Saša Šijak, Full-stack Developer in Belgrade, Serbia
Saša Šijak

Full-stack Developer in Belgrade, Serbia

Member since April 29, 2015
Saša has more than ten years of professional software engineering experience under his belt. He also has strong analytical skills, a wide range of computer expertise, and a history of successful project outcomes. All in all, Saša is organized, attentive-to-details, and a meticulous self-starter who's able to organize and prioritize multiple tasks and stay calm and productive under pressure.
Saša is now available for hire




Belgrade, Serbia



Preferred Environment

Slack, Jenkins, YouTrack, Jira, Git, Linux, IntelliJ

The most amazing...

...project I've done was working with a team that made a distributed search engine which was bought by Facebook.


  • Freelance Software Engineer

    2017 - PRESENT
    Clients (via Toptal)
    • Worked in a team that created an engagement platform designed exclusively for education. My responsibilities ranged from working on the front-end in AngularJS to creating a back-end API in the Play framework and creating Docker-based deployments in between.
    • Developed mobile apps for one of the largest car manufacturers.
    • Implemented a website in React/Gatsby that was (or still is, depending on when you read this) featured on Adidas's homepage and is heavily marketed campaign for sports coaches to teach them how to approach and keep girls in sport (
    • Implemented a web app from scratch for a small business in a timber processing domain. Previously they organized and tracked their business by using very large Excel tables and files, shared through email and Dropbox and now they needed something that was more accessible and usable with a larger number of users.
    Technologies: DigitalOcean, PostgreSQL, Spring Boot, CSS, HTML, Docker, Cordova, MongoDB, Linux, SQL, TypeScript, JavaScript, Vue.js, React, Kotlin, Java
  • Owner | Software Engineer

    2016 - PRESENT
    Geek (Sole Proprietorship)
    • Created the website from scratch using GatsbyJS, React, GraphQL, Netlify CMS, and hosted it on Netlify; also implemented the SEO and marketing campaigns.
    • Developed a multiplayer and multiplatform soccer game.
    • Built a multiplayer turn-based browser game implemented using Vue.js and Firebase platform.
    • Created an open-source Kotlin-based startup template.
    Technologies: CSS, HTML, Gradle, Kotlin, Spring, Java, Vue.js, GatsbyJS, GraphQL, React
  • Software Engineer

    2013 - 2019
    Mozzart Bet
    • Pushed for and led the implementation of a new land-based casino system that replaced a legacy one—resulting in new business opportunities and better architecture. This system is used in more than 700 hundred betting shops and six countries and more than 3,500 gambling machines, all connected.
    • Hacked a proprietary communication protocol for gambling machines on my own; after which we could make our own solution pushing out 3rd parties, cutting on expenses, and opening the doors for exciting possibilities.
    • Implemented the jackpot animation and notification system on my own; which replaced a third-party solution and enabled us to push features that were never implemented by the third party. This app is used in every casino and it boosted profits and improved player experience.
    • Strove continuously to improve the business and system—going beyond basic requirements and pushing the boundaries of the old enterprise system and company. Provided many proposals and solutions to more than just technical problems.
    • Implemented a large part of the betting machines/terminals project. This machines are placed into betting shops or cafes. It involved integrating several hardware products, making an API, deployment system to several hundred machines on random locations, server and machine administration.
    • Implemented a messaging system which allows our managers to interact with players and customers, and which also sends automated messages based on some configurable rules.
    Technologies: CSS, HTML, Docker, Server Administration, Linux, Redis, ActiveMQ, Oracle, Spring, Vue.js, AngularJS, Java
  • Software Engineer

    2017 - 2017
    • Implemented new-and-supported old features for Quadwrangle (engagement platform targeting universities).
    • Developed a new deployment system using Docker.
    • Integrated several different APIs.
    Technologies: CSS, HTML, Cordova, Linux Administration, AngularJS, MongoDB, Docker, Play Framework, Java
  • Software Engineer | Team Lead

    2012 - 2013
    • Designed the back-end architecture for a scalable, high-performing online casino system.
    • Led the team which implemented the back-end from scratch. The back-end was then used by various clients, and tens of thousands of players making big profits.
    • Implemented various gaming providers and communicated with external providers.
    Technologies: Redis, Linux, ZeroMQ, MySQL, Python
  • Software Engineer

    2008 - 2011
    • Improved the app performance by profiling and optimizing a Java virtual machine.
    • Created the first version of the Wowd iOS app.
    • Handled QA and testing.
    Technologies: Linux, Java
  • Associate Lecturer

    2009 - 2010
    Union University, School of Computing
    • Organized lectures for the operating systems class.
    • Implemented various operating system parts with students in a simulator—improving their understanding and knowledge.
    • Earned one of the best student evaluations given during my time there.
    Technologies: Operating Systems, Java
  • Software Test Engineer

    2008 - 2008
    • Built a custom testing tool for Asseco banking software.
    • Designed and wrote various test cases.
    • Organized the Asseco programming challenges in the university.
    Technologies: Microsoft SQL Server, C#


  • Website for Adidas (Development)

    I implemented a website that was (or still is, depending on when you read this) featured on Adidas' homepage and is heavily marketed campaign for sports coaches to teach them how to approach and keep girls in sport.

    As the sole developer, I built the website using React/Gatsby along with implementing the custom styling and design.

  • Customer Messaging System (Development)

    I implemented a messaging system; this included both the back and front end along with an integration with the rest of the system. This enabled our managers to interact with players and customers. It also sends automated messages to customers when relevant events occur.

