Migrating foundational software to the 21st century

We help teams build in Rust.

legacy.c potentially unsafe

                                1 char *msg = malloc(6);
                            
                                2 strcpy(msg, "hello");
                            
                                3 free(msg);
                            
                                4 puts(msg); // use-after-free bug

modernized.rs borrow-checked

1 let msg = String::from("hello")?;

2 drop(msg);

3 println!("{}", msg);

4 // compile-time error: borrow of moved value: `msg`

Proudly European based in Amsterdam

Member of

Explore the move to Rust without the expensive middlemen. Work together with two talented, passionate and approachable engineers instead

+31 20 3242 150 hello@definedonce.com@definedtwicenow.com

Systems Engineer

Elias

Passionate about solving difficult problems with motivated people, not letting any layer of abstraction untouched while doing so.

Systems Engineer

Max

Holds a strong passion for programming language design and an equally strong fascination for the origins of computer architecture.

Elias and Max are ready to walk the extra mile. They are efficient organizers and motivators, think in solutions and out-of-the-box, and never let you down.

Dr. Natalia Silvis-Cividjian Client · DBugIT, VU Amsterdam

We believe that now is the time for reliable software. Made in Europe

Read why

Our vision is shared through RFDs. Shared in public and open to be challenged

All RFDs

005 Working with Sensitive Data

This document describes how to handle sensitive data during our work. It does not include novel ideas, but summarizes relevant rules of thumb, tools and example…

004 Principled Version Control

Source management with git allows for ample opportunity of dangerous, unhelpful or plain annoying experiences as a software engineer. While this is bad enough w…

003 Software Guidelines

A small collection of guidelines on writing software. Programs must be written for people to read, and only incidentally for machines to execute. — Gerald Jay …

We are a fully privately owned company. In our blog, we write about our work and the technical background

All posts

Elias (left) and Max (right) at RustWeek 2026 in Utrecht

May 19, 2026

And We're Live - Launching at RustWeek 2026

We're officially launching Defined Once at RustWeek 2026 in Utrecht. Here is who we are, what we do, and why we do it.

Abstract visualization of a gap between private data (a sovereign cloud) and a shady corporate cloud, illustrated by a bridge between secured data storage and a crumbling bridge to a dark landscape with a cloud that shows a question mark.

Apr 17, 2026

Pushing for digital sovereignty as a member of NLdigital

Why we do and do not believe that being part of NLdigital is beneficial to our values and operation.

Abstract visualization of two people on a computer screen, with charts and technical icons on the background in Rusty colors.

Mar 28, 2026

Why this might be the worst time to start a consultancy

With the world on the verge of an economic crisis and AI getting better and better, now might be the worst time to launch a development consultancy. Why are we determined to proceed anyway?

+31 20 3242 150

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hello@definedonce.com@definedtwicenow.com

We typically respond within one day.