The CloudEvents specification defines a set of metadata attributes than can be used when transforming a generic event into a CloudEvent. The list of attributes specified in that document represent the minimal set that the specification authors deemed most likely to be used in a majority of situations.
This document defines some addition attributes that, while not as commonly used as the ones specified in the CloudEvents specification, could still benefit from being formally specified in the hopes of providing some degree of interoperability. This also allows for attributes to be defined in an experimental manner and tested prior to being considered for inclusion in the CloudEvents specification.
Implementations of the CloudEvents specification are not mandated to limit their use of extension attributes to just the ones specified in this document. The attributes defined in this document have no official standing and might be changed, or removed, at any time. As such, inclusion of an attribute in this document does not need to meet the same level of maturity, or popularity, as attributes defined in the CloudEvents specification. To be included in this document, aside from the normal PR review process, the attribute needs to have at least two Voting member organizations stating their support for its inclusion as comments in the PR. If the author of the PR is also a Voting member, then they are allowed to be one of two.
Support for any extension is OPTIONAL. When an extension definition uses RFC 2199 keywords (e.g. MUST, SHOULD, MAY), this usage only applies to events that use the extension.
Extensions always follow a common placement strategy for in-memory formats (e.g. JSON, XML, Protocol Buffers) that are decided by those representations. Transport bindings (e.g. HTTP, MQTT, AMPQ, NATS) provide default placement for extensions, but an extension MAY require special representation when transported (e.g. tracing standards that require specific headers). Extension authors SHOULD only require special representation in transport bindings where extensions integrate with pre-existing specs; extensions with custom transport bindings are much more likely to be dropped by middleware that does not understand the extension.
As a convention, extensions of scalar types (e.g. String, Binary,
URI-reference, Number) document their Value and structured types document
their Attributes.