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  1. OMG Issue

CORBA34 — Codec Interface Deficiencies

  • Key: CORBA34-195
  • Legacy Issue Number: 7730
  • Status: open  
  • Source: Zuehlke Engineering ( Frank Pilhofer)
  • Summary:

    CORBA 3, chapter 13.8, defines the Codec interface to encode
    arbitrary data values into CORBA::OctetSeq "blobs" and vice
    versa. This interface can be used, e.g., to supply and retrieve
    ServiceContext data using the PortableInterceptor interfaces.

    In practice, the Codec interface is also being used for data
    serialization, i.e., to store and retrieve arbitrary values in
    files or other databases.

    However, the interface is deficient in that it does not consider
    all possible variables that are needed for interoperability.
    It supports setting the CDR version that is to be used, but
    neglects byteorder and codeset settings.

    Consequently, the encoded values are platform-specific. If a
    value was encoded on a little-endian system, it will not decode,
    or worse, decode erroneously, on a big-endian system. The same
    caveats apply to codesets, e.g., when an ISO-8859-1 encoded
    blob is decoded using UTF-8 or Windows-1252.

    To support interoperability, the Codec interface needs to be
    extended.

    My recommendation is to extend the CodecFactory interface,
    so that it supports creating CDR version-, byteorder-, and
    codeset-specific Codec instances, either supplying user-
    provided values for each, or informing the user about chosen
    defaults.

    Example:

    module IOP {
    const EncodingFormat ENCODING_DEFAULT = -1;

    typedef short ByteorderFormat;
    const ByteorderFormat BYTEORDER_DEFAULT = -1;
    const ByteorderFormat BYTEORDER_BIGENDIAN = 0;
    const ByteorderFormat BYTEORDER_LITTLEENDIAN = 1;

    struct EncodingExt

    { EncodingFormat format; octet major_version; // set to 0 for default octet minor_version; ByteorderFormat byteorder; CONV_FRAME::CodeSetId char_data; // set to 0 for default CONV_FRAME::CodeSetId wchar_data; // set to 0 for default }

    ;

    local interface CodecFactory

    { // create_codec remains as before Codec create_codec_ext (inout EncodingExt enc) raises (UnknownEncoding); }

    ;
    };

    The create_codec_ext operation would create an appropriate
    Codec instance, if available; it will then set all "default"
    members of the EncodingExt structure to their actual values,
    so that the application can store this information along
    with any encoded values.

    One potential criticism of the above is that the encoding
    format's parameters depend on the encoding format. For example,
    there may be encoding formats that are byteorder-independent,
    or that consistently use UTF-32 for strings, thus not needing
    codeset parameters. Also, they may use wildly different
    versioning. So a "better" solution might involve passing
    the EncodingFormat, and an Any with a format-specific data
    type.

    That could look like:

    module GIOP {
    typedef short ByteorderFormat;
    const ByteorderFormat BYTEORDER_DEFAULT = -1;
    const ByteorderFormat BYTEORDER_BIGENDIAN = 0;
    const ByteorderFormat BYTEORDER_LITTLEENDIAN = 1;

    struct CDREncodingParameters

    { octet major_version; // set to 0 for default octet minor_version; ByteorderFormat byteorder; CONV_FRAME::CodeSetId char_data; // set to 0 for default CONV_FRAME::CodeSetId wchar_data; // set to 0 for default }

    ;
    };

    module IOP {
    const EncodingFormat ENCODING_DEFAULT = -1;

    local interface CodecFactory

    { // create_codec remains as before Codec create_codec_ext (inout EncodingFormat format, inout Any parameters) raises (UnknownEncoding); }

    ;
    };

    Once we have consensus on the approach, I will gladly volunteer
    to come up with a full set of editing instructions

  • Reported: CORBA 3.0.3 — Thu, 9 Sep 2004 04:00 GMT
  • Updated: Tue, 8 Oct 2019 18:02 GMT