    Technologies: Java, Spring, Vue.js, ActiveMQ, Oracle, Linux

  • Betting Machine and Terminal (Development)

    I implemented a large part of the betting machine/terminal project. These machines are placed in betting shops or cafes. It involved working with the hardware approval process, integrating several hardware products, making an API and integration with the rest of the system, deployment system to several hundred machines on random locations, server and machine administration, and so on.

    Technologies: Java, Spring, Vue.js, ActiveMQ, SQLite, Oracle, Linux

  • Land-based Casino Management System (Development)

    I led the design and implementation of a land-based casino management system which was used in the largest betting operator in Southeastern Europe. It is used in several countries to manage more than 500 betting shops and more than 3,000 gambling machines in real-time. It is used to manage everything from machine information to jackpots and everything going-on in real-time. It was presented with large success at the London ICE Totally Gaming fair.

    Technologies: Java, Spring Framework, AngularJS, Oracle Database, Redis, Linux, ActiveMQ

  • Online Casino Management System (Development)

    I designed and implemented the back-end for the online casino management system. This system was later used on many websites and had tens of thousands of users. It was very flexible and offered some crucial features that competitors did not have. It made a good profit for the clients and is still used today on many websites.

    Technologies: Python, ZeroMQ, MySQL, SQLAlchemy, Redis, Linux

  • Amazing Multiplayer Soccer Game (Development)

    I worked with a team to create a multiplayer turn-based soccer game which was played by tens of thousands of players and attracted investors.

    Technologies: Java, JavaScript, Vert.x

  • Hacked Proprietery Protocol (Other amazing things)

    I hacked the proprietary protocol for communication with gambling machines by sniffing the traffic with modified serial cables and traffic analysis. Later, I wrote the server which implemented that protocol and which is successfully in use today.

  • | Online Presence (Development)

    I set up the website, Facebook page, Instagram account, the brand's whole online presence, SEO and marketing campaigns for a personal trainer's business. At first, the website was created with WordPress because of the very high deadline pressure and later I rewrote it in GatsbyJS (a React and GraphQL-based static site generator) and hosted it on Netlify. There is also a protected part of the website which is using Firebase Auth and Firestore.

    Technologies: React, GatsbyJS, Firebase, WordPress, Contentful, Netlify

  • Jackpot System with Roulette (Development)

    With a colleague, we implemented a first-of-its-kind jackpot system used with roulette in land-based casinos in our business region of interest.

    Technologies: Java, Spring, ActiveMQ, Oracle, Linux

  • Flat (Apartment) Ad Scraper (Development)

    I needed to find a flat (apartment) to rent and 90% of the ads on the internet were from the really bad agencies—I spent a month trying to manually find a flat.

    To solve this, I wrote a program that scrapes websites with flat ads every few hours, filters out all the agency and fishy ads, and showed me all good ones in the part of the city and other parameters that I was interested in.

    In the end, I found a flat in one day. I used this program again two years later and had the same result—I also found a flat in one day. Then I decided to share it with friends.

    Technologies: Java

  • Fun Factory Apps on Android (Development)

    I tested various earning methods on Google Play (Google Market before the switch), marketing strategies, and app positioning in the search results. The app earns a very good dollar return per hour due to the applied techniques.

    Technologies: Java, Android


  • Languages

    Java, HTML, SQL, JavaScript, CSS, Sass, YAML, C#, Python, TypeScript, Kotlin, GraphQL
  • Frameworks

    Spring, Spring Boot, Spring 5, Tailwind CSS, JPA, Play Framework, Cordova, Flask, AngularJS, Bootstrap, Ktor, Redux
  • Libraries/APIs

    GatsbyJS, React, Vue.js, Vuex, Spring REST, MobX, ZeroMQ, JOOQ, SQLAlchemy, Google Cloud API, Styled-components, Emotion Library, Google Maps, Google Maps Geocoding API
  • Tools

    JetBrains, Gatsby, Jira, Terminal, Slack, Git, ActiveMQ, Maven, IntelliJ, Docker Hub, Vagrant, YouTrack, Jenkins, Subversion (SVN), Gradle, PostCSS, JSX
  • Platforms

    Netlify, Linux, Contentful, Oracle, DigitalOcean, Firebase, Kubernetes, Docker, AWS EC2, Amazon Web Services (AWS)
  • Storage

    Redis, H2, PostgreSQL, Cloud Firestore, Firebase Realtime Database, Spring Data, Microsoft SQL Server, Google Cloud, Memcached, Oracle 12c, MySQL, MongoDB, MySQL/MariaDB
  • Other

    Single-page Applications (SPA), Full-stack, Web App Deployment, Creative Problem Solving, Software Design, APIs, Algorithms, Software Architecture, Server Administration, Browsers, Data Structures, Operating Systems, Linux Administration, Message Queues, Analytics, Administration, Cloud, Styled Components
  • Paradigms

    Scrum, Design Patterns, REST, SOA, Object-oriented Programming (OOP), Continuous Integration (CI), DevOps, Test-driven Development (TDD), Functional Programming, On-page SEO, Testing, Hosting
  • Industry Expertise

    Web Development


  • Bachelor's degree in Computer Science
    2006 - 2010
    Union University, School of Computing - Belgrade, Serbia


  • Google Associate Cloud Engineer
    NOVEMBER 2019 - NOVEMBER 2021

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