Kostas is available for hire
Hire KostasKostas Familonidis
Verified Expert in Engineering
Ruby Developer
Athens, Central Athens, Greece
Toptal member since June 18, 2014
Bio
Kostas is a seasoned full-stack engineer with 10+ years of experience developing web applications using Ruby, JavaScript, and Node.js in multiple sectors. He recently focused on SaaS integrations, internationalization projects, and cross-platform mobile application development. He is experienced with various projects and platforms, from business process automation to finance and beyond. Kostas has project management experience in the software industry and can drive teams to success.
Portfolio
Red Hat
JavaScript, React, Agile, Test Management, Project Management, HTML5...
LGMar
Agile, JavaScript, Artificial Intelligence (AI), Project Management...
FieldPulse
Ruby, Requirements Analysis, Project Timelines, Project Planning...
Experience
- Ruby - 13 years
- Ruby on Rails (RoR) - 13 years
- RSpec - 11 years
- Full-stack - 10 years
- JavaScript - 10 years
- React - 9 years
- Node.js - 8 years
- D3.js - 2 years
Preferred Environment
Unix, Linux, Middleware
The most amazing...
...thing I've developed is a CRM that integrates mail services in mobile devices.
Work Experience
Senior Software Engineer
2025 - 2026
Red Hat
- Modernized the front end using React, Redux, PatternFly, and Ruby on Rails.
- Collaborated with a global team to reduce technical debt in the upstream Foreman and Katello projects.
- Engaged with the open-source community by reviewing contributions and demonstrating new features.
- Partnered with Quality Engineering to ensure high product reliability and standards.
Technologies: JavaScript, React, Agile, Test Management, Project Management, HTML5, Front-end Development, User Interface (UI), Claude Code, System Integration
Senior Software Engineer
2024 - 2025
LGMar
- Developed Ports Dashboard, an interactive tool for monitoring and managing port-related data, including advanced filtering, data presentation, and integration with external data sources.
- Built a robust Orders System with full back-end API support, enabling creation, search, audit logging, and management of customer orders.
- Designed and implemented an AI Assistant prototype enabling natural language interaction to assist personnel and streamline internal workflows.
- Delivered a scalable Fleet Management Interface supporting data overview, reporting workflows, and system extensibility for fleet operations.
- Created a high-performance Quick Search Interface with dynamic filtering and auto-complete, integrated across modules (orders, customers, fleet) to enable real-time access to large datasets.
- Ensured all deliverables met technical and functional specifications, accompanied by documentation and post-delivery support.
Technologies: Agile, JavaScript, Artificial Intelligence (AI), Project Management, Large Language Models (LLMs), HTML5, Front-end Development, User Experience (UX), User Interface (UI), Python, AI Agents, System Integration, Automation
Ruby Developer
2024 - 2024
FieldPulse
- Optimized the existing codebase based on metrics analysis using Ruby on Rails.
- Improved feature development that relies on real-time user interactions with the application servers.
- Participated in the planning of new features and optimization tasks.
Technologies: Ruby, Requirements Analysis, Project Timelines, Project Planning, Discovery Workshops, Code Review, Agile Sprints, Software as a Service (SaaS), Module Integrations, Middleware, System Integration, Automation
Full-stack Web Developer
2021 - 2023
Getaway House
- Developed the application stack for enabling multiple payment processors Stripe, and Braintree, using Ruby, Ruby on Rails (RoR), JavaScript, and React.
- Improved code quality to support message queuing under the CQRS pattern.
- Designed robust services based on task-based UI (user interface) analysis.
- Integrated 3rd-party platforms API such as Twillio and Zendesk.
- Provided accurate estimates with scope analysis and management to provide state-of-the-art delivery standards.
- Designed and implemented custom solutions for improving the customer UX (user experience), such as waitlists and a loyalty program.
- Implemented TDD (test-driven development) and extensive acceptance-test coverage in line with Agile development practices.
