Pantelis Vratsalis, Developer in Sparti, Laconia, Greece
Pantelis is available for hire
Hire Pantelis

Pantelis Vratsalis

Verified Expert  in Engineering

FastAPI Developer

Sparti, Laconia, Greece
Toptal Member Since
January 8, 2014

Pantelis is a full-stack engineer with experience building complex products using a wide array of technologies (Python, Ruby, JavaScript, Java, Go) and tools (Flask, FastAPI, Vert.x, React, Vue, Ruby on Rails, and SQL and NoSQL databases). Apart from coding, Pantelis has worked as a team lead and engineering manager in the last few years—building, growing, and managing teams and individual contributors. His work includes hiring, performance management, mentoring, and overseeing team projects.


Python, PostgreSQL, Flask, FastAPI, RabbitMQ, Kubernetes, React
Go, PostgreSQL, React
Prometheus, Vue, React, Ruby on Rails (RoR)




Preferred Environment

Ruby, Python, Git, Linux, Kubernetes, React, Vue, Aerospike, PostgreSQL, MySQL

The most amazing...

...thing I've coded is a retargeting DSP for programmatic advertising. It was a great challenge due to the complexity and traffic scale (150,000 reqs/second).

Work Experience

Senior Software Engineer and Team Lead

2020 - PRESENT
  • Bootstrapped a new team and set the direction (North Star) for the company's security and authentication/authorization.
  • Built microservices for authentication and authorization (as a platform team) to enable other services to be extracted from the monolith.
  • Set up security best practices and automated tests in SDLC.
Technologies: Python, PostgreSQL, Flask, FastAPI, RabbitMQ, Kubernetes, React

Engineering Lead

2022 - 2023
  • Led a platform (web/desktop) team and created systems to gather customers' and internal stakeholders' requirements in a structured way, focusing on the highest impact work at any given point.
  • Oversaw the professional growth of all team members. Led several engineers to promotion through personal goal setting and coaching in a short period.
  • Migrated our codebase from JavaScript to TypeScript.
Technologies: Go, PostgreSQL, React

Tech Lead

2019 - 2020
  • Co-founded and tech-led AskBea, a Shopify partner that builds apps to help store owners grow their sales and better manage their stores.
  • Handled the first Shopify app, a post-purchase and abandoned cart NPS survey scheduling and reporting software.
  • Used machine learning to build the recommendation algorithm for the second app, a product recommendation engine.
Technologies: Prometheus, Vue, React, Ruby on Rails (RoR)

Senior Engineer and Tech Lead

2017 - 2019
  • Worked on retargeting dynamic audience segmentation and personalized ads using Java, Scala, Vert.x, Kafka, Aerospike, and big data for analytics and reporting.
  • Built APIs that power UIs for reporting and campaign management purposes using Node.js and GraphQL.
  • Deployed and scheduled web applications using Docker, Nomad, Consul, and Kubernetes.
Technologies: GraphQL, Node.js, Kubernetes,, Apache Kafka, Vert.x, Java

Tech Lead

2016 - 2017
  • Co-founded and led technical projects. Used technologies including Ruby on Rails for the back-end APIs, React for the front end, PostgreSQL, Redis, and Roda for the landing pages and marketing website.
  • Integrated multiple external services for things like email, SMS, geolocation (e.g., Twilio, Nexmo, Sendgrid, Mailgun).
Technologies: PostgreSQL, React, Ruby on Rails (RoR)

Senior Software Engineer

2015 - 2016
  • Worked with a team of six developers to build the dashboard and front pages of booking experience (tour guides and more).
  • Built a single-page application with React and Ruby on Rails on the back end.
Technologies: PostgreSQL, React, Ruby on Rails (RoR)

Full-stack Web Developer

2014 - 2015
  • Owned most of the features of the website (API development and front-end AngularJS work).
  • Collaborated with a small team of three reporting directly to the CEO.
  • Contributed to the development of the back end (PHP Yii framework, MongoDB) and front end (Angular 1.4).
Technologies: Ansible, AngularJS, PHP

Web Developer and Team Lead

2013 - 2014
  • Contracted for a Swiss online learning platform for sales people. Developed the platform in Python with the Pyramid framework.
  • Developed for a social wifi startup. Built several aspects of the application visible to visitors and the control panel for the clients (wifi network owners).
  • Led a team with Stratagem Technologies, Ltd. Technologies used include Python, Node.js, AngularJS MongoDB, and Redis. Used big data from the sports betting industry for statistical analyses and other purposes. Managed a distributed team of six developers. Built and maintained REST APIs and API consumers using Python.
Technologies: Pytest, Python

Online Market Research Project

Currently, the website has suspended its functions. This website was online for three years (beginning 2011 till the end of 2013) for an online market research firm, but it has currently suspended its functions. It had a panel to recruit online members (around 4,000 members). I created this as a startup venture and developed both the front and back end (admin interface and user interface). I also handled marketing efforts (SEO and SEM). The web application was developed with PHP and CodeIgniter, and the data was stored using MySQL.

Online Polls Website

This project enables the creation of web polls and subsequent embedding of them on a website. It is also capable of obtaining the URL/link/QR code of the poll to share it and get responses. The project was developed with PHP and the Kohana framework and uses MySQL to store data.

Eduno is an online learning platform for salespeople based in Switzerland. It allows companies and individuals to educate themselves in sales-related subjects with online videos and interactive quizzes. It also has a recommendation engine for marketing purposes. I'm working on the development team of this ongoing project with six remote individual contractors. Development is in Python with the Pyramid framework. For storage, we used the ZODB NoSQL database.

Collection Feature for Freelancer Platform

I developed the collections feature for an online platform for freelance developers, displaying curated collections of freelancer services. It includes an administrative interface where services are grouped into collections. Freelancers can request to be included in one or more collections.

Quovoo Quotes API

Famous quotes API, available through Rapid API marketplace. Technologies used: Fastify (Node.js) for the back end with a PostgreSQL database.

AskBeat Shopify App

AskBeat Easy Email NPS Surveys is a Shopify app for sending NPS surveys to store customers. The surveys are sent via email either manually (uploading a list of customers) or automatically every time an order is placed (integrating with Shopify's API). Technologies used include Ruby on Rails, Vue.js, and Bulma.

Vegetarian Keto Recipes Android App (Recipeno)

A native Android mobile app for vegetarian keto recipes. Built with Java and communicates with an API built with Ruby on Rails for fetching data. is a local theater performance search engine. Visitors can easily search among theaters and performances, using multiple criteria (e.g., genre, day of the week), to find performances that interest them.

Technologies used: Ruby on rails 5, jQuery and vanilla JavaScript, SQLite 3, Sass, and Webpack for asset bundling.

Integrations were made with Mailgun's API and with various affiliate APIs.


Python, Ruby, Java, GraphQL, JavaScript, Go


Ruby on Rails (RoR), Flask, Vert.x


React, Vue, Node.js


Test-driven Development (TDD), Unit Testing, REST


Linux, Apache Kafka, Kubernetes


PostgreSQL, Aerospike, MySQL,, MongoDB


Pytest, RabbitMQ


Physics, FastAPI, Prometheus

2003 - 2003

Master’s Degree (Exchange Program) in Business Administration (MBA)

Rotterdam School of Management - Rotterdam, Netherlands

2001 - 2003

Master's Degree in Business Administration (MBA)

Athens University of Economics and Business - Athens, Greece

1997 - 2001

Bachelor of Science Degree in Physics

University of Crete - Heraklion, Greece