DENIS FEJZA Get in touch
Open to backend & full-stack roles — Belgrade or remote

I build systems that hold up when they actually get used.

I'm Denis Fejza — a backend and full-stack engineer in Belgrade. Computer engineering degree from ETF Belgrade, three years shipping production platforms for paying clients, and enough systems-level work — a RISC-V kernel, a compiler, a CPU in SystemVerilog — that I understand what runs beneath the framework.

Email me → See the work GitHub
B.Sc.
Electrical Engineering & Computing, ETF Belgrade
3+ yrs
Production work across web, backend, AI and Web3
40%
Faster page loads on a live platform via query & caching work
Top Rated+
Upwork Top Rated Plus — top tier of vetted freelancers

01 / Selected Work

What I've built.

Client platforms that carry real users and real money, alongside systems projects where I built the layer everything else runs on. Source is public where the client allows it.

Client Platform · Backend

ShivWorks Network

Audit score 4/10 → 8.5/10

Rebuilt the account and content systems of a live membership platform: fixed broken access control, introduced admin role management, secured video delivery, and repaired email deliverability that was silently dropping member mail.

Node.jsAuth & RBACCloudflare

Client Platform · Full-Stack

gradi.rs

40% faster page loads

Ongoing development and maintenance of a production platform. Profiled the slow paths, rewrote the expensive queries and added a caching layer — cutting page load times by nearly half without a rewrite.

Full-stackSQL optimizationCaching

Product · Web3 & AI

FUKU AI

Live on Solana & EVM

A DeFi platform with autonomous on-chain agents. I wrote the smart contracts, wired ML models in for predictive analytics, and built the backend microservices behind tokenomics, wallets and transaction monitoring.

SolanaEVMMicroservices

Product · SaaS

InRooms

Events, payments, AI features

A networking platform for tech sales professionals: event discovery and registration, connections and messaging, Stripe-backed subscriptions, LinkedIn OAuth onboarding and an admin panel — built as one coherent React and TypeScript application.

ReactTypeScriptSupabaseStripe
Source on GitHub →

Systems · C++

RISC-V OS Kernel

Written from scratch, no standard library

A multithreaded operating system kernel for RISC-V, running bare-metal: threads and context switching in assembly, counting semaphores, a first-fit memory allocator, and the system-call boundary between user and supervisor mode.

C++RISC-V asmQEMU
Source on GitHub →

Systems · Java

MicroJava Compiler

Full pipeline, source to bytecode

A complete compiler for a Java-like language: JFlex lexer, LALR(1) grammar building a typed syntax tree, semantic analysis against a symbol table, and bytecode generation with backpatched jumps for loops, switches and conditionals.

JavaJFlexCUP
Source on GitHub →

02 / Depth

Four layers, not one.

Most developers work in a single layer and treat the ones below as magic. I've built in all four, which is why a performance problem or a security hole rarely stays mysterious for long.

Layer 04
Agents & chain
Smart contracts and autonomous agents on Solana and EVM, with ML models wired into wallet, tokenomics and transaction-monitoring systems.
SolanaEVMLLM agents
Layer 03
Product systems
Authentication, role-based access control, admin tooling, payment flows and media pipelines — the parts of a platform that decide whether users can trust it.
Auth & RBACStripeMedia delivery
Layer 02
Backend & data
APIs, relational schemas, query optimization and automation pipelines, built for throughput and for the person who has to debug them at 2am.
Node.jsPythonPostgreSQL
Layer 01
Kernel & silicon
A from-scratch RISC-V kernel with threading, semaphores and a custom allocator; a compiler front-to-back; a CPU in SystemVerilog with UVM verification. The ground the rest of it stands on.
C / C++RISC-VSystemVerilog

03 / Stack

What I work with.

Comfortable across the stack, strongest on the backend and in systems languages.

Languages

TypeScriptPythonJava C++CRust SQLDart

Backend & data

Node.jsREST APIsPostgreSQL SupabaseFirebaseMicroservices

Frontend & mobile

ReactViteTailwind Flutter

Tooling & infra

GitDockerLinux CI/CDStripeCloudflare

04 / Experience

Where I've done it.

Three years of contract work means three years of shipping to deadlines, owning production incidents and explaining technical trade-offs to people who are paying for the outcome.

2023 — present

Freelance Software Engineer

Upwork — Top Rated Plus · direct clients

  • Delivered backend and full-stack builds end to end: requirements, architecture, implementation, deployment and handover.
  • Took over existing production codebases and made them faster and safer without rewrites — query optimization, caching, access-control repair.
  • Reached Upwork's Top Rated Plus tier, awarded on sustained client satisfaction and delivery on larger contracts.
  • Worked async with clients across EU and US time zones.
Automation

Python Automation Engineer

SEOTEAM.IO

  • Replaced manual SEO process and reporting work with repeatable Python tooling.
  • Turned a recurring human task into a scheduled, reviewable pipeline.
Product work

Independent Products

InRooms · Zentrix · AskyPrint

  • Built and shipped complete products solo: a SaaS networking platform, a five-platform Flutter media player, and a Windows desktop client for thermal receipt printers.
  • Each one covers the full path — data model, backend integration, interface, packaging and release.

05 / Education

Where the fundamentals came from.

B.Sc. Electrical Engineering and Computing

University of Belgrade — School of Electrical Engineering (ETF), Department of Computer Engineering and Informatics. One of the most demanding engineering programmes in the region, with a curriculum built on mathematics, computer architecture and systems rather than frameworks.

Operating systemsCompilersComputer architecture Algorithms & data structuresDigital design / VLSIDatabases
Degree completed
Serbian — Native
English — Full professional
German — Conversational

Looking for a backend or full-stack engineer?

I'm open to full-time roles in Belgrade or remote. Send a short note about the team and the problem — I'll reply within a day.

Email fejza.denis@gmail.com → LinkedIn

Also available for contract and freelance work — get in touch with your scope.