Cosmin Rusu, Developer in Lausanne, Switzerland
Cosmin is available for hire
Hire Cosmin

Cosmin Rusu

Verified Expert  in Engineering

Full-stack Developer

Lausanne, Switzerland
Toptal Member Since
December 6, 2017

Cosmin worked at Amazon, Google, and Apple on highly-scalable, distributed systems, and machine learning focused on NLP (Siri team). He worked for a startup called Kuende, writing Scala and Go microservices on top of Google Cloud. Cosmin has a strong background in advanced algorithms and data structures in C/C++ and is very passionate about machine learning.


Abundance Digital
Stripe, PostgreSQL, Ruby, Ruby on Rails (RoR), Python, Full-stack, APIs, Linux...
Spotify - Main
React, Ruby, Ruby on Rails (RoR), Tailwind CSS
Ruby on Rails (RoR), Ruby on Rails API, HIPAA Compliance




Preferred Environment

Ruby on Rails (RoR), Data Science, Python, React, Scala, Git

The most amazing...

...thing I've coded was at my internship with Google where I delivered a new type of automatic content creation containing action moments from users' videos.

Work Experience

Stripe Data Consultant

2019 - 2023
Abundance Digital
  • Answered relevant business questions based on available Stripe data for this community-based company. The answers resembled their subscription model and how well it performs.
  • Implemented a checkout page allowing people to pay for the yearly subscription every month using
  • Applied relevant SQL queries in Stripe Sigma so business owners can always run the queries on the latest data.
  • Helped to automate their boarding process with Zapier.
  • Analyzed the subscription data and suggested ways to improve churn rate, revenue, and profit.
Technologies: Stripe, PostgreSQL, Ruby, Ruby on Rails (RoR), Python, Full-stack, APIs, Linux, Web Development, Ruby on Rails API, Ruby on Rails 5, Stripe API

React Developer

2021 - 2022
Spotify - Main
  • Migrated the front end of Megaphone (the podcast-ads business area of Spotify) to a modern stack with React. Worked on re-designing their architecture using current standards.
  • Redesigned the feature flags infrastructure. All our new features were typically launched under feature flags, so they were an important piece that we needed to make sure we got right.
  • Built new components and added storybooks, unit tests, and end-to-end integration tests with Cypress. All new features were built using functional-style React with React Hooks.
  • Helped develop a strategy to upgrade our React version to the latest one.
Technologies: React, Ruby, Ruby on Rails (RoR), Tailwind CSS

Senior Full-stack Engineer

2021 - 2022
  • Worked on an internal project for Kindbody to automate some internal billing processes using Ruby on Rails. The app used server-side Rails templates, but for more complex components, we used React components rendered on the server (think of Next.js).
  • Tracked and implemented features via Trello and discussed with the CTOs and product managers.
  • Deployed the Rails application to the AWS Kubernetes service using Helm and Helmfile.
Technologies: Ruby on Rails (RoR), Ruby on Rails API, HIPAA Compliance

Full-stack Software Engineer

2019 - 2022
Mochila Fulfillment
  • Integrated an external shipment label provider, Shippo.
  • Deployed the ELK stack for our logging infrastructure.
  • Helped the team automate their tasks and adopt Jira.
  • Implemented both front-end in React and back-end in Scala and Java.
  • Contributed to bug fixes and various new features.
  • Migrated the whole infrastructure from DigitalOcean to Google Kubernetes Services (GKE). Used Terraform to provision the infrastructure automatically. Used Helm, Helmfile, and Helm Secrets for secure deployment management.
Technologies: MySQL, Ansible, Rollbar, Bitbucket Pipelines, ELK (Elastic Stack), Docker, Apache Maven, React, Java, Scala, Full-stack, Google Kubernetes Engine (GKE), Google Cloud Platform (GCP), Cloud, APIs, Linux, Web Development, DigitalOcean

