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Apache Avro™ 1.7.6 Documentation
by Sill, Alan 10 May '14

10 May '14

Dear DFDL folks, Thought you would be interested in the following link. I'd be interested in comparisons of this (Apache Avro), the other two systems mentioned (Apache Thrift and Google Protocol Buffers) with DFDL in terms of goals, schema capabilities and general application potential. Alan Topic: Introduction Apache Avro%u2122 is a data serialization system. Avro provides: Rich data structures. A compact, fast, binary data format. A container file, to store persistent data. Remote procedure call (RPC). Simple integration with dynamic languages. Code generation is not required to read or write data files nor to use or implement RPC protocols. Code generation as an optional optimization, only worth implementing for statically typed languages. Schemas Avro relies on schemas. When Avro data is read, the schema used when writing it is always present. This permits each datum to be written with no per-value overheads, making serialization both fast and small. This also facilitates use with dynamic, scripting languages, since data, together with its schema, is fully self-describing. When Avro data is stored in a file, its schema is stored with it, so that files may be processed later by any program. If the program reading the data expects a different schema this can be easily resolved, since both schemas are present. When Avro is used in RPC, the client and server exchange schemas in the connection handshake. (This can be optimized so that, for most calls, no schemas are actually transmitted.) Since both client and server both have the other's full schema, correspondence between same named fields, missing fields, extra fields, etc. can all be easily resolved. Avro schemas are defined with JSON . This facilitates implementation in languages that already have JSON libraries. Comparison with other systems Avro provides functionality similar to systems such as Thrift, Protocol Buffers, etc. Avro differs from these systems in the following fundamental aspects. Dynamic typing: Avro does not require that code be generated. Data is always accompanied by a schema that permits full processing of that data without code generation, static datatypes, etc. This facilitates construction of generic data-processing systems and languages. Untagged data: Since the schema is present when data is read, considerably less type information need be encoded with data, resulting in smaller serialization size. No manually-assigned field IDs: When a schema changes, both the old and new schema are always present when processing data, so differences may be resolved symbolically, using field names. Apache Avro, Avro, Apache, and the Avro and Apache logos are trademarks of The Apache Software Foundation. Link: http://avro.apache.org/docs/current/

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OGF DFDL WG Call Minutes 2014-05-06
by Steve Hanson 06 May '14

06 May '14

Please find minutes from the above call at http://redmine.ogf.org/dmsf_files/13247?download=. Regards Steve Hanson Architect, IBM DFDL, Co-Chair, OGF DFDL Working Group IBM SWG, Hursley, UK smh(a)uk.ibm.com tel:+44-1962-815848

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OGF DFDL WG Call Agenda 2014-05-06
by Steve Hanson 06 May '14

06 May '14

Please find agenda for call on Redmine at http://redmine.ogf.org/dmsf_files/13247?download= Regards Steve Hanson Architect, IBM Data Format Description Language (DFDL) Co-Chair, OGF DFDL Working Group IBM SWG, Hursley, UK smh(a)uk.ibm.com tel:+44-1962-815848

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draft of implementation-specific errata
by Cranford, Jonathan W. 05 May '14

05 May '14

DFDL WG, Attached is my latest errata draft on implementation-specific behavior. This is based on the review we did a while ago. FYI, -- Jonathan W. Cranford Senior Information Systems Engineer The MITRE Corporation (http://www.mitre.org)

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OGF DFDL WG Call Minutes 2014-04-29
by Steve Hanson 01 May '14

01 May '14

Please find minutes from the above call at http://redmine.ogf.org/dmsf_files/13244?download=. Regards Steve Hanson Architect, IBM DFDL, Co-Chair, OGF DFDL Working Group IBM SWG, Hursley, UK smh(a)uk.ibm.com tel:+44-1962-815848

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Re: [DFDL-WG] Action 248 (was Thoughts on a discriminator scenario)
by Steve Hanson 30 Apr '14

