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CloudEvents - Version 0.4-wip

Abstract

CloudEvents is a vendor-neutral specification for defining the format of event data.

Status of this document

This document is a working draft.

Table of Contents

Overview

Events are everywhere. However, event producers tend to describe events differently.

The lack of a common way of describing events means developers are constantly re-learning how to consume events. This also limits the potential for libraries, tooling and infrastructure to aide the delivery of event data across environments, like SDKs, event routers or tracing systems. The portability and productivity that can be achieved from event data is hindered overall.

CloudEvents is a specification for describing event data in common formats to provide interoperability across services, platforms and systems.

Event Formats specify how to serialize a CloudEvent with certain encoding formats. Compliant CloudEvents implementations that support those encodings MUST adhere to the encoding rules specified in the respective event format. All implementations MUST support the JSON format.

For more information on the history, development and design rationale behind the specification, see the CloudEvents Primer document.

Notations and Terminology

Notational Conventions

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 RFC 2119.

For clarity, when a feature is marked as "OPTIONAL" this means that it is OPTIONAL for both the sender and receiver of a message to support that feature. In other words, a sender can choose to include that feature in a message if it wants, and a receiver can choose to support that feature if it wants. A receiver that does not support that feature will then silently ignore that part of the message. The sender needs to be prepared for the situation where a receiver ignores that feature.

Attribute Naming Convention

The CloudEvents specifications define mappings to various protocols and encodings, and the accompanying CloudEvents SDK targets various runtimes and languages. Some of these treat metadata elements as case-sensitive while others do not, and a single CloudEvent might be routed via multiple hops that involve a mix of protocols, encodings, and runtimes. Therefore, this specification limits the available character set of all attributes such that case-sensitivity issues or clashes with the permissible character set for identifiers in common languages are prevented.

CloudEvents attribute names MUST consist of lower-case letters ('a' to 'z') or digits ('0' to '9') from the ASCII character set, and MUST begin with a lower-case letter. Attribute names SHOULD be descriptive and terse, and SHOULD NOT exceed 20 characters in length.

Terminology

This specification defines the following terms:

Occurrence

An "occurrence" is the capture of a statement of fact during the operation of a software system. This might occur because of a signal raised by the system or a signal being observed by the system, because of a state change, because of a timer elapsing, or any other noteworthy activity. For example, a device might go into an alert state because the battery is low, or a virtual machine is about to perform a scheduled reboot.

Event

An "event" is a data record expressing an occurrence and its context. Events are routed from an event producer (the source) to interested event consumers. The routing can be performed based on information contained in the event, but an event will not identify a specific routing destination. Events will contain two types of information: the Data representing the Occurrence and Context metadata providing contextual information about the Occurrence.

Producer

The "producer" is a specific instance, process or device that creates the data structure describing the CloudEvent.

Source

The "source" is the context in which the occurrence happened. In a distributed system it might consist of multiple Producers. If a source is not aware of CloudEvents, an external producer creates the CloudEvent on behalf of the source.

Consumer

A "consumer" receives the event and acts upon it. It uses the context and data to execute some logic, which might lead to the occurrence of new events.

Intermediary

An "intermediary" receives a message containing an event for the purpose of forwarding it to the next receiver, which might be another intermediary or a Consumer. A typical task for an intermediary is to route the event to receivers based on the information in the Context.

Context

Context metadata will be encapsulated in the Context Attributes. Tools and application code can use this information to identify the relationship of Events to aspects of the system or to other Events.

Data

Domain-specific information about the occurrence (i.e. the payload). This might include information about the occurrence, details about the data that was changed, or more. See the Data Attribute section for more information.

Message

Events are transported from a source to a destination via messages.

Protocol

Messages can be delivered through various industry standard protocol (e.g. HTTP, AMQP, MQTT, SMTP), open-source protocols (e.g. Kafka, NATS), or platform/vendor specific protocols (AWS Kinesis, Azure Event Grid).

Type System

The following abstract data types are available for use in attributes. Each of these types MAY be represented differently by different event formats and in transport metadata fields. This specification defines a canonical string-encoding for each type that MUST be supported by all implementations.

