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

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, and 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

We share our vision through RFDs. Publicly tracked 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 …

The latest updates from Defined Once HQ

All posts

RCN launch image with white text on a blue background saying 'Rust Commercial Network Launches to Bring Commercial Users of Rust Language Together'.

June 28, 2026

We're joining the Rust Foundation's Rust Commercial Network (RCN)

Defined Once Engineering B.V. has joined the Rust Commercial Network as a founding member to stay in contact with Rust's commecial users and to give back to the Rust programming language.

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.

+31 20 3242 150

Mon – Fri 08:00 – 18:00 CET

hello@definedonce.com@definedtwicenow.com

We typically respond within one day.