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Signed-off-by: Christoph Neijenhuis <christoph.neijenhuis@commercetools.de>
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CloudEvents - Version 1.x-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 aid 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 Producer and Consumer of a message to support that feature. In other words, a producer can choose to include that feature in a message if it wants, and a consumer can choose to support that feature if it wants. A consumer that does not support that feature will then silently ignore that part of the message. The producer needs to be prepared for the situation where a consumer ignores that feature. An Intermediary SHOULD forward OPTIONAL attributes.

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 Event Data representing the Occurrence and Context metadata providing contextual information about the Occurrence. A single occurrence MAY result in more than one event.

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 Event Data section for more information.

Event Format

An Event Format specifies how to serialize a CloudEvent as a sequence of bytes. Stand-alone event formats, such as the JSON format, specify serialization independent of any protocol or storage medium. Protocol Bindings MAY define formats that are dependent on the protocol.

Message

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

A "structured-mode message" is one where the event is fully encoded using a stand-alone event format and stored in the message body.

A "binary-mode message" is one where the event data is stored in the message body, and event attributes are stored as part of message meta-data.

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).

Protocol Binding

A protocol binding describes how events are sent and received over a given protocol.

Protocol bindings MAY choose to use an Event Format to map an event directly to the transport envelope body, or MAY provide additional formatting and structure to the envelope. For example, a wrapper around a structured-mode message might be used, or several messages could be batched together into a transport envelope body.

Context Attributes

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

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. Attribute names SHOULD be descriptive and terse and SHOULD NOT exceed 20 characters in length.

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 protocol metadata fields. This specification defines a canonical string-encoding for each type that MUST be supported by all implementations.

  • Boolean - a boolean value of "true" or "false".
    • String encoding: a case-sensitive value of true or false.
  • 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 allowable Unicode characters. The following characters are disallowed:
    • the "control characters" in the ranges U+0000-U+001F and U+007F-U+009F (both ranges inclusive), since most have no agreed-on meaning, and some, such as U+000A (newline), are not usable in contexts such as HTTP headers.
    • code points identified as noncharacters by Unicode.
    • code points identifying Surrogates, U+D800-U+DBFF and U+DC00-U+DFFF, both ranges inclusive, unless used properly in pairs. Thus (in JSON notation) "\uDEAD" is invalid because it is an unpaired surrogate, while "\uD800\uDEAD" would be legal.
  • Binary - Sequence of bytes.
    • String encoding: Base64 encoding per RFC4648.
  • URI - Absolute uniform resource identifier.
  • URI-reference - Uniform resource identifier reference.
  • Timestamp - Date and time expression using the Gregorian Calendar.

All context attribute values MUST be of one of the types listed above. Attribute values MAY be presented as native types or canonical strings.

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 protocol 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 protocol 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 protocol type and might be mapped to/from the respective language/runtime types at the producer and consumer ends, and never materialize as a string.

The choice of serialization mechanism will determine how the context attributes and the event data will be serialized. 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.
  • Constraints:
    • REQUIRED
    • MUST be a non-empty string
    • MUST be unique within the scope of the producer
  • Examples:
    • An event counter maintained by the producer
    • A UUID

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
    • MUST be a non-empty URI-reference
    • An absolute URI is RECOMMENDED
  • 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 1.x-wip when referring to this version of the specification.
  • Constraints:
    • REQUIRED
    • MUST be a non-empty string

type

  • Type: String
  • Description: This attribute contains a value describing the type of event related to the originating occurrence. 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_request.opened
    • com.example.object.deleted.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.

datacontenttype

  • Type: String per RFC 2046

  • Description: Content type of data value. This attribute enables data 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 data, and the consumer is informed by this attribute being set to "application/xml". The rules for how data 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.

    For some binary mode protocol bindings, this field is directly mapped to the respective protocol's content-type metadata property. Normative rules for the binary mode and the content-type metadata mapping can be found in the respective protocol

    In some event formats the datacontenttype attribute MAY be omitted. For example, if a JSON format event has no datacontenttype attribute, then it is implied that the data is a JSON value conforming to the "application/json" media type. In other words: a JSON-format event with no datacontenttype is exactly equivalent to one with datacontenttype="application/json".

    When translating an event message with no datacontenttype attribute to a different format or protocol binding, the target datacontenttype SHOULD be set explicitly to the implied datacontenttype of the source.

  • Constraints:

    • OPTIONAL
    • If present, MUST adhere to the format specified in RFC 2046
  • For Media Type examples see IANA Media Types

dataschema

  • Type: URI
  • Description: Identifies the schema that data adheres to. Incompatible changes to the schema SHOULD be reflected by a different URI. See Versioning of Attributes in the Primer for more information.
  • Constraints:
    • OPTIONAL
    • If present, MUST be a non-empty URI

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 cannot 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 algorithm to determine the value used.
  • Constraints:
    • OPTIONAL
    • If present, MUST adhere to the format specified in RFC 3339

Extension Context Attributes

A CloudEvent MAY include any number of additional context attributes with distinct names, known as "extension attributes". Extension attributes MUST follow the same naming convention and use the same type system as standard attributes. Extension attributes have no defined meaning in this specification, they allow external systems to attach metadata to an event, much like HTTP custom headers.

Extension attributes are always serialized according to binding rules like standard attributes. However this specification does not prevent an extension from copying event attribute values to other parts of a message, in order to interact with non-CloudEvents systems that also process the message. Extension specifications that do this SHOULD specify how receivers are to interpret messages if the copied values differ from the cloud-event serialized values.

Defining Extensions

See CloudEvent Attributes Extensions for additional information concerning the use and definition of extensions.

The definition of an extension SHOULD fully define all aspects of the attribute - e.g. its name, type, semantic meaning and possible 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.

Many protocols support the ability for senders to include additional metadata, for example as HTTP headers. While a CloudEvents receiver is not mandated to process and pass them along, it is RECOMMENDED that they do so via some mechanism that makes it clear they are non-CloudEvents metadata.

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 would 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.

Event Data

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

  • Description: The event payload. This specification does not place any restriction on the type of this information. It is encoded into a media format which is specified by the datacontenttype attribute (e.g. application/json), and adheres to the dataschema format when those respective attributes are present.

  • 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: protocol 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 protocols or for events to be re-encoded, the least efficient protocol 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 embedding 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 event 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.

  • Protocol Bindings

    Protocol 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" : "1.x-wip",
    "type" : "com.github.pull_request.opened",
    "source" : "https://github.com/cloudevents/spec/pull",
    "subject" : "123",
    "id" : "A234-1234-1234",
    "time" : "2018-04-05T17:31:00Z",
    "comexampleextension1" : "value",
    "comexampleothervalue" : 5,
    "datacontenttype" : "text/xml",
    "data" : "<much wow=\"xml\"/>"
}
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