7. Optional fields in events ============================ Status ------ Accepted Context ------- `The attrs documentation `_ specifically defines the notion of optionality as, "An optional attribute is one which can be set to None in addition to satisfying the requirements of the sub-validator." Importantly, under this definition, the user must still provide a value for this optional field when instantiating the ``attrs``-decorated class, but this value can be ``None``. Separately, attrs fields can have a ``default`` value, which will provide a value for the field if the user does not supply one at instantiation. `The Avro documentation `_ describes somewhat similar mechanics for its fields. If a field's ``type`` list includes "null," the user can set the value of the field to null/None. Separately, if a field is defined with a ``default`` parameter, then the user does not have to set the value at all. However, it seems the ``default`` parameter may not be fully-supported with Kafka (see https://github.com/confluentinc/kafka-rest/issues/427 ). The Avro documentation, in contrast to that of ``attrs``, uses the default parameter to define optionality. Decision -------- - The Avro schema generation code will convert ``attrs`` fields with a default value of None to Avro fields with ``type = ["null",]`` and ``default = "null"``. Consequences ------------ - The notions of "optionality" and "nullability" will be effectively conflated. This means no distinctions will be made between fields for which ``None`` is a valid value and fields that simply do not need to be supplied. - Attr fields with defaults that are not ``None`` will be treated as required - At the point of writing this ADR, it is unclear exactly how Kafka will treat the addition and removal of optional fields for the purposes of schema evolution (because of the issue linked above). This will require a little more testing. Deferred/Rejected Decisions --------------------------- - Top-level fields will continue to be required, as stated in ":doc:`0006-event-schema-serialization-and-evolution`". We continue to defer the possibility of making these optional. - We are also deferring any work around making an explicit ``Optional`` keyword for attrs fields