[DFDL-WG] Action 287: Find a way to handle a variable path step in DFDL expression - Re: DFDL4S use of wildcard/regex in length path expression
Mike Beckerle
mbeckerle.dfdl at gmail.com
Thu Jan 19 12:16:23 EST 2017
I have analyzed this use of (.*) notation in the
Sentinel2X-bandTMISPData.xsd file.
Below is my discussion of what this is, why it is used, the alternative to
it, and why it is more problematic than it seems at first, as an addition
to DFDL.
There are two instances of (.*) in the Sentinel schema files.
What it expresses is a partial step-name wildcard. This is not a variation
on the XPath * notation, as that matches any step, and can't match a
partial name. In the usage in the Sentinel schema, this wild card is a
regex that matches against a step name, and it matches exactly a single
name. It is not used in a way where it can result in a node-set instead of
a single node.
The question really is why is this (.*) needed - that is what is it trying
to achieve, and whether there is an acceptable alternative for what it is
doing.
I find that this wildcard (.*) is used to achieve a parameterization of the
types TypeISPData and TypeISPData_HKTM. These types are polymorphic, in
that their exact behavior depends on aspects of their surrounding context.
Each of these types incorporates a length of some content that is outside
of its own definition.
As the types are used now, the length comes from a thing outside of them in
the schema that happens to have a particular suffix on its name, which is
"Packet_Secondary_Header" or just "_Secondary_Header".
So these names, while outside of the type, are in some sense being
hard-coded in these types. Even though they are outside of these types, you
cannot change the names of these elements without breaking the ability of
the type to find them via this (.*) notation and a name suffix.
These types, TypeISPData and TypeISPData_HKTM are placed inside other
types, and those are then placed in context with various packet-header
structures. Those structures have various distinguishing prefixes of
sub-elemetnts such as MSI_Packet_Secondary_Header. There are a variety of
different things instead of "MSI_", but there's always something with
suffix "Packet_Secondary_Header" or "Secondary_Header" in it.
The use of this (.*) name wildcard seems convenient, but doesn't offer
anything that isn't better captured by a true parameterization mechanism
which decouples the names used outside the type from those used inside it.
DFDL provides a general mechanism for this sort of parameterization using
variables and dfdl:newVariableInstance.
Let's look at just one of the two instances, TypeISPData.
To use variables, a variable is created which represents this parameter to
the TypeISPData. It is declared in the schema file where TypeISPData is
defined.
<dfdl:defineVariable name="Additional_Content_Length" type="xs:int"/>
The expression within TypeISPData that currently contains
"contentLength(/Packet_Data_Field/(.*)Packet_Secondary_Header, 'bytes')"
That part of the expression instead references the parameter variable
$Additional_Content_Length
At the point of use, where this packet secondary header element is combined
with the element that contains the TypeISPData, at that location, a
dfdl:newVariableInstance is created and bound like so:
...
<xs:element name="MSI_Packet_Secondary_Header" type="TypePacketData_MSI"/>
<xs:sequence>
<xs:annotation><xs:appinfo source="http://www.ogf.org/dfdl">
<dfdl:newVariableInstance
ref="Additional_Content_Length"
value="{ dfdl:contentLength(../MSI_Packet_Secondary_Header, 'bytes')
}"/>
</xs:appinfo></xs:annotation>
<xs:element name="MSI_User_Data_Field" type="TypeUserData"/>
</xs:sequence>
...
This binds the parameter to the needed length around the point of use of
the type that needs it (which is down inside the TypeUserData)
This seems bulky due to the XML and XSD-annotation notational overheads,
but it is just parameter-binding as would occur in an ordinary programming
language when passing an argument.
Using variables decouples the names of the two parts of the schema
entirely. For example, if MSI_Packet_Secondary_Header were to be renamed
to MSI_PSH, it would change only the point of use where the variable
binding occurs, and would not affect the type definition. This also
facilitates testing the types in isolation. It eliiminates the need to have
the complete data structure surrounding them.
Use of (*.) name matching might seem convenient, but really it depends on
an unstated invariant about the way names are chosen in the Sentinel
schema. If the names weren't constructed so uniformly, this notational
trick would be unable to make the necessary distinctions, and you'd have to
fall back on using variables.
So, the above explains an alternative, already in DFDL, that can achieve
the parameterization of types that is needed.
There is an additional issue that makes this (.*) notation problematic.
