MAY define a secondary 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. However, they MUST also include the previously defined primary mapping.

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

3.1.3.1 AMQP Application Property Names

CloudEvent attributes MUST be prefixed with either "cloudEvents_" or "cloudEvents:" for use in the application-properties section.

The '_' separator character SHOULD be preferred in the interest of compatibility with JMS 2.0 clients and JMS message selectors where the ':' separator is not permitted for property identifiers (see section 3.8.1.1 of JMS2.0). Any single message MUST use the same separator for all CloudEvents attributes, but a single queue MAY contain messages which use different separators.

CloudEvents AMQP consumers SHOULD understand the "cloudEvents" prefix with both the '_' and the ':' separators as permitted within the constraints of the client model. JMS 2.0 AMQP consumers MUST understand the '_' separator; they cannot understand the ':' separator as per the cited JMS constraints.

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.

The CloudEvents type system MUST be mapped to AMQP types as follows, with additional notes below.

CloudEvents AMQP
Boolean boolean
Integer long
String string
Binary binary
URI string
URI-reference string
Timestamp timestamp

All attribute values in an AMQP binary message MUST either be represented using the native AMQP types above or the canonical string form.

An implementation

  • MUST be able to interpret both forms on an incoming AMQP message
  • MAY further relax the requirements for incoming messages (for example accepting numeric types other than AMQP long), but MUST be strict for outgoing messages.
  • SHOULD use the native AMQP form on outgoing AMQP messages when it is efficient to do so, but MAY forward values as canonical strings

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: 1.0
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 protocols.

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 and data 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" : "1.0",
    "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
  • JMS20 JSR-343 Java Message Service 2.0