Build systems you can still understand when load, money, and people pressure arrive.
HappiHacking is the technical imprint of Erik "Happi" Stenman: writing, books, tools, courses, experiments, and advisory work around BEAM systems, payments, runtime behavior, and technical judgment.
For the curious: there is a tiny machine hiding in the banner.
Latest Essays
Notes on BEAM, architecture, production systems, software work, and the strange corners that become useful later.
The Shared State of Isolation.
2026-03-29
Why Systems Collapse Under Load (and How to Fix It)
Your Code Has No Memory
2026-03-26
From code change to intent, there and back again
The Late-Night Feeling of Wonder
2026-03-16
The Late-Night Feeling of Wonder In 1980 my ten years older sister started university and brought home a TI-58 programmable calculator. I was ten...
Why I Built a Course on the BEAM
2026-03-11
Understanding the runtime changes how you build on it.
Books and Long-Form Work
The BEAM Book
Erlang runtime internals
A practical reference for understanding scheduling, memory, processes, and the runtime behind Erlang and Elixir systems.
PhD Thesis
Concurrent language implementation
Academic work on efficient implementation of concurrent programming languages and native code compilation for Erlang.
Erlang and OTP in Action
A classic reference
A practical Erlang and OTP book from Richard Carlsson and co-authors, included here because it belongs near this work.
Ongoing Projects
BEAM Course Material
Runtime lessons and exercises
Course notes, slides, and exercises for engineers who want a better mental model of BEAM systems.
N.I.S.S.E.
A small VM in the browser
A tiny interpreter and REPL used to make scheduling, message passing, and process behavior visible.
Talks
Systems, BEAM, and fintech
Conference talks and notes from work on runtimes, high-volume backends, and systems that need to keep running.
Selected Systems Work
The consulting work is part of the archive because it produced lessons worth keeping. These are examples of systems, constraints, and roles.
Klarna
Early payment platform leadership
Technical leadership and core payment platform work during the period when the engineering organization grew from one person to about eighty.
Deutsche Telekom
High-volume IoT data pipeline
Architecture and proof-of-concept work for data ingestion and analysis at telecom scale.
Kappa
Large Erlang codebase analysis
A tool started at Klarna to keep a large Erlang codebase understandable across many teams.
Erlang/OTP
Runtime work over time
Contributions and runtime experience from the HiPE research era through years of production BEAM work.
Hidden Playgrounds and Tools
Some parts of the site are more workshop bench than publication. They are here for people who like to poke at systems directly.
N.I.S.S.E. REPL
Type erl in the site console
Open the console with the tilde key and launch a small VM for process and scheduler experiments.
Word Index
A map of recurring terms
A generated index of words across the essays. It is useful for accidental trails through the archive.
Livebook Exercises
Hands-on BEAM material
Generated notebooks and exercises connected to the BEAM course material.
Work With Erik
I sometimes help teams building difficult systems.
The useful fit is usually a system with real constraints: money movement, high concurrency, unclear failure modes, BEAM runtime questions, technical leadership gaps, agentic development workflows, or code that has become hard to reason about.
The work starts with understanding the system as it exists, then deciding whether a review, a focused implementation effort, a workshop, an interim technical role, or short advisory support makes sense.
