KDE GSOC: Community Bonding and First Coding Period (May 17 - July 11)

Of course the task I described in the last post looks and is quite monumental. That is why I laid some of the groundwork for my GSOC beforehand (in the actual German semester breaks). This work continued in the community bonding and first coding period and will therefore be described here.

But first I want to thank my mentor Jasem Mutlaq for his support, his patience with me and his nerves of steel. My mood levels were somewhat similar to a huge-amplitude sine wave those last weeks.

Now to the meat…

I began by studying the existing deep sky object implementation in KStars to identify what structure the new catalogs should have and what the smallest irreducible core of functionality was I could replace to make integration easier. I discovered that the catalogs were a mix of SQL databases and text files, somehow loaded at startup and then appended to some linked list. There was some deduplication implemented but like most DSO code it was oddly catalog specific. Especially the Messier, IC and NGC catalogs were often mentioned in the code. Also the explicit distinction between stars and DSOs made writing general code complicated but I found a consistent set of data fields shared by all catalog objects which all admitted sane defaults. It wasn’t bad code or anything like that. Just the product of “organic groth” with many thing I wanted already present in some way but somewhat all over the place. I admit that I studied the code just enough to find out what exactly I had to replace and maybe I could have reused more of the existing code but I’ve picked this specific path in the multiverse, so let’s get on with it. Just a shout out to all who did previous work on the DSO code among whom are, just to name a few, Jason Harris, Rishab Arora, Thomas Kabelmann and Akarsh Simha.

With this knowledge I was able to go forward and devise a concrete plan for implementing the new DSO system. First of all, albeit I would love to use std::variant and some kind of entity component system for the different DSO types I settled with a one-for-all type for deep sky objects. The primary reason for this was, that KStars uses C++14 which lacks variants (and the extremely useful std::optional). Furthermore the DSOs all share common structure, so this was just the simpler and thus preferable option. The second design decision was not to load all of the DSOs into memory, but instead to take inspiration from the deep star catalogs. For one they are dynamically loaded from a special trixel indexed format so this already was within the formulated goals of the endeavor. On the other hand the notion of having “canonical” copies of catalog objects in memory and syncing their mutation with the database system seemed overly complicated. The catalog database should be the single source of truth and not the (ephemeral) memory of KStars.

When a specific object is needed, it should just be retrieved from the database locally in the code instead of searching some in memory list in KStars or shooting around with pointers. This notion is somewhat at odds with how things were and are done in KStars which created some interesting problems later on as we shall see. These ideas somewhat dictated the rest of the plan which I (for the first time in my programming career) completely wrote down in advance. The heart of it all is the database manager which abstracts maintaining, reading from and writing to the database. As always one should justify the creation of a special data type. In this case it was the requirement that the database access should be painless and could be handled locally anywhere in the code just by creating another instance of the database manager. The manager should handle retrieving objects and catalog meta information as well as importing, editing and exporting catalogs.

The structure of the database itself was another point of consideration. Naturally each catalog should have its own table. But how should deduplication work? The method I settled on is really quite simple. Each object gets a (relatively stable) hash that is calculated from some of its properties which is henceforth called the ID. When two objects (from different catalogs or otherwise) are the same physical object, then they will both be assigned the same object id (OID) which is just the ID of the object in the “oldest” catalog (with the lowest catalog id), trying to make it stable under the introduction of new catalogs. Additionally each catalog is assigned a priority value which is just a real number (conventionally between zero and one). When loading objects from the database into KStars and there are multiple objects with the same OID only the one from the catalog with the highest priority will be loaded. This simple mechanism should cover the requirements of KStars quite well and is relatively easy to implement.

There I ran into an issue that demanded some research and table in the database. The simplest option would be just to create a benchmarking. Remember that each catalog is represented by its own so-called view, a dynamic “virtual” table that combines all the catalog into one homogeneous table. SQL magic could automatically perform the deduplication algorithm outline above and everything would be fine and dandy. However, benchmarking revealed that actually writing the view into its own table, henceforth called the master catalog/table, increased the performance quite considerably, enough so to justify the increased complexity in the implementation. And then I discovered SQL indexes. A gift from the heavens! They increased performance on loading objects in a trixel from the master catalog roughly threefold and I was sold on the master catalog approach. So to summarize it all; a deduplicated view of the combined catalog tables is being created and then written into the master table. This has to be done for every modification of the catalogs but is relatively fast (just not fast enough to be done 20 times per second). Later experimentation showed that this approach could accommodate catalogs up to a million objects in size.

