On 03/12/2018 18:13, Juergen Schoenwaelder wrote:
On Mon, Dec 03, 2018 at 09:56:48AM -0800, Andy Bierman wrote:
On Mon, Dec 3, 2018 at 3:36 AM Juergen Schoenwaelder <
j.schoenwael...@jacobs-university.de> wrote:
On Mon, Dec 03, 2018 at 10:21:20AM +0000, Robert Wilton wrote:
On 30/11/2018 19:28, Juergen Schoenwaelder wrote:
I doubt it makes sense to carry an entire yang library schema with the
instance data. The modules are actually known via the namespaces.
Knowing the module name or namespace isn't sufficient to be able to parse
the data:
- E.g. if the contents are in the format of RFC7895bis, but the client
tries to parse it using the schema from RFC7895, then it will fail.
- Once NBC changes are allowed via YANG versioning it will become more
necessary to know the revision or version of the modules to be able to
parse
the data. E.g. not even just the latest module revision will necessarily
work. It is entirely plausible that different server revisions may be
generating instance data using slightly different schema.
Perhaps this is a problem of the versioning solution then. ;-)
If the server has deviations then the client may also need to know those
deviations to correctly parse the file.
So, I think that it is pretty clear that knowing the right schema is
required to be able to correctly parse and interpret the instance data.
I think that there are 4 ways that this can be achieved:
1) embedded the necessary schema information into the YANG instance
data.
2) put the necessary schema information online somewhere and have a URI
reference to it.
3) some combination of (1) and (2), e.g. packages defined centrally,
with
deviations listed in the file.
4) the schema is determined using some out of bound mechanism, or
possibly
it is hard coded.
Perhaps we need to define first what "parse" means. I am not sure
parsing requires to know all schema details. Anyway, if all schema
details are needed, then the simplest solution seems to be to read the
schema from another instance file. This then only requires to
bootstrap the schema model version.
I agree with Rob.
The solution has to support accurate parsing of the instance data.
This means that the parser has the correct schema tree to use for
validating an instance document against the schema.
Since the new yang-library can have a different schema tree for each
datastore,
clearly the option of specifying the datastore is needed.
We need to get the terminology straight. I can parse XML and JSON
files without the need to have schema information available. So I
think the word 'parsing' is quite misleading here. And I can extract
data out of XML and JSON files that I find interesting without schema
information as well. So its a certain type of tools that may take
advantage of being schema aware but not all tools need to be schema
aware.
I don't think that there is a one size fits all answer here. I think
that
that it depends on the use case. Certainly, I think that facilitating 1
-3
is useful, but they should be optional rather than mandatory. I.e.
defining
nodes for these doesn't seem to cost much if a server isn't obliged to
populate them.
I do think that YANG packages (themselves defined in instance data
documents) could be very helpful here. I.e. rather than listing all the
modules, instead list the packages + any deviations from that. I'm
presuming here that the definitions of the packages are available via a
URI.
Yes -- this has been my goal for YANG Packages since the start.
By using nested packages and offline caching, the entire YANG library for a
device
could be recognized in a few URIs.
An instance file storing the schema tree is an offline cache. It falls
out of the instance file format for free. Yes, packages are yet
something entirely different but so far no work in this direction is
chartered so we should get instance file formats defined without
solving another bigger problem first.
In the IETF 103 discussion on YANG versioning, it seemed to be the
consensus in the room that the versioning design team need to consider
also versioning of sets of modules, rather than just individual
modules. I.e. something along the lines of YANG packaging.
Thanks,
Rob
If
you want to capture the schema, dump the relevant yang library into
another instance file.
That just means another file to carry around and manage.
Yes, but compared to solutions that require new and/or much more
elaborate data formats, this is very cheap and efficient, in
particular for systems where the schema itself is relatively
static. Also in terms of engineering time this is a rather cheap
solution since you do not need to invent a new way to document and
communicate a schema.
I agree the cost of an extra instance document is not a problem, especially
since
it would be optional and only used if the defaults are not sufficient:
- latest revision of module will be OK
- assume all features present will be OK
- assume no deviations will be OK
If the defaults are not OK, then the parser will incorrectly accept or
reject certain instance data,
unless the correct schema tree is obtained somehow.
Or the tool processing instance file content just does not care about
the schema and it simply assumes that a certain (module,path)
combination has fixed semantics. I am big fan of being able to use
generic tools to filter and process XML or JSON encoded data.
/js
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