An indicator of the behavior of a given field (for example, that a field is required in requests, or given as output but ignored as input). This does not change the behavior in protocol buffers itself; it only denotes the behavior and may affect how API tooling handles the field.
Note: This enum may receive new values in the future.
Conventional default for enums. Do not use this.
Specifically denotes a field as optional. While all fields in protocol buffers are optional, this may be specified for emphasis if appropriate.
Denotes a field as required. This indicates that the field must be provided as part of the request, and failure to do so will cause an error (usually
Denotes a field as output only. This indicates that the field is provided in responses, but including the field in a request does nothing (the server must ignore it and must not throw an error as a result of the field's presence).
Denotes a field as input only. This indicates that the field is provided in requests, and the corresponding field is not included in output.
Denotes a field as immutable. This indicates that the field may be set once in a request to create a resource, but may not be changed thereafter.
Denotes that a (repeated) field is an unordered list. This indicates that the service may provide the elements of the list in any arbitrary order, rather than the order the user originally provided. Additionally, the list's order may or may not be stable.
Denotes that this field returns a non-empty default value if not set. This indicates that if the user provides the empty value in a request, a non-empty value will be returned. The user will not be aware of what non-empty value to expect.