I also created a catalog file format, which is just an sqlite database file with the application id set to a special value with almost the same structure as the catalog database proper. The application id enables KStars to check if the database is really a catalog file and not to rely just on the structure of the contained database for that. In the future the file command and other utilities like file managers could be made aware of this special application id to recognize the catalog files. We will leave it this level of detail for now. For more details please refer to my notes.

Of course the operations on catalogs have to somehow be accessible in the GUI of KStars so this was another point of action. Before that however the glue between the database manager and the usual sky composite system had to be implemented. In KStars different types of objects (Stars, Comets, Asteroids, etc. pp.) all are implemented as components with a unified interface. These components provide methods for loading and drawing objects, as well as utilities to find objects near a certain point on the sky and similar things. The loading and drawing part was relatively simple to implement. The drawing code could be straight up reused from the old implementation and the loading was essentially covered by the database manager but with a twist.

To support very large catalogs it would be desirable to only have objects in memory which are currently visible. Thus a LRU cache was implemented with the trixel id, which essentially labels a portion of the sky, as key. This cache is fully unit tested and relies completely on standard library containers so not a single pointer appears in the code.¹ As an added bonus, the cache is completely transparent by default and only takes effect if configured to so and therefore includes the typical use case of comparatively small catalogs up to ten-thousands of objects.

But here the culture clash between the new DSO implementation and the traditional KStars way of things became apparent. In many places KStars expects pointers to so called SkyObjects with no real clue as to where they are actually stored and how their memory is managed and with the implication that the object is expected to live forever. Well, the DSOs from the catalogs aren’t supposed to be kept around forever and thus a compromise is in order. So whenever a pointer to an object is required, it is inserted into a linked list² in a hash table with the trixel as index or is taken from there if it’s already present. I hope that we can eventually transition away from raw pointers and manage life time either explicitly or with smart pointers.

With this done and basic drawing working I went on to implement a basic GUI for catalog management³. I also wrote unit tests for the database functionality which proved itself as very useful later on. After that I couldn’t delay anymore. Back when I implemented the component for the new DSOs I went as far as getting it to compile and not much further⁴.

Now I had to go around and find out what broke. A lot broke and I did not find all of it until the big merge :P. A rather interesting source of work happened to be the way metadata like observation notes and image links were stored. They came from a text file and then were loaded into the sky objects at startup and somehow synchronized with the text files on mutation. This, of course, played not well with the new DSOs as they were ephemeral. So I replaced the whole shebang with a hash table which incidentally improved startup performance. The rest of the integration work was similarly interesting and continues today. I will not go into it further but feel free to look at the KStars commit history.

Just yesterday I added a feature back in that I had axed accidentally to the dismay of its original author. That showed me that I am not entitled to judge the merit of individual features and whether they could be sacrificed for the “greater good”. The answer is: They cannot! Another lesson I’ve learned is, that too much magic just ain’t no good. I had created a variadic template wrapper for the QSqlQuery type for syntactical convenience and shot myself in the food with it. It ended up obscuring an error message and prevented me from reproducing a crash that users on certain platforms were experiencing. After a not-so-great couple of days I, with the help of two kind people, finally found the lowest common denominator of the problem: an old, but still supported version of QT which bundled an old version of sqlite which in turn did not support the NULLS FIRST directive that I was using. Turtles all the way down. Although I tested all my changes on KDE Neon (I am on NixOS primarily) the wise thing would have been to develop or at least test everything with an older QT version from the get go.

Also, although I had put in version checking into the database code, I didn’t provide a mechanism for upgrading the database format to new versions. This I now remedied by introducing a simple mechanism that applies database modifications successively for each version upgrade. So if I go from version two to version four it will be upgraded from version two to three and then to four which I understand is the way those things are usually done.

Now, I did do at least some “constructive” work, adding a (admittedly ugly) CSV importer so that users can import arbitrary CSV-ish catalogs. The greater chunk however I will cover next week: The python catalog package tooling with continuous integration and deduplication. The catalogs churned out by that framework are then installed via the KNewStuff framework. I discovered two interesting bugs in this framework because KStars seems to be almost the only program using the framework in this specific way.

If you made it this far, I applaud and thank you for your endurance. See you next time.

Cheers, Valentin

P.S. Currently I am working on documenting both the new DSO GUI and the python tooling. I hope eventually they will pass the “noob test” :P. But, as you may have recognized above, I am not the best explainer.

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1. As a matter of fact, I set out with the goal not to do any manual memory management and not to use a single pointer in the new code. I have been successful thus far if you would be so lenient not to count glue code for legacy KStars systems. ↩︎

2. References to objects in linked lists are stable. ↩︎

3. See the KStars Handbook. ↩︎

4. I really appreciate c++ as a compiled language. ↩︎