Ladislav Lhotka <lho...@nic.cz> wrote: > Martin Bjorklund <m...@tail-f.com> writes: > > > 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 > > This is impossible if we want to call it XPath. With an explicit > namespace/prefix declaration, for example > > "namespace": [ > { > "prefix": "if", > "uri": "urn:ietf:params:xml:ns:yang:ietf-interfaces", > "default": true > }, > { > "prefix": "ip", > "uri": "urn:ietf:params:xml:ns:yang:ietf-ip" > } > ] > > it would be > > /interfaces/interface[name='eth0']/ip:ipv4 > > which is not too bad either.
Well, "ncc myhost --get -x /interfaces/interface[name='eth0']/ipv4" is a one-liner; if I also have to specify the namespace list for every filter, it suddenly is not that convenient. /martin > > Lada > > > > > > 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 > > -- > Ladislav Lhotka > Head, CZ.NIC Labs > PGP Key ID: 0xB8F92B08A9F76C67 > _______________________________________________ netmod mailing list netmod@ietf.org https://www.ietf.org/mailman/listinfo/netmod