Konstantinos Karachalios, Developer in Barcelona, Spain
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Konstantinos Karachalios

Verified Expert  in Engineering

Software Developer

Barcelona, Spain
Toptal Member Since
May 8, 2017

Konstantinos is a programmer, but he doesn't just work as one—he's been coding since he was 12 on BASIC. He dropped out of a filmmaking university to return to programming. Since then, he's never stopped exploring different ways to code and for stuff to code. Konstantinos has lived around Europe and California while working on many web services, designing ETL pipelines, and leading a cross-functional team, all of which have allowed him to develop a great sense of adaptability.


Ruby on Rails (RoR), Git, Docker, Amazon Web Services (AWS), MySQL, Redis...
a Startup Company
JavaScript, Node.js, PostgreSQL, PostGraphile, GraphQL, WebRTC, pgTAP, Git...
Ruby, Ruby on Rails (RoR), Scala, React, Jira, Git, Linux, Sidekiq...




Preferred Environment

Trello, Git, Linux

The most amazing...

...work I've done was to rewrite a part of a Perl monolith in Rails and deploy it with zero downtime despite the millions of requests it served each day.

Work Experience

Ruby on Rails Engineer

2021 - PRESENT
  • Upgraded a Rails 3 app to Rails 7, with no impact on other team members' work.
  • Introduced integration testing via Selenium and championed a TDD ethos.
  • Mentored a fellow engineer on AWS, especially in relation to ECS and Docker.
  • Led the team by example through an Agile transformation.
Technologies: Ruby on Rails (RoR), Git, Docker, Amazon Web Services (AWS), MySQL, Redis, Sidekiq, Selenium, Unit Testing, Integration Testing, Agile, Linux, Jira, Web Development, Ruby, Full-stack, Test-driven Development (TDD), Scrum, JavaScript, Project Management, Code Review, Software Architecture, Object-oriented Programming (OOP), Databases, APIs

Co-founder and Back-end Engineer

2020 - 2022
a Startup Company
  • Developed a voice-first communication desktop app from scratch, using WebRTC and other web standard technologies.
  • Organized and prioritized the work needed to achieve an MVP with Trello and GitHub projects.
  • Architected the deployment on Linode virtual machines, emphasizing security and performance.
Technologies: JavaScript, Node.js, PostgreSQL, PostGraphile, GraphQL, WebRTC, pgTAP, Git, Trello, GitHub, Linode, Mediasoup, Docker, Tauri, Minimum Viable Product (MVP), Software Architecture, Linux, Databases

Freelance Senior Software Engineer

2021 - 2021
  • Designed a secure and performant round-trip computation of accounting corrections and adjustments between modern and legacy systems.
  • Mentored a junior colleague in the mailing system of Ruby on Rails and background jobs processing and batching.
  • Integrated a CI tool via its API with a code editor used by various colleagues to provide more direct visibility into the company's work progress.
Technologies: Ruby, Ruby on Rails (RoR), Scala, React, Jira, Git, Linux, Sidekiq, Apache Kafka, Protobuf, CircleCI, Web Development, Full-stack, Test-driven Development (TDD), Microservices, Docker, Code Review, RESTful Microservices, Object-oriented Programming (OOP), Databases

Data Engineer

2018 - 2020
  • Designed and implemented two high-traffic ETL projects.
  • Facilitated the upgrade, migration, and refactoring of numerous ETL pipeline jobs.
  • Interviewed dozens of candidates for various teams and skillsets.
  • Introduced a valuable vendor to the company, who ended up being the linchpin of the company's future technology strategy.
Technologies: Amazon Web Services (AWS), Databricks, MySQL, Ruby on Rails (RoR), Ruby, Delta Lake, Apache Hive, Apache Spark, Git, Test-driven Development (TDD), Redis, Agile, Microservices, Project Management, Code Review, Minimum Viable Product (MVP), Software Architecture, RESTful Microservices, Sidekiq, Linux, Databases, APIs

Staff Engineer

2017 - 2018
  • Designed and developed the digital banking solutions on a microservices architecture.
  • Interviewed potential hires to expand the team to about double the size.
  • Aided in the transformation towards Agile development.
  • Promoted to staff engineer for the latter half of my job.
Technologies: GoCD, Git, RESTful Development, Ruby, Agile, Web Development, Test-driven Development (TDD), Scrum, Microservices, Docker, REST APIs, Sinatra, Project Management, Code Review, Minimum Viable Product (MVP), Software Architecture, RESTful Microservices, Object-oriented Programming (OOP), Sidekiq, Linux, Databases, APIs

