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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 ID counter maintained by the producer
    • A UUID

CloudEvents-NL

  • Constraints:
    • If an ID is available that can durable identify the event, producers SHOULD use that ID. (for example so that consumers MAY use id to request information about the event from the source).
    • If no ID is available that can durable identify the event producers SHOULD use a random ID:
      • SHOULD use a UUID.
      • MUST describe the limitations (eg that it's just a random ID and no identification of the event occurred).
  • Examples:
    • 'doc2021033441' (ID of the document created as a result of an event that occurred).
    • 'f3dce042-cd6e-4977-844d-05be8dce7cea' (UUID generated with the sole function of being able to uniquely identify the event.

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

CloudEvents-NL

  • Constraints:
    • SHOULD be a URN notation with 'nld' as namespace identifier.
    • SHOULD contain consecutive a unique identifier of:
      • the organization that publishes the event
      • the source system that publishes the event.
    • involved organizations SHOULD agree on how organizations and systems are uniquely identified (e.g. via the use of OIN, KVK-nummer or for organization identification);
      • In line with API Designrules identification :
        • SHOULD use the "organisatie-identificatienummer" (OIN) for identifying Dutch government organizations
        • SHOULD use the KvK-nummer for identifying Dutch non-government organizations (companies, associations, foundations etc...)
        • SHOULD use the eIDAS legal identifier in the EU context. national, European or worldwide)
      • one SHOULD choose an abstraction level that can be used sustainably; even if the initial scope expands (e.g. from domain specific to more general).
    • SHOULD NOT be used to reference an external data location (see extension attribute Dataref for that purpose).
  • Examples:
    • urn:nld:oin:00000001823288444000:systeem:BRP-component
    • urn:nld:kvknr:09220932.burgerzakensysteem
    • urn:nld:gemeente-nijmegen.burgerzakensysteem
    • urn:nld:gemeente-Bergen%20%28L%29.burgerzakensysteem Comment: The use of (unique) descriptions increases recognisability, but also has disadvantages such as occurred changes or required encoding (like in the above example where "Bergen (L)" requires encoding).

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 1.0 when referring to this version of the specification.

    Currently, this attribute will only have the 'major' and 'minor' version numbers included in it. This allows for 'patch' changes to the specification to be made without changing this property's value in the serialization. Note: for 'release candidate' releases a suffix might be used for testing purposes.

  • Constraints:

    • REQUIRED
    • MUST be a non-empty string

CloudEvents-NL

  • MUST be '1.0'

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 CloudEvents 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

CloudEvents-NL

Constraints:

  • MUST be Reverse domain name notation
  • MAY be specified by adding as a suffix (for example: nl.brp.verhuizing.binnengemeentelijk)
  • Producers MUST facilitate consumers to request the exact meaning.
  • SHOULD stay the same when a CloudEvent's data changes in a backwardly-compatible way.
  • SHOULD change when a CloudEvent's data changes in a backwardly-incompatible way.
  • The producer SHOULD produce both the old event and the new event for some time (potentially forever) in order to avoid disrupting consumers.
  • The producer decides if versioning is used.
  • If versioning is used semantic versioning SHOULD be used; a prefix of 'v' SHOULD be used to indicate it is a version number (e.g. 'v1.2.3')
  • In descending order of preference one SHOULD use the name of a:
    • data source (for example: 'nl.brp.persoon-verhuisd)
    • domain (for example: nl.natuurlijke-personen.persoon-verhuisd); for domain designation plural MUST be used.
    • law or rule (for example: nl.amsterdam.erfpacht.overdracht)
  • Names of organizations SHOULD NOT be used (because they are not durable). Examples:
  • nl.vng.zgw.zaken.status.create or nl.overheid.zaken.zaakstatus-gewijzigd (context is relevant when defining type)
  • nl.brp.huwelijk-voltrokken or nl.brp.persoon-gehuwd (be specific because exact meaning can differ)
  • nl.vng.zgw.zaak-toegevoegd-aan-document or nl.vng.zgw.document-toegevoegd-aan-zaak (perspective is relevant)
  • nl.brp.huwelijk-voltrokken.v0.1.0 (for initial development, anything may change)

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

CloudEvents-NL

Constraints:

  • JSON-format SHOULD be used (The Nederlandse API Strategie knows API-Design Rules Extensions in development. Part of this is the intention to name JSON as the primary representation format for APIs. Because APIs play an important role in communicating events (eg when using the webhook pattern) the JSON format is preferred to use for payload data).

