DaCHS 2.5: Check your UCDs

2021-11-17 Markus Demleitner

In the background of the DaCHS 2.5 release picture: UCDs grabbed from the Registry. The factual background: DaCHS 2.5 will now moan at you when you invent or mistype UCDs

This afternoon, I have released DaCHS 2.5. As usual, I will discuss the more important changes in a blog post – this one.

A change many of you will not like too much is that DaCHS now validates UCDs you give it, and it will warn you when you do not follow the UCD rules. This may seem like nit-picking, but as blind discovery is on the verge of becoming usable in the VO, making sure these strings actually are what they should be is becoming operationally important: If I want to find resources that give errors for their photometry, I have to know whether it's stat.error;phot.mag.b or phot.mag.b;stat.error, or else I will miss half the resources out there.

So, I'm sorry if DaCHS starts complaining about half of your RDs after you update, but it's for a good cause. And don't feel bad about the complaints: DaCHS complained about close to half of my RDs after I had put in that feature.

By the way, this comes as part of a larger effort on the side of the Operations IG to improve the validity of UCDs and units in the VO, an effort that has unearthed bugs in the SSAP and SLAP specifications in that they require UCDs forbidden by the UCD standard. DaCHS 2.5 still follows SSAP and SLAP, and hence external tools like stilts will protest because of bad UCDs even if DaCHS is happy. Errata for the specifications are being worked on, and once they are accepted, DaCHS and stilts will finally agree on UCD validity, or so I hope.

Code-wise, a much more intrusive change was that asynchronous services (in particular, async TAP) now use the same formalism for parsing parameters as their synchronous counterparts. It may seem odd that that hasn't been the case up to now, but there were good reasons for that; for instance, with async, people can post incomplete parameter sets that would be rejected by normal sync processing.

Unless you are running User UWS services, you should not notice anything. If you do run User UWS services, please contact me before upgrading. I would like to work with you on how these should look like in the future.

Another change that might break your services is that DaCHS now actually complies to VOUnits, which has always forbidden whitespace of all kinds in unit strings. DaCHS, on the other hand, has foolishly encouraged putting whitespace between scale factors and pure units, as in 1e-10 m. That's not interoperable, and hence DaCHS now rejects such units. This may lead to hidden failures when dachs val doesn't notice something is a unit, and things only break during execution. I'm aware of one place where that's relevant: spectral cutout services that need to know the spectral unit If you're running those, make double sure that the spectralUnit in the SSAP mixin does not contain any whitespace. It's 0.1nm according to VOUnits, not 0.1 nm.

An update that should silently make your services more compliant is that DaCHS' representation of EPN-TAP is updated to what is currently under IVOA review. After you upgrade, DaCHS will try to update your EPN tables' metadata, which in turn should make stilts taplint a lot happier. It will also make DaCHS pass on the new, IVOA table utype to the Registry, which is how people should in the future find EPN-TAP data.

DaCHS now also contains some code that may help you import data from HDF5 files. For one, there is the HDF5 grammar, which rather directly pulls data from HDF5s written by astropy or vaex. But, really: HDF5 is a rather low-level format not particularly well suited for relational data, and it is virtually impossible to write generic code for doing something sensible with it. The two flavours DaCHS supports have very little in common, and it is therefore almost certain that if you have HDF5s coming from somewhere else, hdf5Grammar will not understand them. Still, let us know what you've got, we may be able to put support for it in.

Hdf5grammar is written in Python, and thus imports perhaps a few thousand rows per second. For Gigarow-sized data collections, that's nowhere near fast enough, and hence for vaex-written HDF5s, there is booster support. As before, if you have bulk data in HDF5 that you want to put into a database and that was not written by vaex, let us know and we'll see what we can do.

A surprisingly minor change enabled DaCHS to deal with materialised views, database views that are turned into actual tables by postgres. See the corresponding section in the tutorial for how you can use them. We do not have any materialised views in our Heidelberg data center yet. So, if you use them and notice something is clunky, your feedback is particularly appreciated.

