Time Series Cube DM - IVOA Note

CresitelloDittmar, Mark mdittmar at cfa.harvard.edu
Mon Feb 27 16:56:04 CET 2017


I'm on leave this week.. so just a quick reply.
This approach is basically butting an instance against a role across model
boundaries, instead of defining a relationship.  This is a very different
approach than the vo-dml and compliant model work has been taking over the
past few years.  I don't even know how to express that in the model.  It
seems a bit late in the game to be expressing this concern so urgently.

(Harrumph),  off to enjoy my vacation,

On Monday, February 27, 2017, Markus Demleitner <
msdemlei at ari.uni-heidelberg.de> wrote:

> Hi Mark,
> On Fri, Feb 24, 2017 at 08:50:24PM -0500, CresitelloDittmar, Mark wrote:
> > Can you make a statement about how you think this proposed arrangement
> > would effect
> >   1) Validation
> You separately validate against each version of each DM that is in
> the annotation.  This is actually, I claim, what matters to clients.
> Say I'm a simple plot program -- I'm not at all concerned about
> PubDIDs or about objects observed, all I need is figure out which
> axis make up the cube in what way.  So, I need valid NDCube-1.x, but
> I don't break if the data provider chose to annotate with Dataset-1.0
> or Dataset-2.0.
> Say I'm a component that can transform coordinates between different
> frames.  I'm concerned about correct STC-2.x annotation, but I
> couldn't care less if the coordinates I'm converting are axes of an
> NDCube or are source positions in a source DM, or yet something else,
> and again I don't care at all about any other DM that might be
> instanciated in that particular dataset.
> Say I'm a cube analysis program.  In that case I'll use NDCube to
> understand the structure, STC to do reprojections and regridding,
> perhaps photometry to convert the units of the dependent axis, or
> some other DM if the cube values require it.  And for all of these, I
> can simultaneously support multiple versions (independently from each
> other and thus relatively cheaply), so I can maintain backwards
> compatibility for a long time.
> >   2) Interoperability
> Interoperability is actually what this is about.  If we build
> Megamodels doing everything, we either can't evolve the model or will
> break all kinds of clients needlessly all the time -- typcially,
> whatever annotation they expect *would* be there, but because their
> positition in the embedding DM changed, they can't find it any more.
> Client authors will, by the way, quickly figure this out and start
> hacking around it in weird ways, further harming interoperability;
> we've seen it with VOTable, which is what led us to the
> recommendations in the XML versioning note.
> Keeping individual DMs small and as independent as humanly possible,
> even if one has to be incompatibly changed, most other functionality
> will just keep working and code won't have to be touched (phewy!).
> I'd argue by pulling all the various aspects into one structure,
> we're following the God object anti-pattern
> (https://en.wikipedia.org/wiki/God_object).  Ok, since we're using
> composition rather than a flat aggregation, it's not a clear case,
> but I maintain we're buying into many of the issues with God objects
> without having to.
> > ???
> > The Cube model has the following links to other models
> >   a) Dataset - defines itself as an extension of ObsDataset
> >   b) Coordsys - where coordinate system objects and the pattern for
> Frames
> > are defined
> >   c) Coords - where the pattern for Coordinates is defined (and
> implemented
> > for several domains, but that is not important here)
> >   d) Trans - where Transform mappings are defined.
> For all of these I'd ask: How does it help clients to have these
> pulled together in one place?  What can it do that it couldn't do if
> these were separate annotations?
> That's actually my personal guideline for DM design: "But does it
> help clients?"
> > You say that cube should not import Coords to identify what a Coordinate
> > is.. that it simply indicates that 'it has Coordinates'.
> > It currently says that an Observable is a coords:DerivedCoordinate ..
> which
> > is an abstract object restricted to
> > follow the pattern defined in that model.  Any model can implement the
> > pattern and declare itself as that type of Coordinate,
> > and be instantly usable in a cube instance.
> >
> > Without this explicit link, then one cannot validate across these
> > boundaries.
> >
> > An instance would have
> >    Element with role cube:DataAxis.observable
> >       Instance with type <whatever implemented "Coordinate" type>  ie:
> > spec:FluxCoord
> >
> > But a validator cannot check if FluxCoord is actually a Coordinate... (I
> > could put a ds:Curation  instance
> > there.. the role and the instance would be individually valid, but the
> > combination is nonsense).
> I'd maintain there's far too much that might work as coordinate
> (Filter name, anyone?) or, even worse, observable to even hope that a
> static validator will provide more benefit that harm (in terms of
> making things hard that clients would otherwise have no issue with).
> In the end, people will annotate something as a Coordinate just to
> shut up the validator and confuse actual users of a Coordinate DM,
> making things worse.  And have mis-pointed axes really been a
> problem in any published dataset?
> So, I'm afraid I find the use case "make sure the thing referenced
> from an NDCube axis is derived from Coordinate" too unconvincing to
> warrant the complication of linking otherwise independent DMs.
> > And.. without the link, there is no binding the various implementations
> of
> > Coordinate to the pattern.
> I have to admit that I find the current artefacts for current STC on
> volute somewhat hard to figure out. But from what I can see I'd be
> unsure how that binding would help me as a client; that may, of
> course, be because I've not quite understood the pattern.
> If this turned out to be true, I'd take that as an indication that
> Coordinate should move into ivoa or, perhaps, a DM of its own, being
> so generic a concept that it actually needs sharing across
> "sub-domains".
> > Interoperability would suffer because there would be no guarantee of
> > compatibility of different Coordinate types.
> > My code that understands the Coords pattern would have no hope of
> > understanding any portion of
> > independently defined Coordinates.
> What does "understand" mean here?  This is not a rhethorical question
> -- I'm actually trying to understand where the complication comes
> from.  What information, in addition to what you get from STC or
> comparable annotation, does your code require, and is there really no
> other way to communicate it without having to have a hard link
> between NDCube and STC (or any other "physical" DM, really)?
> Thanks,
>           Markus
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