Systems Work Archive
A small archive of systems, constraints, roles, and lessons from past work.
Payment platform advisory
Global payment service provider
Context: Klarna's core payment platform was growing under real traffic, product, and operational pressure.
Role: Erik worked as appointed expert advisor for Klarna's Erlang core system. The work included system development, architecture, monitoring, and product development.
Lesson: payment systems need clear state ownership, careful observability, and architecture that can survive increased transaction volume without becoming impossible to explain.
Related blog posts:
High-volume IoT data
Deutsche Telekom
Context: Deutsche Telekom needed a high-volume IoT data platform for customer services.
Role: HappiHacking helped design and implement the architecture and proof of concept for data ingestion and analysis at billions of messages per day.
Lesson: telecom-scale event systems need careful attention to ingestion, backpressure, operational visibility, and GDPR-aware data handling.
“If you are searching for a motivated team that will go the last mile with you, deliver qualitative stuff and understand bad jokes…Happihacking is the right choice!”
Filiz Hazer-Yilmaz, Deutsche Telekom
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Blockchain runtime work
Aeternity
æternity is a European blockchain project, see aeternity.com.
Context: the project needed core Erlang blockchain components that could be reasoned about under consensus, state, and execution constraints.
Role: Erik worked as part of the core team on FATE, a virtual machine for the Sophia smart-contract language, plus block validation and Merkle-tree based state management.
Lesson: virtual-machine and ledger work rewards small contracts, explicit state transitions, and code paths that can be audited without heroic interpretation.
"HappiHacking provides solid engineering even in the most risky projects!"
Michał Zajda, CEO Sennui Lab
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Video chat service
Deutsche Telekom
Context: Deutsche Telekom explored a home video chat service with dedicated hardware and shared media watching.
Role: HappiHacking helped design and implement architecture for a video chat product that combined real-time communication, hardware constraints, and co-watching of movies or TV.
Lesson: consumer-facing systems still depend on the old engineering basics: latency, device behavior, failure modes, and a product architecture that can be tested without guessing.
Related blog posts:
Book about the BEAM
The BEAM Book
Erik Stenman is the author of The BEAM Book, a practical reference for understanding the Erlang runtime system. It is also available for free online.
The book now anchors much of the public BEAM material on HappiHacking: course notes, runtime essays, workshop material, and the N.I.S.S.E. experiments.
"Recent discussion reminded me about this great book on lower-level working of ERTS/BEAM by @erik_stenman. Highly recommended, I think I'll reread it myself :-) blog.stenmans.org/theBeamBook"
Saša Jurić @sasajuric
Related blog posts:
Mobile Puzzle Game
Taba
Taba is a mobile puzzle game built by HappiHacking.
It uses Flutter on the front end and Erlang in the back end. See taba.quest or get the game on your mobile from your store.
Lesson: small games are useful laboratories for product loops, distributed backends, mobile UX, and the unglamorous parts of keeping a live system pleasant.
“My new favorite game, it is SUPER FUN and addictive!!!”
User on AppStore
Related blog posts:
Communications platform
Nynja
Nynja was a communications platform where the work required quick technical reading, ownership, and practical delivery across an existing system.
“Impressive! This firm dug into our tech and got started fast! Their work was excellent and more importantly, diligent! They stood behind their work and took ownership! I give them 5 stars for sure!!! ”
JR Guerrieri, founder
Erlang language development
Context: WhatsApp was interested in typed Erlang ideas and language-level improvements around large Erlang systems.
Role: HappiHacking helped investigate possible language features and directions for typed Erlang.
Lesson: language work needs respect for production code, existing developer habits, and the cost of making useful types fit a dynamic runtime culture.
Early technical leadership
Klarna
Before HappiHacking, Erik was Klarna's first CTO. He helped scale the engineering organization from one person to about eighty while working on the development and operation of a 24/7 payment system.
The work connected architecture, hiring, product pressure, runtime behavior, and C-level decision making during a period of rapid growth.
"Erik Stenman was the first CTO of Klarna and, without a doubt, one of very few key people behind its great success. During Klarnas early years Eriks ability to transform C-level Management requirements in to comprehensive and strategic development tasks as well as being a part of the development team that performed those tasks was an invaluable asset for Klarna. As Klarna grew Erik, very smoothly, grew with the company and his responsibilities, handling people and larger strategic issues with the same professionalism and positive attitude as ever."
Peter Elasyeh Head of Development Klarna AB
Erik also mentored some of the CTOs and CIOs that came after him.
"I really enjoyed our lunches during the year, you helped me a lot whether you realized it or not :)"
Yuval Samet, Chief Information Officer
Related blog posts:
Interim CTO
Travel SaaS
Context: a German travel SaaS startup was in a critical MVP phase and needed a technical roadmap that could connect product ambition to engineering execution.
Role: Erik joined as interim CTO, reviewed the initial plan, shaped target architecture, worked with the Head of Engineering, and helped build a reliable engineering team.
Lesson: in early product companies, architecture is useful only when it gives the team a clearer path to delivery, hiring, and ownership.
“We were able to convince Erik to join our SaaS start-up as interim CTO in a critical MVP development phase. His track record speaks for itself, and what we found most appealing was his humble attitude and pragmatic approach in dealing with us, the company founders, as well as our board.
He grasped quickly that we were missing a proper technical roadmap and took initiative immediately. Over time, Erik worked closely with our Head of Engineering and engineers, helping us form a reliable and motivated engineering team.”
Marco Krebs, Founder
Technical Advisor
Healthcare Payments
Context: Sika Health was building in health payments, where early architecture, roadmap, and hiring choices had long-term consequences.
Role: Erik advised the founder on technical roadmap, engineering-team shape, and fintech/product judgment during the early company phase.
Lesson: advisory work is most useful when it helps a founder connect business constraints, technical architecture, and the next hires the system will need.
“Erik supported my company Sika Health in early days as a Technical Advisor. His background having lead and scaled technical teams at Klarna was a distinguishing factor since our business in the health payments space has many elements in common with the Klarna product. He helped me develop an initial technology roadmap so that I could understand who I needed to bring onto my Engineering team to deliver on this roadmap.
Additionally, he’s been a trusted advisor to me personally as a first-time founder/CEO building in fintech. I’ve been able to talk with him about a variety of topics as I’ve built the foundation for growth in my business. I would highly recommend Happi to any early stage founders and executives building in tech who need a thoughtful, warm, and experienced executive advisor to help them start or grow their business.”
Ami Kumordzie, Founder/CEO
