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1c1d152 Nov 8, 2018
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HTTP Transport Binding for CloudEvents - Version 0.1

Abstract

The HTTP Transport Binding for CloudEvents defines how events are mapped to HTTP 1.1 request and response messages.

Status of this document

This document is a working draft.

Table of Contents

  1. Introduction
  1. Use of CloudEvents Attributes
  1. HTTP 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 HTTP 1.1 requests and response 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 HTTP

This specification does not prescribe rules constraining the use or handling of specific HTTP methods, and it also does not constrain the HTTP target resource that is used for transferring or soliciting events.

Events can be transferred with all standard or application-defined HTTP request methods that support payload body transfers. Events can be also be transferred in HTTP responses and with all HTTP status codes that permit payload body transfers.

All examples herein that show HTTP methods, HTTP target URIs, and HTTP status codes are non-normative illustrations.

This specification also applies equivalently to HTTP/2 (RFC7540), which is compatible with HTTP 1.1 semantics.

1.3. Content Modes

This 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 HTTP request or response body using an event format.

In the binary content mode, the value of the event data attribute is placed into the HTTP request or response body as-is, with the contenttype attribute value declaring its media type; all other event attributes are mapped to HTTP headers.

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, but MAY support any additional, including proprietary, formats.

1.5. Security

This specification does not introduce any new security features for HTTP, or mandate specific existing features to be used. This specification applies identically to HTTP over TLS.

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 HTTP 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 HTTP 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 to HTTP.

3. HTTP Message Mapping

The event binding is identical for both HTTP request and response messages.

The content mode is chosen by the sender of the event, which is either the requesting or the responding party. Gestures that might allow solicitation of events using a particular 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 header value. 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. HTTP Content-Type

For the binary mode, the HTTP Content-Type value maps directly to the CloudEvents contenttype attribute.

3.1.2. Event Data Encoding

The data attribute byte-sequence is used as the HTTP message body.

3.1.3. Metadata Headers

All CloudEvents attributes with exception of contenttype and data MUST be individually mapped to and from distinct HTTP message headers, with exceptions noted below.

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

An extension specification that defines a diverging mapping rule for HTTP, 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 HTTP Header Names

Except for attributes [explicitly handled in this specification] (#2-use-of-cloudevents-attributes), the naming convention for the HTTP header mapping of well-known CloudEvents attributes is that each attribute name MUST be prefixed with "ce-".

Examples:

* `time` maps to `ce-time`
* `id` maps to `ce-id`
* `specversion` maps to `ce-specversion`

Map-typed CloudEvents attributes MUST be flattened into a set of HTTP headers, where by the name of each header carries the prefix "ce-", an infix reflecting the map attribute followed by a dash ("-"), and the name of the map entry key, e.g. "ce-attrib-key".

Note: per the HTTP specification, header names are case-insensitive.

3.1.3.2 HTTP Header Values

The value for each HTTP header is constructed from the respective attribute's JSON value representation, compliant with the JSON event format specification.

Some CloudEvents metadata attributes can contain arbitrary UTF-8 string content, and per RFC7230 Section 3, HTTP headers MUST only use printable characters from the US-ASCII character set, and are terminated by a CRLF sequence.

Therefore, and analog to the encoding rules for Universal character set host names in URIs RFC3986 3.2.2, the JSON value MUST be encoded as follows:

Non-printable ASCII characters and non-ASCII characters MUST first be encoded according to UTF-8, and then each octet of the corresponding UTF-8 sequence MUST be percent-encoded to be represented as HTTP header characters, in compliance with RFC7230, sections 3, 3.2, 3.2.6. The rules for encoding of the percent character ('%') apply as defined in RFC 3986 Section 2.4..

JSON objects and arrays are NOT surrounded with single or double quotes.

3.1.4 Examples

This example shows the binary mode mapping of an event with an HTTP POST request:

POST /someresource HTTP/1.1
Host: webhook.example.com
ce-specversion: "0.1"
ce-type: "com.example.someevent"
ce-time: "2018-04-05T03:56:24Z"
ce-id: "1234-1234-1234"
ce-source: "/mycontext/subcontext"
    .... further attributes ...
Content-Type: application/json; charset=utf-8
Content-Length: nnnn

{
    ... application data ...
}

This example shows a response containing an event:

HTTP/1.1 200 OK
ce-specversion: "0.1"
ce-type: "com.example.someevent"
ce-time: "2018-04-05T03:56:24Z"
ce-id: "1234-1234-1234"
ce-source: "/mycontext/subcontext"
    .... further attributes ...
Content-Type: application/json; charset=utf-8
Content-Length: nnnn

{
    ... 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. HTTP Content-Type

The HTTP Content-Type header MUST be 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 HTTP message body.

3.2.3. Metadata Headers

Implementations MAY include the same HTTP headers as defined for the binary mode.

All CloudEvents metadata attributes MUST be mapped into the payload, even if they are also mapped into HTTP headers.

3.2.4 Examples

This example shows a JSON event format encoded event, sent with a PUT request:


PUT /myresource HTTP/1.1
Host: webhook.example.com
Content-Type: application/cloudevents+json; charset=utf-8
Content-Length: nnnn

{
    "specversion" : "0.1",
    "type" : "com.example.someevent",

    ... further attributes omitted ...

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

This example shows a JSON encoded event returned in a response:


HTTP/1.1 200 OK
Content-Type: application/cloudevents+json; charset=utf-8
Content-Length: nnnn

{
    "specversion" : "0.1",
    "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
  • RFC2818 HTTP over TLS
  • RFC3629 UTF-8, a transformation format of ISO 10646
  • RFC3986 Uniform Resource Identifier (URI): Generic Syntax
  • 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
  • RFC7159 The JavaScript Object Notation (JSON) Data Interchange Format
  • RFC7230 Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing
  • RFC7231 Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content
  • RFC7540 Hypertext Transfer Protocol Version 2 (HTTP/2)