Dear collegues,
I would appreciate to hear your opinion concerning an XSD design problem. The goal is a general pattern for modelling web service responses which are required to either deliver a normal payload or, instead, deliver error diagnostics. The problem at hand is whether or not to use a choice between an "Errors" element and a normal payload,
Until now I followed a pattern consisting of such a choice:
<xs:choice>
<xs:element name="Errors" type="ErrorsType"/>
<xs:sequence>
<xs:element name="SomeData" type="xs:string"/>
<xs:element name="MoreData" type="xs:string"/>
</xs:sequence>
</xs:choice>
The advantage is the clear semantics: EITHER error OR payload. Never none or both.
Unfortunately, the choice (including the error branch) canot be shifted into a common base type, as any extension of such a base type would come after the choice, rather than extend the payload branch. Java developers complain, as the error structure is thus (from their point of view) repeated unduly.
So I wonder about the following alternative, which replaces the choice by a sequence: a nillable (or, alternatively, optional) error element is followed by the optional payload, which is provided by the response type extending the base type
<xs:complexType name="BaseRSType" abstract="true">
<xs:sequence>
<xs:element name="RS_Metadata" type="RS_MetadataType"/>
<xs:element name="Errors" xsi:nil="true" type="ErrorsType"/>
</xs:sequence>
</xs:complexType>
<xs:complexType name="FooRSType">
<xs:complexContent>
<xs:extension base="BaseRSType">
<xs:sequence minOccurs="0">
<xs:element name="SomeData" type="xs:string"/>
<xs:element name="MoreData" type="xs:string"/>
</xs:sequence>
</xs:extension>
</xs:complexContent>
</xs:complexType>
[Note the minOccurs=0 on the sequence representing a normal payload.]
Advantage: the error structure has been shifted into a common base type of all responses. Disadvantage: the either-error-or-payload semantics is not any more captured by the schema, which would allow also an error plus a payload.
Should you have an opinion whether to prefer the choice model or the sequence model, I would like to know.
Kind regards,
Hans-Juergen Rennau
