Qin Wu <bill...@huawei.com> wrote: > -----邮件原件----- > 发件人: netmod [mailto:netmod-boun...@ietf.org] 代表 Ladislav Lhotka > 发送时间: 2018年10月22日 21:12 > 收件人: Martin Bjorklund > 抄送: netmod@ietf.org > 主题: Re: [netmod] xpath expressions in JSON > > On Mon, 2018-10-22 at 14:56 +0200, Martin Bjorklund wrote: > > Ladislav Lhotka <lho...@nic.cz> wrote: > > > Martin Bjorklund <m...@tail-f.com> writes: > > > > > > > Hi, > > > > > > > > Going back to the most urgent issue, what is this WG's > > > > recommendation for the subscribed-notifications draft in NETCONF > > > > wrt/ their usage of > > > > yang:xpath1.0 in filters? > > > > > > > > To summarize: > > > > > > > > We already have > > > > > > > > o instance-identifier in XML uses prefixes from the XML document > > > > o instance-identifier in JSON uses module names as prefixes > > > > o XPath in NETCONF filter uses prefixes from the XML document > > > > o XPath in JSON query filter uses module names as prefixes > > > > > > > > > > > > Alternative A: > > > > -------------- > > > > > > > > Use different encodings for "stream-xpath-filter" as well, > > > > depending on if it is XML or JSON. > > > > > > > > We would do in SN: > > > > > > > > o If the node is encoded in XML, the set of namespace > > > > declarations are those in scope on the > > > > 'stream-xpath-filter' leaf element. > > > > > > > > o If the node is encoded in JSON, the set of namespace > > > > declarations is the set of prefix and namespace pairs > > > > for all supported YANG modules, where the prefix is > > > > > > Is "supported" the same as "implemented", or something else? > > > > It should be "implemented". > > > > > > the YANG module name and the namespace is as defined > > > > by the "namespace" statement in the YANG module. > > > > > > > > Pro: the format is consistent within each encoding. > > > > > > > > Con: unclear how to handle other encodings. > > > > Con: we keep using context-depending encodings. > > > > > > Con: XPath expressions in JSON can get pretty long (I assume it's not > > > just an instance identifier but may contain predicates etc.). We > > > cannot use the trick with the default namespace as in YANG, so all > > > data node names will have to carry the prefix. > > > > Yes. > > > > > > We could probably add that CBOR uses the same representation as JSON. > > > > > > > > Example in XML: > > > > > > > > <stream-xpath-filter > > > > xmlns:if="urn:ietf:params:xml:ns:yang:ietf-interfaces" > > > > xmlns:ip="urn:ietf:params:xml:ns:yang:ietf-ip"> > > > > /if:interfaces/if:interface/ip:ipv4 > > > > </stream-xpath-filter> > > > > > > > > Example in JSON: > > > > > > > > "stream-xpath-filter": > > > > "/ietf-interfaces:interfaces/ietf-interfaces:interface/ietf-ip:ipv4" > > > > > > > > > > > > > > > > Alternative B: > > > > -------------- > > > > > > > > Use a non-context depending encoding, with the module name as prefix. > > > > > > > > We would do in SN: > > > > > > > > o The set of namespace > > > > declarations is the set of prefix and namespace pairs > > > > for all supported YANG modules, where the prefix is > > > > the YANG module name and the namespace is as defined > > > > by the "namespace" statement in the YANG module. > > > > > > > > Pro: the format is independent from the protocol encoding > > > > > > > > Con: in XML, this leaf is treated differently from other XPath > > > > expressions, such as get-config filter and nacm rules. > > > > > > > > Example in XML: > > > > > > > > <stream-xpath-filter> > > > > /ietf-interfaces:interfaces/ietf-interfaces:interface/ietf-ip:ipv4 > > > > </stream-xpath-filter> > > > > > > > > Example in JSON: > > > > > > > > "stream-xpath-filter": > > > > "/ietf-interfaces:interfaces/ietf-interfaces:interface/ietf-ip:ipv4" > > > > > > > > > > > > My proposal is A. I think it is more important with consistency > > > > within each encoding than across encodings. > > > > > > I would suggest to consider declaring prefixes & namespaces > > > explicitly in the data, as in the schema mount document. It is > > > independent of encoding and the expressions can be kept short. In > > > fact, one of the namespaces can be declared as default, so this use > > > of XPath would then be very similar to YANG. > > > > Ok, so this is another alternative that works today, and achieves the > > goal of being encoding-independent. It is still context-dependent > > though. > > Yes, every module that uses XPath in data will have to deal with this. There > may potentially be multiple independent prefix declarations (this is actually > a con). > > > > > BTW, when used in filters, it is nice to let an unprefixed name to > > match any namespace; i.e., treat "foo" as syntactic sugar for > > "local-name(.) = 'foo'". ("*:foo" is not legal...) > > Hmm, I think this is a bad idea because it departs even further from the > original XPath semantics. Such chameleon names should IMO be pretty rare, and > if they are needed, local-name() is always available. > > [Qin]: Agree with Lada, Referencing RFC8407, section 4.6.2, I think the below > guideline is relevant. > " > The "local-name" function SHOULD NOT be used to reference local names > outside of the YANG module that defines the must or when expression > containing the "local-name" function. Example of a "local-name" > function that should not be used: > > /*[local-name()='foo']
This guideline is for must/when expressions *within* YANG modules. I'm talking about a different use case, namely filtering. It is pretty convenient for users to send a filter: /interfaces/interface[name='eth0'/ipv4 and get back what they expect. Even in the rare case of local name clashes, this filter works and gives back what was expected (+ additional nodes). I have no plans on writing up this as a proposal; I'm just pointing out that when XPath is used in filters, this is convenient. /martin _______________________________________________ netmod mailing list netmod@ietf.org https://www.ietf.org/mailman/listinfo/netmod