A strongly-typed programming model that represents a CloudEvent or any extension MUST be able to convert from and to the canonical string-encoding to the runtime/language native type that best corresponds to the abstract type.

For example, the time attribute might be represented by the language's native datetime type in a given implementation, but it MUST be settable providing an RFC3339 string, and it MUST be convertible to an RFC3339 string when mapped to a header of an HTTP message.

A CloudEvents transport binding or event format implementation MUST likewise be able to convert from and to the canonical string-encoding to the corresponding data type in the encoding or in transport metadata fields.

An attribute value of type Timestamp might indeed be routed as a string through multiple hops and only materialize as a native runtime/language type at the producer and ultimate consumer. The Timestamp might also be routed as a native transport type and might be mapped to/from the respective language/runtime types at the producer and consumer ends, and never materialize as a string.

  • Integer - A whole number in the range -2,147,483,648 to +2,147,483,647 inclusive. This is the range of a signed, 32-bit, twos-complement encoding. Event formats do not have to use this encoding, but they MUST only use Integer values in this range.
  • String - Sequence of printable Unicode characters.
  • Binary - Sequence of bytes.
    • String encoding: Base64 encoding per RFC4648.
  • Map - String-indexed dictionary of Any-typed values.
  • URI-reference - Uniform resource identifier reference.
  • Timestamp - Date and time expression using the Gregorian Calendar.
  • Any - A variant type that can take the shape of either an Integer, String, Binary, Map, URI-reference or Timestamp. The type system is intentionally abstract, and therefore it is left to implementations how to represent the Any type.

Context Attributes

Every CloudEvent conforming to this specification MUST include context attributes designated as REQUIRED and MAY include one or more OPTIONAL context attributes.

These attributes, while descriptive of the event, are designed such that they can be serialized independent of the event data. This allows for them to be inspected at the destination without having to deserialize the event data.

The choice of serialization mechanism will determine how the context attributes and the event data will be materialized. For example, in the case of a JSON serialization, the context attributes and the event data might both appear within the same JSON object.

REQUIRED Attributes

The following attributes are REQUIRED to be present in all CloudEvents:

id

  • Type: String
  • Description: Identifies the event. Producers MUST ensure that source + id is unique for each distinct event. If a duplicate event is re-sent (e.g. due to a network error) it MAY have the same id. Consumers MAY assume that Events with identical source and id are duplicates.
  • Examples:
    • An event counter maintained by the producer
    • A UUID
  • Constraints:
    • REQUIRED
    • MUST be a non-empty string
    • MUST be unique within the scope of the producer

source

  • Type: URI-reference

  • Description: Identifies the context in which an event happened. Often this will include information such as the type of the event source, the organization publishing the event or the process that produced the event. The exact syntax and semantics behind the data encoded in the URI is defined by the event producer.

    Producers MUST ensure that source + id is unique for each distinct event.

    An application MAY assign a unique source to each distinct producer, which makes it easy to produce unique IDs since no other producer will have the same source. The application MAY use UUIDs, URNs, DNS authorities or an application-specific scheme to create unique source identifiers.

    A source MAY include more than one producer. In that case the producers MUST collaborate to ensure that source + id is unique for each distinct event.

  • Constraints:

    • REQUIRED

  • Examples

specversion

  • Type: String
  • Description: The version of the CloudEvents specification which the event uses. This enables the interpretation of the context. Compliant event producers MUST use a value of 0.4-wip when referring to this version of the specification.
  • Constraints:
    • REQUIRED
    • MUST be a non-empty string

type

  • Type: String
  • Description: Type of occurrence which has happened. Often this attribute is used for routing, observability, policy enforcement, etc. The format of this is producer defined and might include information such as the version of the type - see Versioning of Attributes in the Primer for more information.
  • Constraints:
    • REQUIRED
    • MUST be a non-empty string
    • SHOULD be prefixed with a reverse-DNS name. The prefixed domain dictates the organization which defines the semantics of this event type.
  • Examples
    • com.github.pull.create
    • com.example.object.delete.v2

OPTIONAL Attributes

The following attributes are OPTIONAL to appear in CloudEvents. See the Notational Conventions section for more information on the definition of OPTIONAL.

datacontentencoding

  • Type: String per RFC 2045 Section 6.1

  • Description: Describes the content encoding for the data attribute for when the data field MUST be encoded as a string, like with structured transport binding modes using the JSON event format, but the datacontenttype indicates a non-string media type. When the data field's effective data type is not String, this attribute MUST NOT be set and MUST be ignored when set.