Technologies: Ruby, JavaScript, API Integration, Full-stack, Ruby on Rails (RoR), HTML, Architecture, Back-end Development, Customer Relationship Management (CRM), Back-end, Zendesk, Stripe API, Auditing, Web Development, Node.js, JSON, Deployment, RSpec, Project Management, React, Axios, Full-stack Development, Stripe, BrainTree, System Architecture, Agile, Twilio, Heroku, CI/CD Pipelines, Front-end, Bootstrap, Booking, Requirements Analysis, Project Timelines, Project Planning, Discovery Workshops, Code Review, Stripe Payments, Agile Sprints, Amazon Web Services (AWS), Redux Toolkit, Technical Writing, Documentation, Technical Documentation, Payment APIs, Software Architecture, Software as a Service (SaaS), Workflow, CRM, Enterprise Resource Planning (ERP), Module Integrations, Test Management, Middleware, HTML5, Front-end Development, User Experience (UX), User Interface (UI), System Integration, Automation
Full-stack Web Developer
2019 - 2021
Procore
- Developed the application's stack to support internationalization across the platform.
- Modernized existing parts of the codebase using the latest Ruby and JavaScript patterns, RSpec, and UI (user interface) functional testing.
- Performed auditing and regression tests analysis on tools enforcing internationalization and localization standards.
- Reviewed code and supported the team's internal QA (quality assurance) process.
- Led the engineering team to document and apply code best practices across the organization.
- Implemented TDD (test-driven development) and extensive acceptance-test coverage in line with Agile development practices.
Technologies: RSpec, Functional Programming, Unix, Object-oriented Programming (OOP), HTML, Git, Redis, JavaScript, Ruby on Rails (RoR), Ruby, React, Webhooks, Full-stack, Node.js, APIs, SaaS, Code Architecture, REST APIs, API Integration, Architecture, Back-end Development, Back-end, TypeScript, Auditing, Stripe, Storybook, Jest, Web Development, JSON, Cypress, Deployment, Project Management, Axios, Full-stack Development, Express.js, Agile, Front-end, Requirements Analysis, Project Timelines, Project Planning, Discovery Workshops, Code Review, Management, Agile Sprints, Redux Toolkit, Technical Writing, Documentation, Technical Documentation, Software Architecture, Software as a Service (SaaS), Workflow, Enterprise Resource Planning (ERP), Test Management, Middleware, HTML5, Front-end Development, User Interface (UI), Automation
React Developer
2016 - 2018
Vertical Scope, Inc.
- Developed cross-platform native mobile applications in iOS and Android using JavaScript and React Native.
- Developed and integrated React components into a forum website using Redux architecture.
- Improved UX (user experience) and UI (user interface) performance on mobile applications.
- Reviewed code and supported the team's internal QA (quality assurance) process.
- Implemented TDD (test driven development) with Agile development practices.
- Provided accurate time estimates with delivery with the team.
Technologies: Functional Programming, Unix, HTML, CSS, JavaScript, React Native, React, Webhooks, Node.js, Project Management, APIs, Code Architecture, Redux, Redux-Saga, REST APIs, Architecture, Material UI, Web Development, JSON, Deployment, Axios, Full-stack Development, Express.js, Agile, Android, iOS, Front-end, Requirements Analysis, Project Timelines, Code Review, Agile Sprints, Redux Toolkit, Documentation, Software Architecture, Software as a Service (SaaS), HTML5, Front-end Development, User Experience (UX), User Interface (UI)
Full-stack Web Developer
2017 - 2017
Pivot.io, Inc.
- Developed an application for automating business processes in Ruby, Ruby on Rails, JavaScript, and GraphQL.
- Developed and integrated React components using Redux and sagas for concurrent data structures.
- Transitioned REST (representational state transfer) APIs and micro-services in GraphQL implementations.
- Provided scope analysis and execution plan in line with industry best practices.
Technologies: Unix, Object-oriented Programming (OOP), Git, GraphQL, React, Ruby on Rails (RoR), Ruby, Webhooks, Full-stack, Node.js, APIs, SaaS, Code Architecture, Redux, Redux-Saga, Architecture, Back-end Development, Back-end, Web Development, Deployment, Full-stack Development, System Architecture, Agile, Project Planning, Code Review, Software Architecture, Software as a Service (SaaS), Enterprise Resource Planning (ERP), Test Management, Middleware, Automation
Software Engineer
2015 - 2015
Sky UK, Ltd.
