Max Hollmann, Developer in Hamburg, Germany
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Max Hollmann

Verified Expert  in Engineering

Machine Learning Developer

Hamburg, Germany
Toptal Member Since
March 4, 2016

Max is a seasoned full-stack developer with a background in data science. With extensive experience in Ruby on Rails, he has spent the last few years primarily working in Python-based environments. Max collaborated with large teams on back-end solutions for sizable software systems and designed and implemented complete tech stacks for early-stage startups from the ground up. His diverse skill set makes him an incredibly versatile engineer.


BlueSky API
Python, FastAPI, Zarr, Kubernetes, GitLab CI/CD, Argo CD, Terraform...
Sum.cumo Sapiens GmbH
Ruby on Rails (RoR), Kubernetes, Docker, GraphQL, Dry-rb, PostgreSQL...
Python, Machine Learning, Chef, Sidekiq, Ruby on Rails (RoR), Ruby...




Preferred Environment

Git, Emacs, Unix

The most amazing...

...web app I've built uses NLP and machine learning to find out who got interviewed on a podcast episode—building a people-centered podcast directory.

Work Experience


2020 - PRESENT
BlueSky API
  • Built a high-performance public API using FastAPI, streamlining access for users to historical weather forecast data.
  • Designed and implemented the ETL process to aggregate weather data from various sources into our Zarr-based format.
  • Developed a 2-tiered storage system, employing write-optimized chunking for data ingestion and read-optimized chunking for data retrieval.
  • Automated tests, Docker builds, infrastructure, and deployments using GitLab CI, Kubernetes, and Argo CD for a fast development cycle.
  • Designed and implemented the project's website using Vue.js with Nuxt.js.
Technologies: Python, FastAPI, Zarr, Kubernetes, GitLab CI/CD, Argo CD, Terraform, Data Engineering, Docker, Vue, Software Design, Architecture, Full-stack, SaaS, Leadership, Machine Learning, Git, Python 3, NumPy, Jupyter, Jupyter Notebook, Pandas, OpenAPI, APIs, Flask, Hosting, Geospatial Data

Senior Ruby on Rails Developer

2019 - 2022
Sum.cumo Sapiens GmbH
  • Led implementation of a decentralized scheme to handle ownership of discount classes between car insurances.
  • Saved weeks of developer time by developing a framework to generate test data for the above semantically.
  • Designed and implemented a multichannel customer communication framework.
Technologies: Ruby on Rails (RoR), Kubernetes, Docker, GraphQL, Dry-rb, PostgreSQL, GitLab CI/CD, Software Design, Architecture, SaaS, Leadership, Git, MySQL, APIs, Caching, Ruby

Lead Developer

2015 - 2019
  • Developed a podcast directory and scaled it to 2.5 million episodes and 800,000 interviewees.
  • Employed an NLP model to extract names of people appearing on the podcasts and various machine learning models to categorize the type of appearance.
  • Implemented a system to process new data from thousands of podcasts in parallel using Sidekiq.
  • Set up an Elasticsearch server to support fast and powerful searches.
  • Managed the server infrastructure on DigitalOcean using Chef.
Technologies: Python, Machine Learning, Chef, Sidekiq, Ruby on Rails (RoR), Ruby, Elasticsearch, Luigi, Natural Language Processing (NLP), Data Science, Deep Learning, Software Design, Architecture, Full-stack, SaaS, Git, Python 3, NumPy, Jupyter, Jupyter Notebook, Pandas, APIs, Caching, Hosting


2015 - 2016
  • Improved the quality of the existing code base and database design, and established sustainable design patterns for future development.
  • Implemented a complete redesign of the front-end with better responsiveness and all-around user experience.
  • Created a report system for the outcomes of online votings and the demographics of the voters, both as HTML and as CSV.
Technologies: Heroku, PostgreSQL, Ruby on Rails (RoR), Ruby, Architecture, Full-stack, SaaS, Git

Lead Developer

2013 - 2016
  • Provided technical guidance from the startup's idea to its first big clients.
  • Implemented the back-end and API of a complex CMS/CRM solution.
  • Set up a reminder service using push notifications, interfacing directly with the Apple Push Notification Service and Google Cloud Messaging.
  • Coordinated the development of the iOS and Android apps.
  • Interviewed and screened mobile app developers.
Technologies: Push Notifications, Heroku, Memcached, PostgreSQL, Ruby on Rails (RoR), Ruby, Software Design, Architecture, Full-stack, SaaS, Leadership, Git, APIs, Caching, Hosting