30 Apr '14

248 Discriminators and potential points of uncertainty (Steve) 28/1: Steve to write up a proposal to prevent a discriminator from behaving in a non-obvious manner when used with a potential point of uncertainty that turns out not to be an actual point of uncertainty. 5/2: Steve sent an email to check whether choice branches, unordered elements and floating elements should always be actual points of uncertainty, as there are times when there is no uncertainty, eg, last choice branch; all floating elements found. It was decided that they are always actual points of uncertainty. To do otherwise will complicate implementations and result in fragile schemas. Steve will proceed with the proposal on that basis. Based on the above, which reflects the email discussion below, here is what I propose to resolve this action. If a potential point of uncertainty is always an actual point of uncertainty (choice branch, element in unordered sequence, floating element, ock 'parsed') then a discriminator that applies to it will only ever resolve that point of uncertainty. It never has an effect on any enclosing point of uncertainty. If a potential point of uncertainty is sometimes an actual point of uncertainty (ock 'implicit') then a discriminator that applies it will only ever resolve, or have no effect on, that point of uncertainty. It never has an effect on any enclosing point of uncertainty. If a potential point of uncertainty is never an actual point of uncertainty (ock 'fixed', 'expression', 'stopValue') then a discriminator that applies to it will never have an effect on that point of uncertainty. Nor does it ever have an effect on any enclosing point of uncertainty. I think 1 and 2 are not controversial, but there is an alternative for 3: 3. If a potential point of uncertainty is never an actual point of uncertainty (ock 'fixed', 'expression', 'stopValue') then a discriminator that applies to it will never have an effect on that point of uncertainty. Instead the discriminator is applied to any enclosing point of uncertainty. The alternative means that changing an element from (say) ock 'parsed' to ock 'expression' has the same effect on a discriminator as changing the element to (1,1). The discriminator that applied to it now applies to any enclosing pou. Regards Steve Hanson Architect, IBM DFDL Co-Chair, OGF DFDL Working Group IBM SWG, Hursley, UK smh(a)uk.ibm.com tel:+44-1962-815848 From: Steve Hanson/UK/IBM To: Tim Kimber/UK/IBM@IBMGB, Cc: dfdl-wg(a)ogf.org, dfdl-wg-bounces(a)ogf.org Date: 05/02/2014 12:04 Subject: Re: [DFDL-WG] Action 248 (was Thoughts on a discriminator scenario) Thanks Tim, all good points. Comments to your comments. Regards Steve Hanson Architect, IBM DFDL Co-Chair, OGF DFDL Working Group IBM SWG, Hursley, UK smh(a)uk.ibm.com tel:+44-1962-815848 From: Tim Kimber/UK/IBM To: Steve Hanson/UK/IBM@IBMGB, Cc: dfdl-wg(a)ogf.org, dfdl-wg-bounces(a)ogf.org Date: 05/02/2014 11:01 Subject: Re: [DFDL-WG] Action 248 (was Thoughts on a discriminator scenario) A couple of comments below. regards, Tim Kimber, IBM Integration Bus Development (Industry Packs) Hursley, UK Internet: kimbert(a)uk.ibm.com Tel. 01962-816742 Internal tel. 37246742 From: Steve Hanson/UK/IBM@IBMGB To: dfdl-wg(a)ogf.org, Date: 05/02/2014 10:50 Subject: [DFDL-WG] Action 248 (was Thoughts on a discriminator scenario) Sent by: dfdl-wg-bounces(a)ogf.org 248 Discriminators and potential points of uncertainty (Steve) 28/1: Steve to write up a proposal to prevent a discriminator from behaving in a non-obvious manner when used with a potential point of uncertainty that turns out not to be an actual point of uncertainty. 5/2: With Steve I started on this by reading section 9.3.3 on points of uncertainty, which lists the potential PoUs. Here's the list to save getting the spec out. 1. An xs:choice branch 2. All xs:elements in an unordered xs:sequence (dfdl:sequenceKind is 'unordered') 3. An optional xs:element 4. An array xs:element 5. All xs:elements in an xs:sequence containing one or more floating xs:elements. The section then looks at each in turn and gives the circumstances when it is an actual PoU or not. As currently written, it is only 3 and 4 where a potential PoU might not be an actual PoU. For 1, 2 and 5 it says they are always actual PoUs. But I'm not sure that's correct. A deeper analysis of what is actually going on with 1, 2 and 5 says to me that there are times when there might not be an actual PoU. 1. Given that there is no concept in DFDL of optional choice branches, then if the last branch is reached then there is no longer a PoU. It must be that branch else it is a processing error. TK: I think of it slightly differently. It is a PoU, even if the branch is the only remaining branch. If we say that the final choice branch is not a PoU then diagnostics become confused - the parser reports the error code as 'error while parsing root/choice/lastBranch/field1' when the correct error code would be 'none of the branches of root/choice were found in the data'. SMH: I see your point. My thinking was that choices have finite branches and a choice is (1,1). If I have got to the last branch then I am not one of the other branches so I must be this one. If there is any other possibility then the model is missing a branch, even if it is just one that contains an empty sequence with an assert {fn:false()}. In practice of course users forget to add that last branch (there's no XSDL equivalent to the 'default' branch of a switch/case statement), so yes they could end up with an unclear diagnostic. 