- Developed full-stack applications using microservices in Ruby, Sinatra, and JavaScript.
- Utilized A/B testing to validate product efficiency on customer retention rates.
- Maintained and developed multiple custom rack middlewares in Ruby.
- Managed maintained the project's scope, and planned according to the customer needs.
- Implemented TDD (test driven development) with Agile development practices using RSpec.
Technologies: RSpec, Functional Programming, Unix, Object-oriented Design (OOD), Object-oriented Programming (OOP), Ubuntu, Rack, HTML, TeamCity, Git, SQL, Redis, jQuery, Sinatra, Ruby, Webhooks, APIs, SaaS, REST APIs, Architecture, Back-end, Web Development, JSON, Deployment, Axios, Agile, CI/CD Pipelines, Requirements Analysis, Project Planning, Code Review, Agile Sprints, Technical Writing, Documentation, Technical Documentation, Software Architecture, Software as a Service (SaaS), CRM, Test Management, Middleware
Lead Developer
2013 - 2015
Ringley, Ltd.
- Developed full-stack applications on the intranet/extranet deployed to the cloud.
- Handled back-end system administration and infrastructure configuration to prepare for distributed computing.
- Developed a contract management system for tracking job completion and staff performance.
- Implemented and designed a back-end storage system for optimized performance in MongoDB and MySQL.
- Implemented and designed an Elasticsearch cloud-based search engine solution.
- Modernized existing Ruby on Rails applications by extracting business logic, using unit testing, and making use of JavaScript, Ruby, and Ruby on Rails engines.
- Exposed secure web APIs to integrate with mobile applications.
- Developed a financial application that monitors and calculates cash flows, payments fees, and percentage fees.
- Restructured and introduced effective project management methodologies in the organization.
Technologies: RSpec, Functional Programming, Object-oriented Design (OOD), Unix, Ubuntu, GitLab, Sinatra, Elasticsearch, Redmine, HTML, Memcached, D3.js, Git, SQL, MySQL Daemon, Redis, jQuery, Red Hat Linux, CSS, JavaScript, Ruby on Rails (RoR), Ruby, Webhooks, Full-stack, Project Management, APIs, SaaS, Code Architecture, REST APIs, API Integration, Architecture, Customer Relationship Management (CRM), Back-end, Web Development, JSON, Deployment, MongoDB, Full-stack Development, Corporate Finance, System Architecture, Agile, Rails Engines, Finance, Front-end, Bootstrap, Requirements Analysis, Project Timelines, Project Planning, Discovery Workshops, Code Review, Management, Agile Sprints, Amazon Web Services (AWS), Technical Writing, Documentation, Technical Documentation, System Administration, Payment APIs, Software Architecture, Software as a Service (SaaS), CRM, Enterprise Resource Planning (ERP), Module Integrations, Test Management, Middleware, HTML5, Front-end Development, User Interface (UI), System Integration
Software Developer
2013 - 2013
Farncobe
- Developed a back-end authentication system for a mobile application.
- Integrated Facebook and Twitter API on the application UI.
- Developed a full database-driven implementation for multi-language support.
- Implemented TDD (test driven development) and extensive acceptance-test coverage using RSpec.
Technologies: RSpec, Object-oriented Design (OOD), Unix, PostgreSQL, Ubuntu, HTML, Git, SQL, Redis, Ruby on Rails (RoR), Ruby, Webhooks, APIs, SaaS, REST APIs, API Integration, Back-end, Web Development, JSON, Single Sign-on (SSO), Deployment, Project Management, Agile, CI/CD Pipelines, Bootstrap, Requirements Analysis, Project Timelines, Project Planning, Agile Sprints, Amazon Web Services (AWS), Technical Writing, Documentation, System Administration, Software as a Service (SaaS), Workflow, Automation
PMO/Projects Coordinator
2008 - 2012
Intrasoft International
- Oversaw SLA budgets in accordance with EU general directives.
- Maintained the database backlog for tracking work in progress using Ruby, MySQL, and Linux.
- Consolidated contracts and delivery of technical documents.