Web Developer

2011 - 2011
Empuxa GmbH
  • Implemented a platform giving patients the chance to rate their care providers on various criteria.
  • Implemented a Facebook app for a customer support software to add a channel of communication between companies and their clients.
Technologies: Facebook, Amazon Web Services (AWS), MongoDB, Ruby on Rails (RoR), Ruby, Full-stack, SaaS, Git, APIs, Hosting


2009 - 2009
  • Developed a tool to visualize the accuracy of the company’s 3D reconstruction software.
  • Implemented a library to convert MPEG4 videos into single frames.
Technologies: Video Processing, Image Processing, FFmpeg, C++

Autonomous GPT Software Engineer
An experiment to make GPT-4 work autonomously on software projects by giving it access to the shell, the ability to edit files, giving it a task, and putting it in a continuous loop. It turned out that GPT-4 is still not good enough to be useful for this because prompting it and fixing its mistakes takes more time than doing it yourself.

A web application to convert YouTube videos into podcast episodes that I built from scratch and used as an opportunity to play around with Dry-rb. The back end runs on Ruby on Rails with a GraphQL API, and the front end is a Vue application with live progress updates via WebSockets, i.e., Action Cable.

A side project to scratch my own itch, provides an interface to YouTube that is suitable for parties—search for songs, queue them in the playlist, and play them all from one page. Given that design is not my area of expertise, I'm quite happy with how the interface turned out.

A joint initiative with a friend who was backpacking in Australia. With the aim of mitigating the widespread problem of employers taking advantage of backpackers, we developed a platform where travelers can share their experience with previous employers. The website is slowly gaining steam by word of mouth and being consulted by more and more travelers looking for short-term work every day.

Ruby Gem: Rails API Explorer
A plugin for Ruby on Rails that lets developers easily create API documentation, including an interactive interface to test and play around with the API.
2016 - 2018

Master of Science Degree in Statistics and Data Science

Leiden University - Leiden, Netherlands

2011 - 2014

Bachelor's Degree in Psychology

University of Groningen - Groningen, Netherlands


REST APIs, Sidekiq, jQuery, ActiveRecord, Ruby on Rails API, Zarr, Node.js, Keras, TensorFlow, Luigi, Vue, NumPy, Pandas, Google Maps, Twitter API, Facebook Open Graph API, FFmpeg, Ractive.js, D3.js, React, SciPy, OpenAPI


Git, Emacs, RSpec, Jupyter, GitHub, TensorBoard, GitLab CI/CD, Apple Push Notifications, Google Cloud Messaging, NGINX, MATLAB, Chef, Terraform


Ruby on Rails (RoR), Slim, Capybara, Bootstrap 3, Bootstrap, Vuetify, Flask, ActionCable


Bash, R, Python, Ruby, CoffeeScript, JavaScript, Sass, Haml, HTML5, CSS, HTML, Python 3, ECMAScript (ES6), SQL, C++, CSS3, Java, GraphQL


Agile Software Development, Test-driven Development (TDD), Data Science, Object-oriented Design (OOD), Plain Old Ruby Object (PORO), DevOps, Hosting


Linux, Unix, Heroku, Docker, Jupyter Notebook, Kubernetes, Amazon Web Services (AWS), DigitalOcean


JSON, PostgreSQL, Databases, MySQL, Memcached, Redis, MongoDB, Elasticsearch


Machine Learning, FastAPI, Software Design, Architecture, Full-stack, SaaS, APIs, Statistics, Deep Learning, OpenAI GPT-4 API, Data Engineering, Leadership, Caching, Geospatial Data, Image Processing, Video Processing, Push Notifications, Facebook, Natural Language Processing (NLP), WebSockets, Dry-rb, Artificial Intelligence (AI), Generative Pre-trained Transformers (GPT), Argo CD, OpenAI GPT-3 API, Bayesian Statistics, Science, Psychology, Psychology & Mental Health, Cognitive Psychology

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