Particle Physicists Pulling Themselves From The Swamp

This post originally appeared on the Software Carpentry website.

What does it mean to work on a modern particle physics experiment like ATLAS (wikipedia, public) or CMS (wikipedia, public) at the Large Hadron Collider in the 21st century? It's fun, it's collaborating with great and interesting people, it's challenging, it's making you enjoy finding things out, it's what I always wanted to do. Also: it is painful, discouraging, and tends to suck the life out of a young mind. Confused? Let's rewind...

Five years ago I was a young man entering a field that I knew from textbook excerpts and stories of Nobel prizes. This field centered around a handful of large laboratories worldwide that host some of the most advanced technology mankind has ever build: particle accelerators. Billions are invested to build a machine that accelerates particles onto a near light speed orbit in order to bring them to a high energetic collision. Around these fixed collision points, multi-purpose experiments of the size of a multi-apartment houses record the particles thus produced to reconstruct what happened during the event and obtain deeper knowledge of the underlying physics.

YouTube video produced by the ATLAS collaboration explaining what we do, ATLAS Experiment (c) 2014 CERN

If I say multi-apartment houses, you might imagine that analyzing the data of these experiments is far from trivial. The collaborations running these machines provide physicists with object-oriented frameworks (cmssw, gaudi, athena) of the order of 7-10 kLOC (thousands of lines of code) that facilitate the read-out, simulation, filtering and analysis of this data. Sophisticated statistics software is used further down the pipeline to produce publication grade plots, make statistical inferences, and eventually extract knowledge.

Why is it so frustrating then? University curricula for physicists hardly ever contains programming skills, much less conceptual knowledge of object-oriented programming, revision control, software design, parallel computing, and the like. Neither do prep courses in early stages of a PhD. Most of the time, wiki pages allowing copy-and-paste style learning-by-doing that get you going but never explain underlying principles. But I am a scientist: I need to understand what is going on and use my tools to the best of my knowledge to make inferences.

ATLAS experiment open detector, ATLAS Experiment (c) 2014 CERN

One and a half year into my thesis, I personally was so frustrated of spending hours in front of the computer understanding other people's code, that I was dedicated to change the situation. At the same time, a federal German funding agency initiated a German network for particle physics, the Helmholtz Alliance "Physics at the Terascale", to induce higher scientific throughput just in time with the LHC coming online in 2009. This funding program also contained budgets for training of staff. Motivated by a never-ending and daily struggle to work with object-oriented frameworks that were poorly documented and yet state-of-the-art, I started to inquire about the possibility of organizing a workshop on software design principles. It was my hope that by transferring knowledge from computer science to physics students at the keyboard, we could pull ourselves out of the swamp and finally understand why we program as we do, how we can work effectively with code (both our own and others') and as such be more productive. After all, I chose to be a PhD student to do physics, not fight code!

Lord Muenchhausen pulls himself and his horse from the swamp (German tall tale)

Due to the support of my supervisor and the motivation of my fellow students and post-doc, we were able to find potent speakers. The only thing left was getting the money in and advertising the idea to principal investigators so they'd send their students around. And I can tell you, there were many PIs not willing to back us. After all, most of the group leaders made their day during the Fortran age in particle physics. So there was a deep cultural canyon between their view of how people should work and what everyone faced in everyday development.

It took a year to setup the first workshop in 2010 that welcomed 25 participants. We started by recapping object-oriented programming, then introduced UML, and finally climbed the hill to discuss design patterns, class-design and package-design principles. The week-long workshop was concluded by a student exercise project that lasted almost an entire day. We also had two keynote speakers from the trenches of the local software industry (which was quite a clash of cultures, but a very insightful experience).

Given today's standards, many of the details of the workshop were not very well thought through, I believe. It was a lot of content for 4.5 days and was not always paired with exercises or the like. But we ingested a lot of cool stuff to feed our curiosity. Finally, someone taught us the fundamental concepts we needed so desperately to know in order to grasp what were doing every single day. This made my (professional) life a lot happier than it was before. And I would even claim, it made me much more productive as a scientist.

Many participants gave us very positive feedback:

This workshop should be on everyone's curriculum in Particle Physics.