- Produced and maintained all project documentation, including project plan and project scope.
- Organized, prepared, and followed up the invoicing procedure, including infrastructure procurement and software license updates.
Technologies: Linux, Redmine, Git, MySQL Daemon, CSS, HTML, SQL, Ruby, Bash, Red Hat Linux, Project Management, Customer Relationship Management (CRM), Web Development, JSON, Corporate Finance, Project Timelines, Project Planning, Management, Technical Writing, Documentation, Technical Documentation, System Administration, CRM, Enterprise Resource Planning (ERP), Module Integrations
Experience
Basic Skeleton for an Object-oriented JavaScript Framework
https://gist.github.com/kfamilonidis/200221fb1fa6373c778dc3997c94d4d8I undertook the task of creating a fundamental framework for object-oriented JavaScript programming. My focus was on minimalistic object initialization, emphasizing scope management and extensibility. The initialization function is activated by a self-executing anonymous function which I designed to transfer the scope to the initialization method within the object. This design approach enables the creation of multiple objects while ensuring a significant reduction in scope, enhancing efficiency and clarity.
React and React Native
Developed and enhanced user experiences of multiple forum sites utilizing React and React Native frameworks so the end codebase was shared between web, mobile, and native devices. The solution improved the general organization of the project and the users' UI.
GraphQL Implementation
Built a GraphQL implementation from scratch to utilize very complex data structures and report statistics with minimum time delay. Developed both back- and front-end logic that delivered a complete solution.
Basic Setup for Loading TSort in Ruby
https://gist.github.com/kfamilonidis/e193240de686c7971505e582845e0e04Topological sorting or TSort is a built-in method in Ruby 3.1. Its capability is to solve complex dependency problems that can be represented as graphs. One of its uses is in library dependencies resolution, but its use can be extended to many fields, such as task management.
Education
2000 - 2004
BSc Degree in Business Administration (Technology)
University of Patras - Patras, Greece
Certifications
JULY 2020 - PRESENT
AI Programming with Python Nanodegree
Udacity
SEPTEMBER 2012 - SEPTEMBER 2015
Red Hat Linux System Administrator
Red Hat
Skills
Libraries/APIs
React, jQuery, Node.js, Redux-Saga, REST APIs, Stripe API, Stripe, Rails Engines, Rack, D3.js
Tools
RSpec, Git, TeamCity, Redmine, GitLab, Redux Toolkit, Claude Code
Languages
HTML, GraphQL, JavaScript, CSS, Ruby, Python, HTML5, Bash, SQL, TypeScript
Frameworks
React Native, Sinatra, Ruby on Rails (RoR), Redux, Express.js, Bootstrap, Material UI, Next.js, Flask, Jest, Cypress
Paradigms
Object-oriented Programming (OOP), Object-oriented Design (OOD), Agile, Management, Automation, Functional Programming, Metaprogramming, Requirements Analysis
Platforms
Linux, Ubuntu, Red Hat Linux, Heroku, Unix, Zendesk, Android, iOS, Twilio, Amazon Web Services (AWS)
Storage
Elasticsearch, Redis, MySQL Daemon, JSON, PostgreSQL, MongoDB, Memcached
Industry Expertise
Project Management
Other
Webhooks, Full-stack, APIs, SaaS, Code Architecture, API Integration, Architecture, Back-end Development, Customer Relationship Management (CRM), Back-end, Auditing, Web Development, Single Sign-on (SSO), Deployment, Computer Science, Axios, Full-stack Development, Corporate Finance, BrainTree, System Architecture, Finance, Front-end, Project Timelines, Project Planning, Code Review, Agile Sprints, Technical Writing, Documentation, Technical Documentation, System Administration, Payment APIs, Software Architecture, Software as a Service (SaaS), CRM, Enterprise Resource Planning (ERP), Module Integrations, Front-end Development, User Experience (UX), User Interface (UI), AI Agents, System Integration, Machine Learning, Artificial Intelligence (AI), CI/CD Pipelines, Booking, Discovery Workshops, Stripe Payments, Workflow, Test Management, Large Language Models (LLMs), Storybook, Middleware
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