    The "Base64" value for the Base64 encoding as defined in RFC 2045 Section 6.8 MUST be supported. When set, the event-format-encoded value of the data attribute is a base64 string, but the effective data type of the data attribute towards the application is the base64-decoded binary array.

    All other RFC2045 schemes are undefined for CloudEvents.

  • Constraints:

    • The attribute MUST be set if the data attribute contains string-encoded binary data. Otherwise the attribute MUST NOT be set.
    • If present, MUST adhere to RFC 2045 Section 6.1

datacontenttype

  • Type: String per RFC 2046

  • Description: Content type of the data attribute value. This attribute enables the data attribute to carry any type of content, whereby format and encoding might differ from that of the chosen event format. For example, an event rendered using the JSON envelope format might carry an XML payload in its data attribute, and the consumer is informed by this attribute being set to "application/xml". The rules for how the data attribute content is rendered for different datacontenttype values are defined in the event format specifications; for example, the JSON event format defines the relationship in section 3.1.

    When this attribute is omitted, the data attribute simply follows the event format's encoding rules. For the JSON event format, the data attribute value can therefore be a JSON object, array, or value.

    For the binary mode of some of the CloudEvents transport bindings, where the data content is immediately mapped into the payload of the transport frame, this field is directly mapped to the respective transport or application protocol's content-type metadata property. Normative rules for the binary mode and the content-type metadata mapping can be found in the respective transport mapping specifications.

  • Constraints:

    • OPTIONAL
    • If present, MUST adhere to the format specified in RFC 2046

  • For Media Type examples see IANA Media Types

schemaurl

  • Type: URI-reference
  • Description: A link to the schema that the data attribute adheres to. Incompatible changes to the schema SHOULD be reflected by a different URL. See Versioning of Attributes in the Primer for more information.
  • Constraints:
    • OPTIONAL

subject

  • Type: String

  • Description: This describes the subject of the event in the context of the event producer (identified by source). In publish-subscribe scenarios, a subscriber will typically subscribe to events emitted by a source, but the source identifier alone might not be sufficient as a qualifier for any specific event if the source context has internal sub-structure.

    Identifying the subject of the event in context metadata (opposed to only in the data payload) is particularly helpful in generic subscription filtering scenarios where middleware is unable to interpret the data content. In the above example, the subscriber might only be interested in blobs with names ending with '.jpg' or '.jpeg' and the subject attribute allows for constructing a simple and efficient string-suffix filter for that subset of events.

  • Constraints:

    • OPTIONAL
    • If present, MUST be a non-empty string

  • Example:

    • A subscriber might register interest for when new blobs are created inside a blob-storage container. In this case, the event source identifies the subscription scope (storage container), the type identifies the "blob created" event, and the id uniquely identifies the event instance to distinguish separate occurrences of a same-named blob having been created; the name of the newly created blob is carried in subject:

time

  • Type: Timestamp
  • Description: Timestamp of when the occurrence happened. If the time of the occurrence can not be determined then this attribute MAY be set to some other time (such as the current time) by the CloudEvents producer, however all producers for the same source MUST be consistent in this respect. In other words, either they all use the actual time of the occurrence or they all use the same alogorithm to determine the value used.
  • Constraints:
    • OPTIONAL
    • If present, MUST adhere to the format specified in RFC 3339

Extension Context Attributes

CloudEvents producers MAY include additional context attributes in the event that might be used in ancillary actions related to the processing of the event. See CloudEvent Attributes Extensions for additional information concerning the use and definition of extensions.