There are many smaller changes and improvements; let me mention what the changelog euphemistically calls ”better systemd integration”, which really means that so far systemctl restart dachs simply didn't do anything at all. Apologies. And shame on everyone who was bewildered but failed to report this to dachs-support.

Also, you can use float arrays in boosters now, and DaCHS' ADQL has just leared about COALESCE. That's a SQL feature that lets you deal sensibly with NULLs in some cases: COALESCE(arg1, arg2, ...) will return the first non-NULL argument it encounters. That may sound like a slightly exotic function. Until you need it, at which point you wonder how ADQL could reach its ripe age without COALESCE.

Finally, let me mention something that is not part of the release, though it is DaCHS-related and is new since the last release: I have cleaned up the access log processing machinery we have used in Heidelberg in the past 15 years or so, and I have packaged it up for general consumption. It is, of course, a DaCHS RD that you can just check out and use in your own DaCHS installation if you have to keep access logs and want to do that with at least some basic respect for your user's rights. See http://docs.g-vo.org/DaCHS/tutorial.html#access-logs for details.

And the Solar System, too

2017-03-30 Markus Demleitner

Virtual Observatory technologies are increasingly being adopted outside of “core” astronomy in the vicinity of the optical band (to which they have had, I'll have to admit, a certain slant) . An excellent example for that trend is the Europlanet community. Their goal is to make solar system data accessible without fiddling, and they are employing a wide range of VO standards for that. At the heart of their efforts are TAP and the VO Registry.

While the usual VO client software will of course work fine with their services, they are offering a nice web-based discovery tool executing queries against an increasing number of services. Such uniform quering over many services is possible is because all of them implement TAP and host EPNcore tables. The resulting interface, also known as EPN-TAP, allows for very flexible discovery and retrieval of solar system data products, much like ObsTAP does for astronomical observations outside of the solar system.

Since quite a few EPN-TAP services are built using GAVO's DaCHS publication suite, I was invited to this week's VESPA implementation workshop 2017 in Graz to help the data providers set up their services.

I can't deny that I'm somewhat excited when I see how our software is used to publish spectra of the ice blocks in Saturn's ring taken by the lonely Cassini spacecraft still orbiting the gas giant, or data transmitted by Rosetta, now (and for who knows how long) sitting on comet 67P/Churyumov-Gerasimenko. There's even an upcoming archive of solar system alerts that may, according to its builders, include events like meteor showers on Mars. I can almost hear my code whisper “I've archived signals of C-beams glitter in the dark near the Tannhäuser Gate”.

Even documentation can become otherworldly in this business: Already in February, DaCHS has learnt to procude GeoJSON, a format common in the GIS community and also adopted by Europlanet – planetology has lots of common ground with geoinformatics. And in the reference documentation on annotating tables to enable that, when I wrote “standards-compliant GeoJSON clients will interpret your coordinates as WGS84 on Earth if you leave [frame annotation] out”, I was severely tempted to add “which is probably not what you want” and feel like Spaceman Spiff.

Romantic space adventures aside, after this intense week, not only are there several additional or improved EPN-TAP services from places ranging from Pasadena to Villafranca to Warsaw in the pipeline, the close interaction with the data providers has also led to very significant improvements to DaCHS' EPN-TAP support. The tutorial chapter on EPN-TAP and the reference documentation linked from there already reflect the results of this workshop. You'll need a current DaCHS beta package for that to work, though; we expect this stuff to go into our release packages around July.

If any of the workshop participants read this: Thanks a lot for your patience with DaCHS' sometimes somewhat cryptic diagnostics. If, on the other hand, you missed the Graz workshop and have solar system data: Please talk to us or the kind and friendly Europlanet folks – either us will be delighted to support your publication project. And perhaps we'll meet you at the next such workshop, planned for 2018 in the Czech Republic.

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