The issue is thes semantics relative to QNames and ordinary XML namespace
management for name conflict control, The Sentinel schema is all a "no
namespace" schema. In that world a path step is just an identifier with no
structure. However, more generally in DFDL, a path step might be a QName,
and those have a namespace prefix part, and a name part. Name collisions
and conflicts can be managed by use of different target namespaces and
namespace prefixes. Using (.*) prefix regex-matching against QNames is
problematic, as it subverts the ability to use namespace prefixes to
eliminate name conflicts. E.g., does (.*)_Secondary_Header match
"foo:My_Secondary_Header" given that it is in the foo namespace? Matching
against things without taking namespace prefixes into consideration breaks
XSD's ability to use prefixes to manage name collsions. So the answer of
does "(.*)_Secondary_Header" match "foo:My_Secondary_Header" would have to
be that it does not match, and that if one wanted to match that one would
have to write "bar:(.*)_Secondary_Header" where bar prefix is bound to the
same namespace as foo in the schema file where My_Secondary_Header is
defined. This mixture of some plain-textual regex matching, and some
name-prefix-qualified namespace-sensitive matching is not very attractive.
There is certainly nothing like this in XPath.
Last issue is implementation complexity:
So, if one wants to do without the (.*) notation, in terms of making DFDL4S
more complex to implement, you would need to add variables. Fortunately
variables are really one of the simpler things in DFDL to implement at
least for parsing, and is very simple if the dfdl:setVariable annotation is
not implemented since it is not needed in this case.
Each variable has a stack. the dfdl:newVariableInstance pushes a new entry
onto the stack, and in this case, populates it immediately with a value.
Reference to the variable from an expression always takes the value of the
top-of-stack location.
Exiting the scope of the annotation where the dfdl:newVariableInstance
appeared pops the stack.
In summary, given (a) the availability of a robust parameterization
mechanism using parameter variables and newVariableInstance to bind them
(b) the ease of implementing variables, and (c) the complexity of working
out interactions with XML/XSD namespaces/prefixes and name management,.....
given those reasons, I would be disinclined to advocate something like
this (.*) notation to DFDL unless it was first added to XPath, where all
necessary details were worked out for us.
Mike Beckerle | OGF DFDL Workgroup Co-Chair | Tresys Technology |
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 Tue, Sep 13, 2016 at 10:18 AM, Michele Zundo <michele.zundo at esa.int>
wrote:
> Dear Steve,
>
> sorry for the delay due to Summer break and other projects.
>
> Here is the .zip.
>
> Please note that we realised that one of our previous replies date 25 July
> 2016 at 17:45:05 GMT+2
> was a misunderstanding from our part and not applicable.
>
> Regarding your questions please find answers interleaved below
>
> Michele
>
> This message and any attachments are intended for the use of the addressee or addressees only.
> The unauthorised disclosure, use, dissemination or copying (either in whole or in part) of its
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> If you received this message in error, please notify the sender and delete it from your system.
> Emails can be altered and their integrity cannot be guaranteed by the sender.
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> Please consider the environment before printing this email.
>
>
>
>
> Regards
>
>
>
>
>
>
> *From:* Steve Hanson [*smh at uk.ibm.com* <smh at uk.ibm.com>]
> *Sent:* Tuesday, July 26, 2016 3:20 AM
> *To:* Michele Zundo
> *Cc:* Mike Beckerle; *rui.mestre at deimos.com.pt* <rui.mestre at deimos.com.pt>
> *Subject:* Re: Fwd: OGF DFDL WG Call Minutes 2016-07-05
>
>
> ….snip
>
>
> dfdl:length="{/Packet_Primary_Header/Packet_Data_Length + 1 -
> contentLength(/Packet_Data_Fi
> eld/(.*)Packet_Secondary_Header, 'bytes') - 2}"
>
> Firstly, contentLength is a DFDL function so it needs to be in the DFDL
> namespace, eg, dfdl:contentLength().
>
>
> Yes agree with you. We will add the* dfdl:* it in future releases and
> modify the applications accordingly.
>
>
> Secondly, the first argument to dfdl:contentLength() is a path, so you are
> effectively still using regular expressions in path steps.
>
>
> Yes. For now we are using it and expect this to become part of the
> standard.