This specification places no restriction on the type or semantics of the extension attributes. Each definition of an extensions SHOULD fully define all aspects of the attribute - e.g. its name, semantic meaning and possible values or even to indicate that it places no restrictions on its values. New extension definitions SHOULD use a name that is descriptive enough to reduce the chances of name collisions with other extensions. In particular, extension authors SHOULD check the documented extensions document for the set of known extensions - not just for possible name conflicts but for extensions that might be of interest.

Each specification that defines how to serialize a CloudEvent will define how extension attributes will appear.

Here is an example that illustrates the need for additional attributes. In many IoT and enterprise use cases, an event could be used in a serverless application that performs actions across multiple types of events. To support such use cases, the event producer will need to add additional identity attributes to the "context attributes" which the event consumers can use to correlate this event with the other events. If such identity attributes happen to be part of the event "data", the event producer SHOULD also add the identity attributes to the "context attributes" so that event consumers can easily access this information without needing to decode and examine the event data. Such identity attributes can also be used to help intermediate gateways determine how to route the events.

Data Attribute

As defined by the term Data, CloudEvents MAY include domain-specific information about the occurrence. When present, this information will be encapsulated within the data attribute.

data

  • Type: Any
  • Description: The event payload. The payload depends on the type and the schemaurl. It is encoded into a media format which is specified by the datacontenttype attribute (e.g. application/json).
  • Constraints:
    • OPTIONAL

Size Limits

In many scenarios, CloudEvents will be forwarded through one or more generic intermediaries, each of which might impose limits on the size of forwarded events. CloudEvents might also be routed to consumers, like embedded devices, that are storage or memory-constrained and therefore would struggle with large singular events.

The "size" of an event is its wire-size, and includes every bit that is transmitted on the wire for the event: transport frame-metadata, event metadata, and event data, based on the chosen event format and the chosen protocol binding.

If an application configuration requires for events to be routed across different transports or for events to be re-encoded, the least efficient transport and encoding used by the application SHOULD be considered for compliance with these size constraints:

  • Intermediaries MUST forward events of a size of 64 KByte or less.
  • Consumers SHOULD accept events of a size of at least 64 KByte.

In effect, these rules will allow producers to publish events up to 64KB in size safely. Safely here means that it is generally reasonable to expect the event to be accepted and retransmitted by all intermediaries. It is in any particular consumer's control, whether it wants to accept or reject events of that size due to local considerations.

Generally, CloudEvents publishers SHOULD keep events compact by avoiding to embed large data items into event payloads and rather use the event payload to link to such data items. From an access control perspective, this approach also allows for a broader distribution of events, because accessing event-related details through resolving links allows for differentiated access control and selective disclosure, rather than having sensitive details embedded in the event directly.

Privacy and Security

Interoperability is the primary driver behind this specification, enabling such behavior requires some information to be made available in the clear resulting in the potential for information leakage.

Consider the following to prevent inadvertent leakage especially when leveraging 3rd party platforms and communication networks:

  • Context Attributes

    Sensitive information SHOULD NOT be carried or represented in context attributes.

    CloudEvent producers, consumers, and intermediaries MAY introspect and log context attributes.

  • Data

    Domain specific data SHOULD be encrypted to restrict visibility to trusted parties. The mechanism employed for such encryption is an agreement between producers and consumers and thus outside the scope of this specification.

  • Transport Bindings

    Transport level security SHOULD be employed to ensure the trusted and secure exchange of CloudEvents.

Example

The following example shows a CloudEvent serialized as JSON:

{
    "specversion" : "0.4-wip",
    "type" : "com.github.pull.create",
    "source" : "https://github.com/cloudevents/spec/pull",
    "subject" : "123",
    "id" : "A234-1234-1234",
    "time" : "2018-04-05T17:31:00Z",
    "comexampleextension1" : "value",
    "comexampleextension2" : {
        "othervalue": 5
    },
    "datacontenttype" : "text/xml",
    "data" : "<much wow=\"xml\"/>"
}