Lead Senior Software Engineer

2019 - 2020
Go4Ellis LLC
  • Led a team of two developers for this project that connects event operators with athletic trainers for sports events to help in case of any injuries.
  • Upgraded the whole system to the latest Stripe Billing and Stripe Connect API within the given deadline imposed by Stripe.
  • Implemented a HIPAA compliant module for a new feature that would allow users to record and store injury medical records.
  • Applied a CI/CD pipeline for the fast delivery of the Rails app, used Fastlane and CodePush for instant React Native updates.
  • Migrated the whole infrastructure from Heroku to AWS ECS and AWS RDS for Postgres.
  • Integrated and self-hosted Metabase—a full-stack open-source Business Intelligence tool.
  • Improved query times for a variety of API endpoints, fixed N+1 query issues.
  • Took part in the support rotation program, communicated, and fixed users' bugs. Used Sentry for production issues and Zendesk for support tickets.
  • Deployed the back end to Azure Kubernetes Services and built a CI/CD pipeline to automate deployments.
Technologies: Stripe, PostgreSQL, Ruby, Ruby on Rails (RoR), HIPAA Electronic Data Interchange (EDI), Heroku, NGINX, Amazon S3 (AWS S3), Amazon Elastic Container Service (Amazon ECS), React Native, React, Amazon Web Services (AWS), Electronic Medical Records (EMR), Electronic Health Records (EHR), Healthcare, CTO, Architecture, Full-stack, APIs, Linux, Web Development, Ruby on Rails API, Ruby on Rails 5, Stripe API, Azure

Full-stack Software Engineer

2019 - 2019
Testing Psych
  • Built the website from scratch in less than three months. The platform is a testing platform for medical students interested in practicing for their final exam.
  • Used Ruby on Rails 5.2 and explained to the client all of the decisions.
  • Kept the app very lean in terms of deployment (Heroku) and I was able to keep costs as a minimum while not sacrificing performance.
  • Caught production errors through the use of Sentry.
Technologies: Stripe, PostgreSQL, Ruby, Ruby on Rails (RoR), CSS, HTML, Bootstrap, React, Full-stack, APIs, Web Development, Ruby on Rails API, Ruby on Rails 5, Stripe API

Front-end Software Engineer

2019 - 2019
SH Technology
  • Helped this property management early stage startup with their frontend app.
  • Implemented SH Technology's payment flow with Stripe's API.
  • Designed the web application for their different user type: admins, managers, owners, and tenants.
  • Guided the team through best practices on the front end and API work.
Technologies: Stripe, HTML, CSS, JavaScript, React Native, React, APIs, Web Development, Ruby on Rails 5, Stripe API

Software Engineer Intern

2019 - 2019
  • Researched new approaches to build a request predictor - a time-series discrete value model. The problem resembles Intermittent Demand Forecasting. The approaches considered include Croston's method, bootstrapping RNNs, and implementing and testing the performance of different algorithms on previously acquired data in Python Colab.
  • Implemented the method that performed best in our dataset.
  • Proposed different methods to make the project a success proactively.
Technologies: C++, Python, Cloud, Computer Science, Linux

Software Engineer Intern

2018 - 2018
  • Built a fully automated machine learning model training and prediction pipeline. This was the first step in moving from alpha testing where we get the predictions manually by running R scripts, to a beta phase.
  • Ensured the data generated by the pipeline is consumed by an internal dashboard. A strong motivating factor for building an automated pipeline is that the data is always updated and will solve the out-of-sync data problem caused by communication delay between the Engineering, Product and Sales teams.
Technologies: Unix, Machine Learning, R, Protocol Buffers, C++, Cloud, APIs, Computer Science, Linux

Full-stack Developer

2018 - 2018
One Step Software
  • Worked as a full-stack engineer for a sober-living SaaS platform.
  • Built the CI pipeline in order to make sure our service did not introduce any bugs.
  • Collaborated with the team to make our Trello better and advocated for a TDD approach in order to make sure we serve the best software to their clients.
  • Contributed to the Rails web app, introducing new features, implement backlog tasks, and fixing existing bugs.
  • Developed the API for the React Native mobile app and then used that API to build the mobile app.
Technologies: Stripe, PostgreSQL, Ruby, Ruby on Rails (RoR), React Native, Heroku, Full-stack, Linux, Web Development, Ruby on Rails API, Ruby on Rails 5, Stripe API

