Re: [netmod] xpath expressions in JSON
Martin Bjorklund <mbj@tail-f.com> Thu, 11 October 2018 10:21 UTC
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From: Martin Bjorklund <mbj@tail-f.com>
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Subject: Re: [netmod] xpath expressions in JSON
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Andy Bierman <andy@yumaworks.com> wrote: > On Wed, Oct 10, 2018 at 11:39 PM, Martin Bjorklund <mbj@tail-f.com> wrote: > > > Andy Bierman <andy@yumaworks.com> wrote: > > > On Wed, Oct 10, 2018 at 6:59 PM, Reshad Rahman (rrahman) < > > rrahman@cisco.com> > > > wrote: > > > > > > > On 2018-10-10, 9:59 AM, "netmod on behalf of Martin Bjorklund" < > > > > netmod-bounces@ietf.org on behalf of mbj@tail-f.com> wrote: > > > > > > > > Ladislav Lhotka <lhotka@nic.cz> wrote: > > > > > Martin Bjorklund <mbj@tail-f.com> writes: > > > > > > > > > > > Hi, > > > > > > > > > > > > While reviewing restconf-notif, I saw this example: > > > > > > > > > > > > { > > > > > > "ietf-subscribed-notifications:input": { > > > > > > "stream": "NETCONF", > > > > > > "stream-xpath-filter": "/ds:foo/", > > > > > > "dscp": "10" > > > > > > } > > > > > > } > > > > > > > > > > > > Note the "stream-xpath-filter". It has a prefix in the XPath > > > > string. > > > > > > How are prefixes declared when JSON is used? > > > > > > > > > > > > The leaf "stream-xpath-filter" says: > > > > > > > > > > > > o The set of namespace declarations are those in > > > > scope on > > > > > > the 'stream-xpath-filter' leaf element. > > > > > > > > > > > > (I think I provided that text...) > > > > > > > > > > > > This assumes that the encoding is XML, or at leas that the > > encoding > > > > > > can somehow transfer namespace declarations. > > > > > > > > > > It can't. There are two options: > > > > > > > > > > 1. have different representations of this value in XML and JSON, > > > > > analogically to instance indentifiers (sec. 6.11 in RFC 7951). > > > > > > > > > > 2. use a module name rather than a prefix in XML, too. > > > > > > > > > > I would suggest #2. > > > > <RR> But that means making non-backwards compatible change to the XML > > > > representation? > > > > > > > > > > Not really. It means NETMOD WG would be creating its own special variant > > of > > > XPath. > > > > Not at all. What I propose is perfectly fine, legal XPath 1.0. > > > > XPath 1.0 says that an XPath expression is evaluated in a context. > > One item in the context is a set of mappings from <prefix> to <uri>, > > where <prefix> is used to lookup prefixes used in the XPath > > expression, e.g. in "/foo:interfaces" "foo" is the prefix. > > > > It is perfectly fine to say that the prefix mapping set is this: > > > > "ietf-interfaces" -> "urn:ietf:params:xml:ns:yang:ietf-interfaces" > > "ietf-ip" -> "urn:ietf:params:xml:ns:yang:ietf-ip" > > > > and use that to evaluate the expression > > > > /ietf-interfaces:interfaces/ietf-interfaces:interface/ietf-ip/ipv4 > > > > > > > > The XPath expression is normally parsed within an XML instance document. > There are "xmlns" attributes present that map the prefix to a namespace URI. > These mappings will not be present in the JSON at all. > > A custom XPath implementation is required to magically identify the prefix > as a module name and magically find the namespace URI for the module > name. I disagree. You need an XPath implementation + custom code to set up the environment. There is no standard XPath implementation that can just take an XML instance document + YANG module and figure out what to do. Custom code is needed to connect things together. This proposal doesn't change this. /martin > A normal XPath implementation will not find any namespace mapping for the > prefixes. > > An XPath expression has no concept of the "current module" inherited from > the parent > like the JSON encoding. This is problematic for predicates > > /ietf-interfaces:interfaces/interface[name='eth0'] > > XPath says the missing prefixes for 'interface' and 'name' are simply > missing (no namespace). > The JSON encoding says "ietf-interfaces" is used for 'interfaces'. and > 'interface'. > There is no specification for the 'name' node inside a predicate. > > So you must mean the full module name will be used at every node: > > /ietf-interfaces:interfaces/ietf-interfaces:interface[ietf-interfaces:name='eth0'] > > > > > > > /martin > > > > > Andy > > > > > > > > > > > > > > Hmm, so you mean change the leaf "stream-xpath-filter" to say: > > > > > > > > o The set of namespace declarations has one member for > > each > > > > YANG module supported by the server. This member maps > > > > from the YANG module name to the YANG module namespace. > > > > > > > > This means that in the XPath expression, the module > > name > > > > serves as the prefix. > > > > > > > > .... and then also give an example of this. > > > > > > > > This is probably what we need to do in all places where > > yang:xpath1.0 > > > > is used, going forward. Maybe even define a new type > > > > yang:xpath1.0-2 (name?) with the set of namespace declarations > > > > built-in. > > > > > > > > > > > > > We should avoid making off-the-shelf implementations of standards like > > > XPath unusable. > > > At the very least this should be only available if the server supports it > > > (with a capability URI) > > > > > > > > > > > > > <RR> So we need an update to RFC7951? > > > > > > > > Regards, > > > > Reshad. > > > > > > > > > > > > > > Andy > > > > > > > > > > > > > > /martin > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > Lada > > > > > > > > > > > > > > > > > How is this supposed to work with JSON? > > > > > > > > > > > > > > > > > > /martin > > > > > > > > > > > > _______________________________________________ > > > > > > netmod mailing list > > > > > > netmod@ietf.org > > > > > > https://www.ietf.org/mailman/listinfo/netmod > > > > > > > > > > -- > > > > > Ladislav Lhotka > > > > > Head, CZ.NIC Labs > > > > > PGP Key ID: 0xB8F92B08A9F76C67 > > > > > > > > > > > > > _______________________________________________ > > > > netmod mailing list > > > > netmod@ietf.org > > > > https://www.ietf.org/mailman/listinfo/netmod > > > > > > > > > > > > _______________________________________________ > > > > netmod mailing list > > > > netmod@ietf.org > > > > https://www.ietf.org/mailman/listinfo/netmod > > > > > >
- [netmod] xpath expressions in JSON Martin Bjorklund
- Re: [netmod] xpath expressions in JSON Ladislav Lhotka
- Re: [netmod] xpath expressions in JSON Martin Bjorklund
- Re: [netmod] xpath expressions in JSON Reshad Rahman (rrahman)
- Re: [netmod] xpath expressions in JSON Andy Bierman
- Re: [netmod] xpath expressions in JSON Qin Wu
- Re: [netmod] xpath expressions in JSON Martin Bjorklund
- Re: [netmod] xpath expressions in JSON Ladislav Lhotka
- Re: [netmod] xpath expressions in JSON Martin Bjorklund
- Re: [netmod] xpath expressions in JSON Martin Bjorklund
- Re: [netmod] xpath expressions in JSON Andy Bierman
- Re: [netmod] xpath expressions in JSON Vladimir Vassilev
- Re: [netmod] xpath expressions in JSON Robert Wilton
- Re: [netmod] xpath expressions in JSON Martin Bjorklund
- Re: [netmod] xpath expressions in JSON Robert Wilton
- Re: [netmod] xpath expressions in JSON Martin Bjorklund
- Re: [netmod] xpath expressions in JSON Vladimir Vassilev
- Re: [netmod] xpath expressions in JSON Vladimir Vassilev
- Re: [netmod] xpath expressions in JSON Robert Wilton
- Re: [netmod] xpath expressions in JSON Reshad Rahman (rrahman)
- Re: [netmod] xpath expressions in JSON Robert Wilton
- Re: [netmod] xpath expressions in JSON Martin Bjorklund
- Re: [netmod] xpath expressions in JSON Martin Bjorklund
- Re: [netmod] xpath expressions in JSON Andy Bierman
- Re: [netmod] xpath expressions in JSON Robert Wilton
- Re: [netmod] xpath expressions in JSON Andy Bierman
- Re: [netmod] xpath expressions in JSON Reshad Rahman (rrahman)
- Re: [netmod] xpath expressions in JSON Martin Bjorklund
- Re: [netmod] xpath expressions in JSON Robert Wilton
- Re: [netmod] xpath expressions in JSON Robert Wilton
- Re: [netmod] xpath expressions in JSON Andy Bierman
- Re: [netmod] xpath expressions in JSON Reshad Rahman (rrahman)
- Re: [netmod] xpath expressions in JSON Ladislav Lhotka
- Re: [netmod] xpath expressions in JSON Vladimir Vassilev
- Re: [netmod] xpath expressions in JSON Martin Bjorklund
- Re: [netmod] xpath expressions in JSON Ladislav Lhotka
- Re: [netmod] xpath expressions in JSON Robert Wilton
- Re: [netmod] xpath expressions in JSON Reshad Rahman (rrahman)
- Re: [netmod] xpath expressions in JSON Vladimir Vassilev
- Re: [netmod] xpath expressions in JSON Andy Bierman
- Re: [netmod] xpath expressions in JSON Juergen Schoenwaelder
- Re: [netmod] xpath expressions in JSON Martin Bjorklund
- Re: [netmod] xpath expressions in JSON Andy Bierman
- Re: [netmod] xpath expressions in JSON Andy Bierman
- Re: [netmod] xpath expressions in JSON Juergen Schoenwaelder
- Re: [netmod] xpath expressions in JSON Martin Bjorklund
- Re: [netmod] xpath expressions in JSON Andy Bierman
- Re: [netmod] xpath expressions in JSON Ladislav Lhotka
- Re: [netmod] xpath expressions in JSON Martin Bjorklund
- Re: [netmod] xpath expressions in JSON Ladislav Lhotka
- Re: [netmod] xpath expressions in JSON Qin Wu
- Re: [netmod] xpath expressions in JSON Martin Bjorklund
- Re: [netmod] xpath expressions in JSON Ladislav Lhotka
- Re: [netmod] xpath expressions in JSON Martin Bjorklund
- Re: [netmod] xpath expressions in JSON Ladislav Lhotka
- Re: [netmod] xpath expressions in JSON Martin Bjorklund
- Re: [netmod] xpath expressions in JSON Robert Wilton
- Re: [netmod] xpath expressions in JSON Ladislav Lhotka
- Re: [netmod] xpath expressions in JSON Juergen Schoenwaelder
- Re: [netmod] xpath expressions in JSON Robert Wilton
- Re: [netmod] xpath expressions in JSON Juergen Schoenwaelder
- Re: [netmod] xpath expressions in JSON Ladislav Lhotka
- Re: [netmod] xpath expressions in JSON Juergen Schoenwaelder
- Re: [netmod] xpath expressions in JSON Martin Bjorklund
- Re: [netmod] xpath expressions in JSON Kent Watsen