dfdl:largeObjectKind definitely keeps things simple, but does lose flexibility (e.g. maxLength). But that may not be needed. I'm in favor of this. However, one drawback of using xs:string instead of xs:anyURI relates to our TDML test rig. If we make the type of blobs/clobs an xs:string and have it be an opaque identifier then it makes it difficult for our TDML runner to know how to compare actual vs expected blobs. For example: <data xsi:type="xs:string">some unique identifier</data> In this case, the TDML test rig must be schema aware to know that data is not a string and is actually an opaque identifier. And it must know how to use that unique identifier to lookup the bytes to do expected/actual comparisons. By making the type an xs:anyURI and requiring that the identifier is a URI, a TDML runner does not need any knowledge of the schema. Since the xsi:type is an anyURI, it can infer that this must be a blob/clob, and then it can open the URI to determine the bytes and easly compare expected vs actual blobs. And this applies to anyone accessing the infoset as well--not just our TDML runner. Using a type of xs:anyURI provides a hint to infoset users that an element shouldn't be treated like a string, but as a blob handle. - Steve On 8/9/19 10:25 AM, Mike Beckerle wrote:
My suggestions based on this thread are:
I think the dfdlx:blob type is problematic, and we should avoid it in favor of a xs:string with a dfdlx:largeObjectKind property.
I think this should not be a "Type" as in string or hexBinary, because hexBinary is such a misleading term, suggesting textualization, etc. There is nothing "hex" about a BLOB, ever.
I think dfdlx:largeObjectKind="bytes/chars/none" with none the "default" for now, and "chars" as a future capability for character large objects if they prove important. I could be convinced other enums are better than bytes or chars for this. Eg., BLOB, CLOB might be better. Or perhaps this is dfdl:largeObjectRep="binary/text/none" analogous to the dfdl:representation property?
The use of xs:anyURI is unnecessary, and is not a type we have in DFDL as yet. People should treat this string as opaque. The fact that it is potentially a meaningful URI is not relevant, and can be an implementation detail.
I think dfdl:largeObjectDirectory="{ $dfdlx:largeObjectDirectory }" is a nice idea to save for the future. We may find that numerous other parameters are required, so I'd prefer not to predefine this one in advance of clearer direction or whether there are others.
The other thing observed on yesterday's DFDL WG call, was that this has some overlap with the offset/pointer stuff. Unparsing from a blob file is an awful lot like data-source indirection where the source of unparsing is coming from a scattered data structure that is being gathered. There is some conceptual similarity anyway. Not sure how deep this goes or if it is just a superficial observation. And I would not suggest waiting for that to be figured out before proceeding with this experimental BLOB feature.
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>
On Thu, Aug 8, 2019 at 8:40 AM Lawrence, Stephen <slawrence@tresys.com <mailto:slawrence@tresys.com>> wrote:
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 <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 <mailto:mbeckerle.dfdl@gmail.com>> > To: DFDL-WG <dfdl-wg@ogf.org <mailto: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 <mailto:dfdl-wg-bounces@ogf.org>> > > -------------------------------------------------------------------------------- > > > > 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 <mailto:dfdl-wg@ogf.org> > https://www.ogf.org/mailman/listinfo/dfdl-wg > > Unless stated otherwise above: > IBM United Kingdom Limited - Registered in England and Wales with number 741598. > Registered office: PO Box 41, North Harbour, Portsmouth, Hampshire PO6 3AU > > > -- > dfdl-wg mailing list > dfdl-wg@ogf.org <mailto:dfdl-wg@ogf.org> > https://www.ogf.org/mailman/listinfo/dfdl-wg >
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