2. There can come a point in an unordered sequence when all that can be encountered is one element, and if that is (1,1) then there is no longer a PoU. TK: It's still a PoU. The specification says that occursCountKind is 'parsed' for all members of an unordered group, so min/maxOccurs do not come into play. SMH: Interesting. The spec says that if a member is optional or an array then it must be 'parsed'. If it is (1,1) though it does not have an occursCountKind. The specific case I was thinking of is when all members are (1,1), so when you have one element to go there is no PoU. However, the rewrite into a repeating choice has the effect of making everything 'parsed', which is really the point you are making. So I agree with you, it is easier to say that everything is an actual PoU else it complicates the rewrite semantic. 5. If all floating elements are (1,1) and all are encountered, then from that point on there are no longer any PoUs due to floating elements. TK: I suspect that floating elements are somewhat like unordered branches - most users will not want min/maxOccurs to affect the parsing of the group. Schema validation ( or more complex validation applied in the receiving application ) will deal with non-conformances. SMH: Possibly yes. With something like X12 NTE segments, that is the case. But we don't express the floating semantic as a rewrite of the whole sequence like we do for unordered, it's more of a per element thing. And if that is done dynamically as we go through the sequence, having no PoU can result. I'd like us to get straight on this before I proceed with the action proper. Regards Steve Hanson Architect, IBM DFDL Co-Chair, OGF DFDL Working Group IBM SWG, Hursley, UK smh(a)uk.ibm.com tel:+44-1962-815848 ----- Forwarded by Steve Hanson/UK/IBM on 05/02/2014 10:12 ----- From: Steve Hanson/UK/IBM To: dfdl-wg(a)ogf.org, Date: 27/01/2014 17:39 Subject: Fw: Thoughts on a discriminator scenario Been thinking some more on the discriminator scenario below that I mailed out before xmas, and discussing it with the IBM DFDL team. The 'confusing' aspect of the behaviour is that a discriminator within a potential PoU will act on a higher level PoU if the potential PoU is not an actual PoU. In the example, the array element 'Type1' is not an actual PoU for occurrence 1, only for occurrences 2+. So when the discriminator fires for occurrence 1 it will resolve a higher level unresolved PoU if one exists. Perhaps the spec should say that a discriminator can't 'leak' beyond the potential PoU that encloses it ? If so, then for occurrence 1 the discriminator has no effect, and only has an effect for occurrences 2+. This makes for more predictable and robust schemas. We'd need to go through spec section 9.3.3 carefully to see if this does not break any of the potential PoUs that are listed. Regards Steve Hanson Architect, IBM Data Format Description Language (DFDL) Co-Chair, OGF DFDL Working Group IBM SWG, Hursley, UK smh(a)uk.ibm.com tel:+44-1962-815848 ----- Forwarded by Steve Hanson/UK/IBM on 16/01/2014 09:55 ----- From: Steve Hanson/UK/IBM To: dfdl-wg(a)ogf.org, Date: 20/12/2013 13:20 Subject: Thoughts on a discriminator scenario Take the following schema (simplified) for element Type1 (1,10) being a loop for elements A,B,C. Type 1 does not have an initiator so I need to use a discriminator to establish the existence of an occurrence of Type1 so that incorrect backtracking does not occur after an error. Because occursCountKind is 'implicit', the 1st occurrence is not a point of uncertainty so the discriminator acts instead on any enclosing point of uncertainty, but for 2nd and subsequent occurrences it acts on Type1. That is all working as designed, but I think users find will the 1st occurrence behaviour a bit confusing. There are workarounds to avoid the problem, eg, use occursCountKind 'parsed' or split Type1 into two as (1,1) and (0,9). I think this is worth documenting in a tutorial as this is quite subtle stuff. <xs:element name="Type1" maxOccurs="10" dfdl:occursCountKind="implicit"> <dfdl:discriminator test="{fn:exists(A)}" /> <xs:complexType> <xs:sequence> <xs:element name="A" dfdl:initiator="A:" ... /> <xs:element name="B" dfdl:initiator="B:" ... /> <xs:element name="C" dfdl:initiator="C:"... /> </xs:sequence> </xs:complexType> Regards Steve Hanson Architect, IBM Data Format Description Language (DFDL) Co-Chair, OGF DFDL Working Group IBM SWG, Hursley, UK smh(a)uk.ibm.com tel:+44-1962-815848 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 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 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(a)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 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