-- A participant of the 2014 workshop

Since then, I've been lucky enough to organize another workshop one year later. We again had 25 participants with a keynote by a local software consultancy CEO. Further, I was myself able to deliver the knowledge that I had acquired through the past workshop and give a talk on my own on test-driven development. The following year, the workshop started to travel and was hosted by DESY in Hamburg. We had more than 50 participants there, which proved to be a challenge. We were used to having a small number of people, which meant we could manage to cover a lot of material by dynamically adapting our speed and depth. But that approach does not scale! So at the end of a tough week, we again received a lot of positive feedback, but we had to admit that 30-35 people is a good size to go with.

During that time, the focus of the workshop changed a bit. We were still covering object-oriented programming, good design practices, and design patterns, but we added refactoring (thanks to an excellent new contributor) because it relates much more with the day-to-day situation students face: sit down and use the code of others. The number of exercises steadily increased, and I believe we are slowly converging to a 1:1 ratio of exercises to lectures. Finally, the scope of the audience became wider. We had to acknowledge that most particle or nuclear physics related sciences also have substantial need for software training. For example in the field of detector construction, both the nuclear and particle physics community commonly use GEANT (which is again a object-oriented framework) to set up and run detector simulations in a modular fashion. Participants from these fields repeatedly told us that the sort of training we offer is needed.

This year, the workshop was hosted at Munich. Triggered by Greg Wilson's PyCon talk, I tried to use some more interactive teaching elements like sticky notes, an etherpad, and live-coding mixed with a lot of pair work (not pair programming yet). I have to say that especially people in this environment, where they feel they lack competence (and because they are physicists ;) ) are a difficult crowd. Sticky notes were not adopted at all or I didn't "motivate" them enough, so I dropped them half-way through the session.

On the other hand, having the etherpad was great success. It allowed me to bring the teaching into the notebook that everyone likes to hide behind. Live coding also worked out extremely well. I used it to teach C++ template meta programming within one entire morning. The topic is quite complicated, but live coding helped me adapt the speed and ensure that everyone can follow and reproduce my demonstration on their own. There was constant feedback by the participants and people helped each other out. To be honest, I was surprised that live coding worked so well with a crowd of 33 students.

Last, I put all my code and slides on GitHub (see the Performance versus Design C++ repository) for the students to share and fork. I have to say that this did not receive the attention by the participants I hoped it would. But that might have technical or usability reasons or simply that particle physics is mostly an ecosphere of it's own, i.e. GitHub and the like are not yet common tools.

To conclude, I think we are well on on the way to establishing a software development focused training curriculum in the particle physics community. Promoted by this blog post, we will start to publish our experiences if possible at conferences and peer-reviewed journals in order to receive feedback, straighten our quality assurance, and bring our experiences and motivation to the attention of more people. We hope our courses can be adapted in other countries or big laboratories or even lead to a mind change of PIs:

The data volumes at LHC are steadily increasing, thus the analyses are becoming more complex and so become the list of systematic uncertainties to be studied. One is forced to write good code if you want to be flexible and fast.

-- German Particle Physics Group Leader from Bonn (translated from German)

Not only that, but the workshop is also being recognized and appreciated by all involved (PIs and students):

I've been sending students to this workshop for many years. Even though many of the students went to the workshop with a let's-see-if-that-will-help attitude, they always came back full of motivation to code and a lot of important insights how to code. After the workshop, they developed a high enthusiasm for well designed code. Thank you very much for organizing the workshop. It is really well done! Keep the level where it is now.

-- German Particle Physics Group Leader from Aachen (translated from German)

Lastly, I would like to thank the individuals that have made the workshop a success over the last years. The core team that back the workshop and are ready to present annually are: Thomas Schörner-Sadenius (DESY Hamburg), Maria Pia Grazia (INFN Genoa), Stefan Kluth (MPI for Physics, Munich) and myself. Apart from these, I'd like to mention past contributors: Thomas Velz (University of Bonn, now industry) was a participant once in the workshop and this year contributed as a teacher! Benedikt Hegner (CERN) and Eckhardt von Toerne (University of Bonn) both made substantial contributions in the past as well. Also, my gratitude goes towards my supervisor at TU Dresden, Michael Kobel, and my colleagues there (most of all Wolfgang Mader) who supported me to organize the workshops locally and motivated me throughout.

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