>
>
> Regards
>
> Steve Hanson
> *IBM Integration Bus*
> <http://www-03.ibm.com/software/products/en/ibm-integration-bus>,
> 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 at uk.ibm.com* <smh at uk.ibm.com>
> tel:*+44-1962-815848* <%2B44-1962-815848>
> mob:*+44-7717-378890* <%2B44-7717-378890>
>
>
>
> From: Michele Zundo <*michele.zundo at esa.int*
> <michele.zundo at esa.int>>
> To: Steve Hanson/UK/IBM at IBMGB
> Cc: Mike Beckerle <*mbeckerle at tresys.com* <mbeckerle at tresys.com>>
> Date: 25/07/2016 17:10
> Subject: Fwd: OGF DFDL WG Call Minutes 2016-07-05
> ------------------------------
>
>
>
>
> Dear Steve,
>
> Please find below the answer from our developers and example.
>
> Note that we have updated our implementation of DFDL to be as compliant as
> we
> can at this point in time with the exception noted below.
>
> Michele
>
> Begin forwarded message:
>
> *From: *"Rui Mestre (DME)" <*rui.mestre at deimos.com.pt*
> <rui.mestre at deimos.com.pt>>
> *Subject: Re: Fwd: OGF DFDL WG Call Minutes 2016-07-05*
> *Date: *25 July 2016 at 17:45:05 GMT+2
>
> Dear Michele,
>
> I believe that after our DFDL compliance effort the mentioned "use of a
> regex in the path step of a DFDL expression" is no longer in place.
>
> Currently the only extension implemented in DFDL4S regarding the use of
> regular expressions is that implementation of dfdl:contentLength is
> extended to support also regular expressions when specifying the node.
>
> Please find attached a schema file example containing such extension in
> the use of dfdl:contentLength.
>
> Best regards,
> Rui
>
>
> Begin forwarded message:
>
> *From: *Steve Hanson < <smh at uk.ibm.com>*smh at uk.ibm.com* <smh at uk.ibm.com>>
> *Subject: OGF DFDL WG Call Minutes 2016-07-05*
> *Date: *5 July 2016 at 17:49:13 GMT+2
> *To: * <dfdl-wg at ogf.org>*dfdl-wg at ogf.org* <dfdl-wg at ogf.org>
> *Cc: *"Mike Beckerle" < <mbeckerle at tresys.com>*mbeckerle at tresys.com*
> <mbeckerle at tresys.com>>, "Michele Zundo" <*michele.zundo at esa.int*
> <michele.zundo at esa.int>>
>
> Please find minutes from the above call at
> <https://redmine.ogf.org/dmsf_files/13537?download=>
> *https://redmine.ogf.org/dmsf_files/13537?download=*
> <https://redmine.ogf.org/dmsf_files/13537?download=>
>
> *@Michele - please can you send to the WG a schema that shows your use of
> a regex in the path step of a DFDL expression ?*
>
> Next call *Aug 2nd*
>
> Regards
>
> Steve Hanson
> Architect, IBM DFDL,
> Co-Chair, *OGF DFDL Working Group* <http://www.ogf.org/dfdl/>
> IBM SWG, Hursley, UK
> *smh at uk.ibm.com* <smh at uk.ibm.com>
> *tel:+44-1962-815848* <+44-1962-815848>
>
> -----------------------------------------
> Michele Zundo
>
> Head of Ground System Definition and Verification Office
> EOP-PEP
> European Space Agency, ESTEC
> e-mail: <michele.zundo at esa.int>*michele.zundo at esa.int*
> <michele.zundo at esa.int>
>
>
> This message and any attachments are intended for the use of the addressee
> or addressees only.
> The unauthorised disclosure, use, dissemination or copying (either in
> whole or in part) of its
> content is not permitted.
> If you received this message in error, please notify the sender and delete
> it from your system.
> Emails can be altered and their integrity cannot be guaranteed by the
> sender.
>
> Please consider the environment before printing this email.
>
> -----------------------------------------
> Michele Zundo
>
> Head of Ground System Definition and Verification Office
> EOP-PEP
> European Space Agency, ESTEC
> e-mail: *michele.zundo at esa.int* <michele.zundo at esa.int>
>
>
>
>
>
>
>
>
>
>
> 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
>
>
> #### Sentinel2X-bandTMISPData.xsd moved to MyAttachments Repository V3.8 (
> Link
> <notes:///802575AF0030E827/5DE5236E5AD1645685256EE0001BBADF/ABC83CA7B3700B0380258018002AE581>)
> on 23 August 2016 by Steve Hanson.
>
>
> 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
>
>
> -----------------------------------------
> Michele Zundo
>
> Head of Ground System Definition and Verification Office
> EOP-PEP
> European Space Agency, ESTEC
> e-mail: michele.zundo at esa.int
>
>
>
>
>
>
>
>
>
>
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