Senior Consultant

2016 - 2017
Tesco (via ThoughtWorks)
  • Built a robust solution that integrated with many other services as part of a client onboarding flow.
  • Developed and maintained a flexible deployment platform built on AWS.
  • Mentored colleagues on our tech stack and web technologies in general.
Technologies: Amazon Web Services (AWS), Jenkins, Redux, React, JavaScript, Node.js, Agile, Scrum, Web Development, Full-stack, Git, Test-driven Development (TDD), Microservices, Docker, REST APIs, Code Review, Minimum Viable Product (MVP), RESTful Microservices, Object-oriented Programming (OOP), Linux, APIs

Senior Software Engineer

2015 - 2016
  • Migrated a Rails app from Heroku to AWS.
  • Took responsibility of the application's security and data privacy.
Technologies: Amazon Web Services (AWS), Heroku, Redis, PostgreSQL, Angular, Ruby on Rails (RoR), Ruby, Web Development, Full-stack, Git, Agile, Microservices, Code Review, Minimum Viable Product (MVP), RESTful Microservices, Object-oriented Programming (OOP), Linux, Databases, APIs

Software Engineer

2013 - 2015
  • Developed a recruiting tool with the back-end code written in Ruby and Rails and the front-end code in JavaScript and Backbone and later rewrote it in CoffeeScript and Marionette.
  • Migrated one of the most traffic-intensive parts of the platform to Rails and its data to a new schema with zero downtime.
Technologies: MySQL, Riak, AMQP, Backbone.js, Ruby on Rails (RoR), Web Development, Ruby, Full-stack, Git, Scrum, Agile, Microservices, REST APIs, Code Review, Software Architecture, Object-oriented Programming (OOP), Linux, Databases, APIs

Web Developer

2008 - 2012
  • Rewrote a gaming news website from PHP to Rails.
  • Developed a simple bug-tracking tool (Bugja).
  • Took responsibility of the back-office application.
  • Developed a simple visual programming language for the moderators to automate part of their tasks.
Technologies: Elasticsearch, Sphinx Search Engine, Varnish, Memcached, MongoDB, MySQL, Sass, jQuery, Ruby on Rails (RoR), Ruby, Web Development, Full-stack, Git, Redis, Agile, JavaScript, Object-oriented Programming (OOP), Linux, Databases, Sphinx

Racer Maker (Working Title)

A video game where the player is teaching a bot to race. It is still in very early stages of development and has served as a test bed for experimenting with machine learning.
2018 - 2018

Specialization in Deep Learning

Deeplearning.ai | Coursera - Online

2015 - 2015

Course Certificate in Machine Learning

Stanford University | Coursera - Online

2011 - 2011

Statement of Accomplishment in Introduction to AI

Stanford University | Coursera - Onlnine


Sidekiq, Node.js, TensorFlow, Keras, Backbone.js, jQuery, Protobuf, React, AMQP, REST APIs, WebRTC


Git, Jira, Trello, Varnish, Jenkins, CircleCI, Amazon Elastic Container Registry (ECR), AWS CloudFormation, Amazon Simple Email Service (SES), GitHub


Ruby on Rails (RoR), Apache Spark, Sinatra, Angular, Redux, Selenium, pgTAP, Tauri


Ruby, Scala, Python 3, Sass, JavaScript, Clojure, GraphQL

Industry Expertise

Project Management


Microservices, REST, Scrum, Object-oriented Programming (OOP), Test-driven Development (TDD), Agile, RESTful Development, Unit Testing


Docker, Jupyter Notebook, Databricks, MacOS, Heroku, Amazon Web Services (AWS), Apache Kafka, Linux, Linode


MySQL, Redis, Databases, PostgreSQL, Elasticsearch, Riak, Memcached, Sphinx Search Engine, Apache Hive, MongoDB, Amazon S3 (AWS S3)


Web Development, Full-stack, Software Architecture, RESTful Microservices, Minimum Viable Product (MVP), Code Review, APIs, Deep Learning, Delta Lake, Cloud, GoCD, Machine Learning, NixOS, Integration Testing, PostGraphile, Mediasoup, Sphinx

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