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validating expressions on elements in a choice or unordered sequence
by Mark Frost 30 Apr '14

30 Apr '14

When we were implementing unordered sequences, this raised some questions around evaluating relative paths in expressions, for elements in a choice or unordered sequence : DFDL spec: (gwdrp-dfdl-v1.0.4 section 15) "When processing a choice group the parser validates any contained path expressions. If a path expression contained inside a choice branch refers to any other branch of the choice, then it is a schema definition error." 1. I'm not clear what benefit this restriction on path expressions gives. It seems redundant since in any single instance of a choice group, if the branch being processed exists, then by definition none of it's sibling branches exist. Any expression path referring to a non-existent branch would correctly return <empty sequence> If the choice group is inside a repeating structure, then expressions referring to choice branches within other instances of the choice could be useful. Should an expression referring to branches in other instances of a choice cause a schemadef error? Example expression on el_b could be { fn:count(../../el_choice/el_a) } - parent [sequence] - el_choice [minOccurs=5 maxOccurs=5] [choice] - el_a - el_b 2. Should an expression that potentially refers to branches in the choice cause a schemadef error? Example identically named elements in and out of a choice expression on el_c could be { fn:count(../el_a) } - parent [sequence] - el_a - el_b - [embedded choice group] - el_a - el_c Regards, Mark Frost _____________________________________ MP 211, IBM Hursley, Winchester, SO21 2JN Phone: (01962) 817009 or x247009 Email: frostmar(a)uk.ibm.com 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

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Confirmed: DFDL Working Group Call (29 Apr 16:00 GDT in DE3S09)
by Steve Hanson 29 Apr '14

29 Apr '14

OGF DFDL WG weekly call dial-in details. Passcode for Participants: 5381214 Canada Toll-Free 888-426-6840 China Toll-Free 10-800-711-1071 CHINA NETCOM GROUP USERS China Toll-Free 10-800-110-0996 CHINA TELECOM SOUTH USERS France Toll-Free 0800-94-0558 Germany Toll-Free 0800-000-1018 India Toll-Free 000-800-100-1176 Ireland Toll-Free 1-800-943-427 Israel Toll-Free 1-809-417-783 United Kingdom Caller Paid 0-20-30596451 United Kingdom Toll-Free 0800-368-0638 USA Caller Paid 215-861-6239 USA Toll-Free 888-426-6840 Other international numbers available - e-mail smh(a)uk.ibm.com. OGF DFDL Home: http://www.ogf.org/dfdl Redmine DFDL: http://redmine.ogf.org/projects/dfdl-wg Call is on today

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OGF DFDL WG Call Agenda 2014-04-29
by Steve Hanson 29 Apr '14

29 Apr '14

Please find agenda for call on Redmine at http://redmine.ogf.org/dmsf_files/13244?download= Regards Steve Hanson Architect, IBM Data Format Description Language (DFDL) Co-Chair, OGF DFDL Working Group IBM SWG, Hursley, UK smh(a)uk.ibm.com tel:+44-1962-815848

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Re: [DFDL-WG] validating expressions on elements in a choice or unordered sequence
by Steve Hanson 29 Apr '14