Back-end Engineer

2017 - 2018
  • Contributed to microservices for interacting with Google Cloud Storage and Google Cloud PubSub.
  • Developed a GoLang gRPC server that interacts with Google Storage.
  • Built a Scala gRPC client that connects to the Go server. The reason for doing this is that Google Storage SDK does not currently support Scala.
  • Wrote unit, integration, and end-to-end tests for the Go and Scala microservices.
  • Dockerized and deployed each service to Google Kubernetes service using internal proprietary deployment tools.
Technologies: MySQL, Ruby, gRPC, Go, Scala, Google Kubernetes Engine (GKE), Google Cloud Platform (GCP), Cloud, Web Development, Ruby on Rails API, Ruby on Rails 5

Full-stack Web Developer

2017 - 2017
NLV Production
  • Contributed to the implementation of new features and fixing bugs for the website.
  • Used Stripe as a payment processor and investigated the problems they were experiencing with customers being charged more.
  • Implemented a new workflow for the admin dashboard that improved the organization's processes.
  • Proactively provided multiple solutions for every problem that the customer was interested in solving.
  • Fixed memory leaks we experienced on the Heroku dyno.
Technologies: Stripe, PostgreSQL, Ruby, Ruby on Rails (RoR), Mailgun, Sidekiq, Heroku, Full-stack, Amazon Web Services (AWS), APIs, Computer Science, Linux, Web Development, Ruby on Rails API, Ruby on Rails 5, Stripe API

Software Engineer Intern

2017 - 2017
  • Interned in the Google Photos Team. The project involved extending the selection framework for auto-created movies based on user content and implementing a few new concept movies.
  • Created the concept "child friendly" movie. The basic idea is that if we detect that a lot of pictures contain kids, toddlers, or children, we select music that is suitable for a child movie.
  • Designed a second concept movie called 'Outdoor Movie' and it basically tries to match outdoor pictures with suitable outdoor songs.
  • Conceptualized a third movie called 'Highlight Reel' and the main idea is that we want to crop the user's videos so that we retain only some 'action moments'. An action moment is a 5s clip in the movie where we detect some action (i.e. throwing a basketball or shooting a baseball).
Technologies: Protocol Buffers, Java 8

Mobile Developer

2016 - 2017
  • Contributed to the team that develops the Kuende mobile application (available to download on AppStore and Play Store), a social network based on gamification.
  • Implemented a Trie based algorithm for searching people, a wrapper around a list-view so you can easily declare, add filters and comparators to lists, and other small features.
  • Optimized the way we persist our data so that the information is fast accessible and rendered on the phone.
  • Concepted and implemented the Flux architecture on top of C++ and Qt.
Technologies: iOS, Android, Flux, QML, Qt, C++, JavaScript, React Native, APIs, Web Development


2016 - 2016
  • Taught computer science fundamentals to high-school students through the non-profit organization (
  • Presented advanced algorithms and data structures to students interested in the National Olympiad in Informatics.
  • You can find some of the problems that I solved on various competitive programming challenges can be found here:
Technologies: Algorithms

Software Engineer Intern

2016 - 2016
  • Interned in the Robot Detection Research Team at Amazon Development Center Iasi.
  • Created a linkage and score system for robotic attributes. The main challenge was the huge amount of nodes and data that had to be processed every day.
  • Contributed to mitigation of all of the robotic activity that crawls our website, with two things in mind. First, we didn't want to block a real user! Second, we wanted to let some friendly bots crawl the website (eg GoogleBot, BingBot).
  • Designed a system that was able to give a score for each incoming request on the website. Then, based on that score we can decide if the current request was made by a human or a robot. For this approach, we split the request into simple attributes such as user agent, IP, customer id and we independently compute the scores for each such attributes. In the end, we can average the scores to get the request score.
  • Computed the scores as a series of Hadoop Map Reduce jobs that ran on Amazon's Hadoop Clusters and infrastructure. I had a lot of freedom with the project and I learned a lot while designing such a scalable, distributed algorithm.
Technologies: Node.js, JavaScript, Machine Learning, MapReduce, Hadoop, Java, Amazon Web Services (AWS), Web Development

Android Developer

2013 - 2013
  • Built the e-communio app based on the already built local church news website.
  • Understood the main concepts of Android OS and the process of publishing an app.
  • Created the app as a very simple, and at the core, just a list view, containing the latest news that I parsed from a web service written in C# that connects to the database, gathers the news, and splits a JSON array.
  • On the mobile side, I created a request for the news, parsed them and displayed them. When the user clicked one of the items, it will go to another screen where they could read the entire content of the article. There were other nice features such as increasing and decreasing the size of the text, sharing the article, and additional text effects.
Technologies: Android

Data Analysis Project on DST Impact on Users
I've worked on a data analysis project to understand the impact of daylight saving time impact on web usage around the world.

