The intention was that this new type would be an internal built-in type and so no extra properties could be placed on the new simple type. One drawback that I'm realizing as I implement this feature in Daffodil, is that in order to use non DFDL aware XML Validation tools to validate the XML infoset, you need to provide and xs:import this new DFDL schema that defines the dfdlx:blob type, which feels a little awkward to me for something that's considered a built-in for DFDL processors. Maybe an alternative would be to not have a dfdlx:blob type, allow the use of the xs:anyURI type for simple elements, with the implication that we treat the element as if it were xs:hexBinary except for the infoset/blob output. This doesn't easily support CLOB's, but a new DFDL property could determine how an xs:anyURI should be interpreted, e.g.: <xs:element name="myBlobData" type="xs:anyURI" dfdl:largeObjectType="xs:hexBinary" ... /> <xs:element name="myClobData" type="xs:anyURI" dfdl:largeOjectType="xs:string" ... /> So a type of xs:anyURI implies this is going to be some kind of large object representation, and it requires the dfdl:largeOjectType property that must reference a simple type that defines how the content should be turned into an large object. This might also help to support restrictions on the blob data, as well as implicit lengths, e.g.: <xs:simpleType name="blob10"> <xs:restriction base="xs:hexBinary"> <xs:maxLength value="20" / </xs:restriction> </xs:simpleType> <xs:element name="data" type="xs:anyURI" dfdl:objectType="blob10" dfdl:lengthKind="implict" /> DFDL properties could be placed on either the element or the objectType simpleType, with the base type of dfdl:largeObjectType determining which properties are valid/interpreted, rather than the element type (which must be anyURI). But maybe this all adds unnecessary complexity? Regarding specifying the filename via a DFDL property rather than API, we have a use cases where each parse would need to output to a different directory so a property might cause problems with this. But perhaps this could be handled by a variable, e.g.: <xs:element name="data" type="dfdlx:blob" dfdl:blobDirectory="{ $blobDir }" ... /> That said, we had additional use cases where a DFDL blobDirectory property would be too restrictive. For example, maybe the blobs should be put into a database, or pushed to a data store in the cloud, stored in local memory, or not stored anywhere at all but with a special URI with offset+length to the original data. We chose to ignore these use-cases for simplicity, but these different options would probably require a flexible API to support. By going with an API to specify the output directory, it makes it a bit easier to support these different blob outputs in the future if it was needed. On 8/8/19 5:09 AM, Steve Hanson wrote:
Mike
Am I allowed to put DFDL properties on the new simple type, or is the new type considered to be a built-in type? I think the latter is clearer and simpler to implement. Support for 'clob' would then just add a new simple type restriction 'dfdlx:clob'.
Assuming that the feature makes it into a future DFDL 2.0, the schema containing the 'blob' simple type would then be in the standard DFDL namespace. That's the first example of such a schema, as this is the first time we are extending base XML Schema as opposed to defining annotations. If the new type is considered a built-in type, then this schema should be part of the DFDL 2.0 standard and read-only.
Any thoughts on allowing the specification of the filename via DFDL property rather than API call?
Presumably I could create a local restriction of 'dfdlx:blob'? One motivation for so doing would be to validate the length or content of my binary data. There's a problem with that though - validation works against the infoset, so the allowable facets are those applicable to xs:anyUri and would be applied to the file name, not the binary data. It also means that dfdl:lengthKind 'implicit' can't be used. I don't see a way round this.
Regards
Steve Hanson
IBM Hybrid Integration, Hursley, UK Architect, _IBM DFDL_ <http://www.ibm.com/developerworks/library/se-dfdl/index.html> Co-Chair, _OGF DFDL Working Group_ <http://www.ogf.org/dfdl/>_ __smh@uk.ibm.com_ <mailto:smh@uk.ibm.com> tel:+44-1962-815848 mob:+44-7717-378890 Note: I work Tuesday to Friday
From: Mike Beckerle <mbeckerle.dfdl@gmail.com> To: DFDL-WG <dfdl-wg@ogf.org> Date: 12/07/2019 18:14 Subject: [DFDL-WG] BLOB - binary large object proposal - updated Sent by: "dfdl-wg" <dfdl-wg-bounces@ogf.org>
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This concept, ,which has been discussed before, is in high demand in the Daffodil user community to enable DFDL to be used to parse image file formats. The use case is to provide uniform image-metadata access without getting bogged down in the large byte-array that makes up most of the file and would be very large (and pointless) if rendered into XML or JSON.
So our proposal, (which will get turned into an official Experimental feature document), has been simplified and revised and is described here:
_https://cwiki.apache.org/confluence/display/DAFFODIL/Proposal%3A+Binary+Larg...
Mike Beckerle | OGF DFDL Workgroup Co-Chair | Tresys Technology | _www.tresys.com_ <http://www.tresys.com> Please note: Contributions to the DFDL Workgroup's email discussions are subject to the _OGF Intellectual Property Policy_ <http://www.ogf.org/About/abt_policies.php> -- dfdl-wg mailing list dfdl-wg@ogf.org https://www.ogf.org/mailman/listinfo/dfdl-wg
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