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 CloudEvents in the Primer for more information.
  • Constraints:
    • OPTIONAL
    • If present, MUST be a non-empty URI

CloudEvents-NL

Constraints:

  • It SHOULD be prevented that different schedules arise for the same data.
  • The dataschema attribute is expected to be informational, largely to be used during development and by tooling that is able to provide diagnostic information over arbitrary CloudEvents with a data content type understood by that tooling (see: The role of the dataschema attribute within versioning

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:
      • source: https://example.com/storage/tenant/container
      • subject: mynewfile.jpg

CloudEvents-NL

Constraints:

  • Decision on whether or not to use the attribute and/or the exact interpretation is postponed. To be determined partly on the basis of future agreements about subscription and filtering.

Example:

  • source: urn:nld:oin:00000001823288444000:systeem:BRP-component
  • type: nl.brp.persoon-gehuwd
  • subject: 999990342 (citizen service number)

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

CloudEvents-NL

  • The time the event was logged SHOULD be used (in many cases this is the only time that can be determined unambiguously).
  • The exact meaning of time MUST be clearly documented.
  • The time when an event occurred in reality SHOULD NOT be used (if there is a need for this among consumers, this can be included in payload data).
  • If the time when an event occurred in reality is needed for things like routing or filtering, it can be included as a context attribute by the producer.

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.

CloudEvents-NL

  • Two of the extension attributes included by CloudEvents ('dataref' and 'sequence') are included as optional attributes in the CloudEvents-NL profile because it is foreseen that there is often a need to use these attributes.
  • Extension attributes should be kept minimal to ensure the CloudEvent can be properly serialized and transported (e.g. when using HTTP-headers most HTTP servers will reject requests with excessive HTTP header data).

dataref

  • Type: URI-reference

  • Description: A reference to a location where the event payload is stored. The location MAY not be accessible without further information (e.g. a pre-shared secret).

    Known as the "Claim Check Pattern", this attribute MAY be used for a variety of purposes, including:

    • If the Data is too large to be included in the message, the data is not present, and the consumer can retrieve it using this attribute.
    • If the consumer wants to verify that the Data has not been tampered with, it can retrieve it from a trusted source using this attribute.
    • If the Data MUST only be viewed by trusted consumers (e.g. personally identifiable information), only a trusted consumer can retrieve it using this attribute and a pre-shared secret.

    If this attribute is used, the information SHOULD be accessible long enough for all consumers to retrieve it, but MAY not be stored for an extended period of time.

  • Constraints:

    • OPTIONAL

Example

The following example shows a CloudEvent in which the event producer has included both data and dataref (serialized as JSON):

{
    "specversion" : "1.0",
    "type" : "com.github.pull_request.opened",
    "source" : "https://github.com/cloudevents/spec/pull/123",
    "id" : "A234-1234-1234",
    "datacontenttype" : "text/xml",
    "data" : "<much wow=\"xml\"/>",
    "dataref" : "https://github.com/cloudevents/spec/pull/123/events/A234-1234-1234.xml"
}

CloudEvents-NL

  • MAY be used to reference an external data location (for example: a link back to the producer of the event that can be queried for more information about the event).
  • MAY be used to implenment 'informatiearm notificeren' where the consumer of the event receives some minimal information on the nature of the event, but then has to issue a request back to the producer to obtain additional information (the time aspect may deserve attention because changes may occur in the period that consumers are notified and the time of requesting additional information).

Sequence

This extension defines two attributes that can be included within a CloudEvent to describe the position of an event in the ordered sequence of events produced by a unique event source. The sequence attribute represents the value of this event's order in the stream of events. The exact value and meaning of this attribute is defined by the sequencetype attribute. If the sequencetype is missing, or not defined in this specification, event consumers will need to have some out-of-band communication with the event producer to understand how to interpret the value of the attribute.

Attributes

sequence

  • Type: String
  • Description: Value expressing the relative order of the event. This enables interpretation of data supercedence.
  • Constraints
    • REQUIRED
    • MUST be a non-empty lexicographically-orderable string
    • RECOMMENDED as monotonically increasing and contiguous

sequencetype

  • Type: String
  • Description: Specifies the semantics of the sequence attribute. See the SequenceType Values section for more information.
  • Constraints:
    • OPTIONAL
    • If present, MUST be a non-empty string
SequenceType Values

This specification defines the following values for sequencetype. Additional values MAY be defined by other specifications.

Integer

If the sequencetype is set to Integer, the sequence attribute has the following semantics:

  • The values of sequence are string-encoded signed 32-bit Integers.
  • The sequence MUST start with a value of 1 and increase by 1 for each subsequent value (i.e. be contiguous and monotonically increasing).
  • The sequence wraps around from 2,147,483,647 (2^31 -1) to -2,147,483,648 (-2^31).

CloudEvents-NL

  • Attribute 'sequence' can be helpful in situations where:
  • a form of 'pull mechanism' is used ((e.g. periodically fetching events by consumers via HTTP request)) or
  • where there is a need for (re)synchronization (e.g. after errors have occurred).