Road Segmentation

Worked on building a classifier to segment roads in satellite images.


Facebook Messenger chatbot that uses the Facebook Messenger codes.

Scan a QR code in messenger and get different targeting ads/promotions.

API Integration

I have worked for NXT Spots Inc to integrate two of their services they were using: LeagueApps for their payment processor and Copper for their CMS.

Med Prep

I've built an online exam for medicine students web application for this client. The stack I used was Ruby on Rails, PostgreSQL, React.js, ActiveAdmin, and Stripe as a payment processor.

The admin dashboard shows aggregated transactions data.

Watersheds Data Visualization
I've created a visualization tool for people at Stanford to gain more insights into how different experiments affect watershed metrics.

Technologies: D3.js, jQuery, HTML, CSS, Bulma

Babes-Bolyai University of Cluj-Napoca Courses

All my project assignments at university.

The projects there range from web applications to machine learning, parallel and distributed programming, or even math courses like statistics or linear algebra.

I've built a Google Chrome extension that makes it easy to quickly convert date times intent in Gmail and Inbox by Gmail.
2018 - 2020

Master's Degree in Data Science

EPFL | École Polytechnique Fédérale de Lausanne - Lausanne, Switzerland

2015 - 2018

Bachelor's Degree in Computer Science

Babes-Bolyai University - Cluj-Napoca


React, Ruby on Rails API, jQuery, Stripe, Stripe API, Stripe Connect API, Stripe Connect, D3.js, Facebook API, Facebook Messenger API, NumPy, Pandas, PySpark, Scikit-learn, Braintree API, Node.js, Sidekiq, Rollbar, Vue, TensorFlow, Matplotlib, SciPy


Stripe Checkout, GitHub, Amazon Elastic Container Service (Amazon ECS), NGINX, AWS Key Management Service (KMS), Google Kubernetes Engine (GKE), Trello, IntelliJ IDEA, Jekyll, Git, Apache Maven, ELK (Elastic Stack), Ansible, Scikit-image


React Native, Ruby on Rails (RoR), Ruby on Rails 5, Lift, ASP.NET, gRPC, Qt, Spark, Flux, Bootstrap, Django, Flask, Hadoop, Phoenix, Tailwind CSS


Ruby, C++, Java 8, JavaScript, Python, CSS, HTML, PHP, Scala, SQL, Java, QML, R, Go, Bash, GraphQL, Hugo, Elixir


ETL, HIPAA Compliance, DevOps, Agile Software Development, Scrum, Data Science, MapReduce


Docker, Linux, Kubernetes, Google Cloud Platform (GCP), Jupyter Notebook, MacOS, Azure, Android, iOS, Heroku, Unix, Mailgun, Amazon Web Services (AWS), DigitalOcean

Industry Expertise



PostgreSQL, MySQL, Amazon S3 (AWS S3), Google Cloud


Data Engineering, Architecture, Full-stack, Facebook, Web Development, Stripe Subscriptions, Algorithms, Data Structures, Security, HIPAA Electronic Data Interchange (EDI), Chrome Extensions, Cryptography, Applied Cryptography, CTO, Electronic Health Records (EHR), Electronic Medical Records (EMR), APIs, Chatbots, BrainTree, Machine Learning, Protocol Buffers, Bitbucket Pipelines, Computer Science, Cloud, Leadership, Statistics, Data Analytics, Classifier Development

Collaboration That Works

How to Work with Toptal

Toptal matches you directly with global industry experts from our network in hours—not weeks or months.


Share your needs

Discuss your requirements and refine your scope in a call with a Toptal domain expert.

Choose your talent

Get a short list of expertly matched talent within 24 hours to review, interview, and choose from.

Start your risk-free talent trial

Work with your chosen talent on a trial basis for up to two weeks. Pay only if you decide to hire them.

Top talent is in high demand.

Start hiring