Extensible and Dynamic Topic Types for DDS Avatar
  1. OMG Specification

Extensible and Dynamic Topic Types for DDS — Open Issues

  • Acronym: DDS-XTypes
  • Issues Count: 4
  • Description: Issues not resolved
Open Closed All
Issues not resolved

Issues Descriptions

Restrictions on MAP key element type should include ENUM

  • Key: DDSXTY13-6
  • Status: open  
  • Source: Airbus Group ( Oliver Kellogg)
  • Summary:

    DDS-XTypes 1.2 section 7.2.2.4.3 (Collection Types) on page 30 bottom states:

    > * Element type: The concrete type to which all elements conform. (Collection Elements
    > that are of a subtype of the element type rather than the element type itself may be
    > truncated when they are serialized into a Data Representation.)
    > In the case of a map type, this attribute corresponds to the type of the value elements.
    > Map types have an additional attribute, the key element type, that indicates the type of the
    > may key objects.

    I believe there is a typo in "type of the may key objects" - drop the word "may" ?

    The paragraph continues:

    > Implementers of this specification need only support key elements of
    > signed and unsigned integer types and of narrow and wide string types; the behavior of
    > maps with other key element types is undefined and may not be portable.

    Is there a particular reason why enums were excluded from the supported key types?

  • Reported: DDS-XTypes 1.2b1 — Fri, 15 Sep 2017 13:51 GMT
  • Updated: Tue, 19 Sep 2017 19:59 GMT

Typographical corrections

  • Key: DDSXTY13-5
  • Status: open  
  • Source: Airbus Group ( Oliver Kellogg)
  • Summary:

    > 2.2.2 Basic Network Inteoperability Profile

    • "Interoperability"

    > 7.2.3 Type Extensibility and Mutability
    > [...]
    > • A type may be APENDABLE,

    • "APPENDABLE"

    > [...]
    > It is summarized more enerally in Table 12.

    • "generally"

    > 7.2.4.4.1 Assignability of Equivalent Types
    > If two types T1 and T2 are equivalent according to the MINIMAL relation (see Section 7.3.4.7),
    > then thery are mutually assignable,

    • "they"

    > Table 15 – Definition of the is-assignable-from relationship for primitive types
    > [...]
    > UINT64_TYPE
    > BITMASK_TYPE if and only if
    > T2.bound is between 33vand
    > 64, inclusive.

    • "between 33 and 64"

    > Table 20 – Alternative Type Representations
    > [...]
    > XSD [... 2nd column]
    > [...] No direct support for many of the
    > contructs (e.g keys) or the types in
    > the type model

    • "constructs"

    > 7.3.4.6.2 Hash TypeIdentifiers
    > Some TypeIdentifiers include within (directly or indirectly) hashes of one of mre
    > TypeObjects.
    Phrase is unclear, please reword.

    > 7.3.4.6.4 Indirect Hash TypeIdentifiers
    > These are the HASH [...] using a hash
    > TypeIdentifiers . They are distinghished byt:

    • "They are distinguished by:"

    > 7.4.1.1.1 Primitive types
    > [...]
    > • An endianness byte swap shall be performed in case the native system endianness is dif-
    > ferent from the one currently configured in the XCDR stream (XCDR.cendien).

    • "XCDR.cendian"
  • Reported: DDS-XTypes 1.2b1 — Sun, 10 Sep 2017 19:05 GMT
  • Updated: Tue, 12 Sep 2017 19:04 GMT

XSD for XML type representation should not specify default values for attributes representing annotations

  • Key: DDSXTY13-4
  • Status: open  
  • Source: Real-Time Innovations ( Gerardo Pardo-Castellote)
  • Summary:

    The XML type representation (Section 7.3.3) has an associated XSD that defines the legal XML documents that define DDS Types.

    In the XML type representation builtin annotations on members appear as "attributes" on the element that describes the member. For example see example in section 7.3.2.5.1.2 (Members):

    <struct name="structMemberDecl">
       <member name="my_key_field" type="int32" key="true" optional="false"/>
    </struct>
    

    These builtin annotations (key, optional), have default values when they are not present. This is defined in the IDL4+ specification. For example when the annotation "key" is not present, the (default) value is "false" when annotation "optional" is not present, the (default) value is "false". There is also a "default" value when the annotation is present with no parameters, but this is not allowed in the XSD.

    According to the XSD syntax, e.g. see https://www.w3schools.com/xml/schema_simple_attributes.asp

    The "default" value specified for an attribute is the value interpreted when the attribute is not present. Therefore we have two options:

    • Have the XSD specify default values for these attributes to match the "IDL4+ defaults when the attribute is not present
    • Have the XSD specify no default value.

    Currently this is not done correctly for some annotations. For example the XSD for structure members has wrong defaults for all the attributes:

      <xs:complexType name="structMemberDecl">
        <xs:complexContent>
          <xs:extension base="memberDecl">
           <xs:attribute name="id"
                         type="memberId"
                         use="optional"/>
    
            <xs:attribute name="optional"
                          type="xs:boolean"
                          use="optional"
                          default="true"/>
            <xs:attribute name="mustUnderstand"
                          type="xs:boolean"
                          use="optional"
                          default="true"/>
            <xs:attribute name="nonSerialized"
                          type="xs:boolean"
                          use="optional"
                          default="true"/>
            <xs:attribute name="key"
                          type="xs:boolean"
                          use="optional"
                          default="true"/>
          </xs:extension>
        </xs:complexContent>
      </xs:complexType>
    

    However it seems like the best solution is to remove the specification of a default value from the XSD. The problem is that when the default is specified the XSD parsers will fill the default value even if the attribute is not specified and it becomes impossible for the application that uses the parser to know if the attribute was there or not in the first place. This would make it impossible to transform between IDL and XML and back to IDL and get the same result back because all annotations would appear present to the XML parser even if they were not entered buy the user.

    Therefore we recommend removing the specification of 'default" value for all XML attributes that correspond to builtin annotations. This should be done both in Annex A and the machine readable dds-xtypes_type_definition_nonamespace.xsd

  • Reported: DDS-XTypes 1.2 — Mon, 24 Apr 2017 16:16 GMT
  • Updated: Mon, 24 Apr 2017 16:16 GMT

Algorithm to compute autoid is missing from the specification

  • Key: DDSXTY13-2
  • Status: open  
  • Source: Real-Time Innovations ( Gerardo Pardo-Castellote)
  • Summary:

    The specification does not define how memberIDs are computed in the case where the @autoid (or @hashid) annotations are used. This is needed for interoperability.

    Section 7.3.1.2.1.1 (Member IDs) specifies there are thee ways to set them:

    • Automatically following a progression that starts from the most-recently specified member ID.
    • Using the @id annotation
    • As a "hash" on the member name when @autoid(HASH) is specified
    • as a "hash" on a string-parameter when @hashid("string-parameter") is specified

    The use of @autoid refers to sub clause 8.3.1.2 in [IDL41]). But there also there is no specification of how the hash should be computed. Only that a "hashing" algorithm should be used.

    IDL working group discussed this and the preference was to leave it unspecified in IDL4 and instead put it in XTYPES or whichever specification depends on these IDs.

    A proposal would be to use an MD5 (as this is already used for key hashing).
    This needs to be done in a platform-independent manner, for example hashing a serialized representation of the string using a pre-specified endianness. Also per section 7.2.2.4.4.3 (Member IDs) the value/range must be representable in 28 bits.

  • Reported: DDS-XTypes 1.2 — Fri, 7 Apr 2017 00:44 GMT
  • Updated: Fri, 7 Apr 2017 00:44 GMT