29 Apr '14

Tim, thanks for clarifying where you are coming from on this. I interpreted "some component of the static context" differently to you. The components are the 'variables' in the static context, not schema objects. The XPST0001 error says "It is a static error if analysis of an expression relies on some component of the static context that has not been assigned a value. " , so I think this message would be issued if, for example, an XPath processor tried to look up an element in the In-scope schema definitions component but found that the component itself had not been built. I think the error that applies to the case we are discussing is XPST0008 as quoted below. Otherwise I don't see when XPST0008 would be issued. Consequently I am not sure that XPath rules prohibit the genuinely-impossible situation where an expression on choiceElement[1]/A refers to choiceElement[1]/B, because I can't see a crisp definition of in-scope. Maybe I'm missing it? Common sense says that it should be prohibited. But I agree that XPath does not prohibit choiceElement/A referring to choiceElement/B generally. My conclusions about the rules in 23.1 was perhaps too general, and they should be looked at on an individual basis. But the reason they exist is because the parser is not acting on the whole message and in general is prohibited from looking ahead. Clearly the first two rules are needed (assert/discriminator can look down but not ahead, and outputValueCalc can look ahead). The other three are the ones in question. The current wording came from this action which was spun off from the unordered action 199: 214 Expression Language Data Model (All) 16/7: Augment section 23.1 so that it covers the cases where an XPath references an element that is in a different choice branch or that is in the same unordered sequence or that is floating. These cases could be detected statically (though to do this 100% reliably is not easy) or they could be left until runtime and fail if the element does not exist. Both are schema definition errors as explained by errata 2.120. 23/7: Closed. Runtime schema definition error if the element does not exist. Errata taken. (Erratum 2.120 is just the one that categorised all the different errors.) For the choice branch rule: If XPath disallows choice/x[1] referring to choice/y[1] but allows general refs, then I am ok with changing this to match XPath (ie, we drop our rule as it is implied). If it doesn't disallow choice/x[1] referring to choice/y[1] then perhaps we should keep our rule but tighten it up. I think the unordered rule and floating rule both stem from the choice branch rule - because of the rewrite semantic that turns unordered into a choice. Regards Steve Hanson Architect, IBM DFDL Co-Chair, OGF DFDL Working Group IBM SWG, Hursley, UK smh(a)uk.ibm.com tel:+44-1962-815848 From: Tim Kimber/UK/IBM To: Steve Hanson/UK/IBM@IBMGB, Cc: Mark Frost/UK/IBM@IBMGB Date: 28/04/2014 22:03 Subject: Re: [DFDL-WG] validating expressions on elements in a choice or unordered sequence I don't think I am missing the point. DFDL's usage of a partial XDM is not in play here. The static context is defined thus: [Definition: The static context of an expression is the information that is available during static analysis of the expression, prior to its evaluation.] This information can be used to decide whether the expression contains a static error. If analysis of an expression relies on some component of the static context that has not been assigned a value, a static error is raised [err:XPST0001]. This is the definition that is relevant, because the errors are issued by our static analysis ( performed by the DFDL validator ). I believe the rule already prohibits the genuinely-impossible situation where an expression on choiceElement[1]/A refers to choiceElement[1]/B. But it does not prohibit choiceElement/A referring to choiceElement/B - and I don't think DFDL should either. regards, Tim Kimber, IBM Integration Bus Development (Industry Packs) Hursley, UK Internet: kimbert(a)uk.ibm.com Tel. 01962-816742 Internal tel. 37246742 From: Steve Hanson/UK/IBM To: Tim Kimber/UK/IBM@IBMGB, Cc: dfdl-wg(a)ogf.org, dfdl-wg-bounces(a)ogf.org Date: 28/04/2014 13:51 Subject: Re: [DFDL-WG] validating expressions on elements in a choice or unordered sequence It is certainly easier if we can just do the same as XPath 2.0 stipulates. But I think that this misses the point here. The XPath error for statically detecting that an expression refers to something that can never exist is XPST0008, which says: It is a static error if an expression refers to an element name, attribute name, schema type name, namespace prefix, or variable name that is not defined in the static context, except for an ElementName in an ElementTest or an AttributeName in an AttributeTest. The static context has the notion of "In-scope schema definitions" being " a generic term for all the element declarations, attribute declarations, and schema type definitions that are in scope during processing of an expression.". It doesn't define exactly what is meant by "in-scope" but XPath assumes that it acts on a complete instance of an XDM. In DFDL we are different to typical XPath usage as we are applying expressions during parsing when the document is incomplete. We can use that as the justification for applying extra constraints, which is exactly why there are additional rules in section 23.1. So, if there are scenarios where a rule is going to be restrictive then we should consider dropping it. If there are not, but it makes the life of an implementer harder because it is hard to code the rule, then we should consider dropping it. Otherwise keep it. Regards Steve Hanson Architect, IBM DFDL Co-Chair, OGF DFDL Working Group IBM SWG, Hursley, UK smh(a)uk.ibm.com tel:+44-1962-815848 From: Tim Kimber/UK/IBM@IBMGB To: dfdl-wg(a)ogf.org, Date: 11/04/2014 14:03 Subject: Re: [DFDL-WG] validating expressions on elements in a choice or unordered sequence Sent by: dfdl-wg-bounces(a)ogf.org I would be quite uncomfortable with DFDL not being a 'proper subset' of XPath 2.0. I understand the motivation ( having personally been involved in coding a query engine for DFDL ) but I think the cure would be worse than the complaint. Consistent with that, I think I agree with Mark's suggestion - a DFDL processor should just 'do what an XPath processor would do'. regards, Tim Kimber, IBM Integration Bus Development (Industry Packs) Hursley, UK Internet: kimbert(a)uk.ibm.com Tel. 01962-816742 Internal tel. 37246742 From: Mike Beckerle <mbeckerle.dfdl(a)gmail.com> To: Mark Frost/UK/IBM@IBMGB, Cc: "dfdl-wg(a)ogf.org" <dfdl-wg(a)ogf.org> Date: 11/04/2014 13:23 Subject: Re: [DFDL-WG] validating expressions on elements in a choice or unordered sequence Sent by: dfdl-wg-bounces(a)ogf.org Comments inline On Fri, Apr 11, 2014 at 6:22 AM, Mark Frost <FROSTMAR(a)uk.ibm.com> wrote: When we were implementing unordered sequences, this raised some questions around evaluating relative paths in expressions, for elements in a choice or unordered sequence : DFDL spec: (gwdrp-dfdl-v1.0.4 section 15) "When processing a choice group the parser validates any contained path expressions. If a path expression contained inside a choice branch refers to any other branch of the choice, then it is a schema definition error." 1. I'm not clear what benefit this restriction on path expressions gives. It seems redundant since in any single instance of a choice group, if the branch being processed exists, then by definition none of it's sibling branches exist. Any expression path referring to a non-existent branch would correctly return <empty sequence> Typically in XPath, such paths would just be empty-sequence at runtime. Making it an SDE hoists the error to (hopefully) compile time, and making it SDE (non-recoverable) changes the way one must write expressions. You can't write utter nonsense paths and have them be runnable. If the choice group is inside a repeating structure, then expressions referring to choice branches within other instances of the choice could be useful. Should an expression referring to branches in other instances of a choice cause a schemadef error? Should be no issue if you are looking at say, position() - n. If you reach to something that doesn't exist, then you'll get empty sequence. My experience so far with XPath is that this notion that non-existance returns empty sequence is painful at best and a nightmare at worst. Expressions that are utterly nonsense are accepted executed, and silently fail by returning empty sequence. The most common mistake is writing /a/b/c when you needed /ns1:a/ns2:b/ns3:c. Example expression on el_b could be { fn:count(../../el_choice/el_a) } - parent [sequence] - el_choice [minOccurs=5 maxOccurs=5] [choice] - el_a - el_b 2. Should an expression that potentially refers to branches in the choice cause a schemadef error? Example identically named elements in and out of a choice expression on el_c could be { fn:count(../el_a) } - parent [sequence] - el_a - el_b - [embedded choice group] - el_a - el_c I'd love to restrict this, because we're looking at having to create a DFDL expression language implementation for performance reasons, and complex things like this require a very complex implementation tantamount to a query-engine. I would claim that these two el_a elements are different, and we could choose to restrict a DFDL path expression to return only nodes described by the same schema component, with "same schema component" meaning same path from document element to the schema component where an element or group or type reference counts as part of that path. So two different element references to the same global element would be two different schema components. But I suspect that this is too restrictive, and implementations are just going to have to be sophisticated enough to execute queries like this one, and a good implementation will optimize simpler cases for faster execution. ...mikeb-- dfdl-wg mailing list dfdl-wg(a)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(a)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

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