Oneof equivalent?

[elrom-oai]

Looks like the question of having an equivalent to protobuf oneof was raised here: aeron-io/simple-binary-encoding#1043

Has there been a resolution? Alternatively, is there a canonical workaround?

[kleihan]

The issue raised here: aeron-io/simple-binary-encoding#1043 was not discussed by FIX. Currently, there is no active FIX working group to continue the work on SBE 2.0.

Orchestra, the FIX standard for meta-data is able to express the oneOf condition on a logical level. The example from the link above is:

<union name="Car" encodingType="uint8" blockLengthType="uint16">
    <option name="Electric" id="0">
        <type name="capacity" primitiveType="uint16"/>
    </option>
    <option name="Combustion" = id="1">
      <enum name="fuel" encodingType="uint8">
        <validValue name="Gasoline">1</validValue>
        <validValue name="Diesel">2</validValue>
      </enum>
    </option>
</union>

Orchestra could express this as follows:

<fixr:codeSet id="1" name="fuelCodeSet" type="integer">
  <fixr:code id="1" name="Gasoline"/>
  <fixr:code id="2" name="Diesel"/>
</fixr:codeSet>

<fixr:field id="1" name="Electric" type="integer"/>
<fixr:field id="2" name="Combustion" codeSet="fuelCodeSet"/>

<fixr:component name="car" which="oneOf">
  <fieldRef id="1" name="Electric"/>
  <fieldRef id="2" name="Combustion"/>
</fixr:component>

Mapping this to a specific encoding depends on the specific capabilities of the encoding. In the case of SBE or protobuf, the logical representation in Orchestra needs to be mapped to an SBE or protobuf schema respectively. In the case of protobuf, there is a native option to express this whilst this is not the case for SBE.

The objective of SBE was to be for low-latency applications and the ability to directly access data elements in the message. There is a trade-off when adding variable length elements or repeating groups. The proposed union (aka choice component) has a fixed length but also requires additional meta-data as part of the wire format. The alternative is to have bytes for all elements of a union in the wire format as optional and the logical model (Orchestra) tells the application which element to pick, depending on the value of other elements in the message. The length of a message is less of an issue nowadays.

Orchestra can be used to generate code that takes the "oneOf" nature of the component into consideration. Orchestra allows the definition of rules for conditional presence of data elements.