The AMQP Protocol Binding for CloudEvents defines how events are mapped to OASIS AMQP 1.0 (OASIS; ISO/IEC 19464:2014) messages.
- 1.1. Conformance
- 1.2. Relation to AMQP
- 1.3. Content Modes
- 1.4. Event Formats
- 1.5. Security
- 3.2. Binary Content Mode
- 3.1. Structured Content Mode
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.
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.
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.
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 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.
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.
This specification does not introduce any new security features for AMQP, or mandate specific existing features to be used.
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.
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.
The binary content mode accommodates any shape of event data, and allows for efficient transfer and without transcoding effort.
For the binary mode, the AMQP content-type property field value maps
directly to the CloudEvents datacontenttype attribute.
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.
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.
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`
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
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 ...
}
----------------------------------------------
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.
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
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.
Implementations MAY include the same AMQP application-properties as defined for the binary mode.
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 ...
}
}
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- 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