I’m a Senior Software Engineer with a bias towards writing flexible and elegant code to build systems that operation teams can trust and scale and developers find easy to read and maintain. I'm good at keeping tech-debt at bay.

+10/y remote work experience in many different teams. I've worked with teammates from Australia, South America, eastern and western Europe, Asia but mostly the U.S. and Canada.

+23/y total in software engineering strong at OOP used in a functional fashion.

Some remarks

  • I’ve architected and coded software that serves billions transactions per month impacting millions of users in the telecomunications and digital ads industries.
  • I’ve set up a backend update process that did a lossless hot upgrade keeping backward compatibility while serving thousands of transactions per second.
  • I've taken the Cheshire II U.C. Berkeley's paper on probabilistic search (non-boolean) and implemented it as a software feature in a product.
  • I've open-sourced a full-stack framework and a persistence framework, both available in my GitHub account.
  • I've open-sourced Merchant, an API client to perform PCI compliant e-commerce transactions also available in my GitHub account.

Check more interesting stories in my developer story below.

What they say about me

What are the challenges you're currently facing in engineering?

I have familiarity with

Object Oriented Programming, Functional-Programming, JavaScript, ECMAScript-6, Smalltalk, Rust, Swift, XCode, NodeJS, ExpressJS, React, Redux, Vue, Vuex, Angular, Meteor, iOS, macOS, ubuntu, nginx, Apache, MongoDB, PostgreSQL, Docker, Docker-compose, Continuous-Integration, AWS, ElasticBeanstalk, Lambda, GCloud, CloudFlare, CloudFlare Workers, Agile, Design Thinking and Machine Learning.

Here is my developer story


Senior software engineer

November 2021

Joined a team of 4 as a Senior Software Engineer to oversee the operations of a 12-member engineering team and a 50-member customer success team, while also managing the main component of the roaming software. This effort led to a notable achievement in 2022, contributing to the management of nearly 1 Petabyte of network traffic in the North American mobile telecommunications market.

Backend software engineer

September 2019

Engaged in full-time collaboration with the mobile network infrastructure team. One notable aspect of the software being developed here is its commitment to maintaining the utmost standards of stability and availability, while also retaining a foundation of adaptability. This strategic approach is of paramount importance, given the necessity to seamlessly integrate diverse networks across the globe as required. The expectation for code quality remains exceedingly high, with a focus on delivering pristine code in every pull request and master merge.

Machine Learning

ML Certificate of Accomplishment

July 2017

Graded 92% on this online non-credit course authorized by Stanford University and offered through Coursera. We've covered Linear Regression, Logistic Regression, Regularization to prevent overfitting during training, Neural Networks, Support Vector Machines, Unsupervised Learning, Anomaly Detection, Recommender Systems and Photo OCR. The final work of this course was training a network to control the wheel of a vehicle to make it self-drive following the road.

Upgrading a production system in hot and in a massive scale

Backend compatible lossless upgrade

May 2017

Helped to design and implement the migration process of a dynamically scalable service from AWS to GCloud. Once the service was migrated to GCloud, the upgrades where backward compatible and lossless. The clients started to consume the new API version organically and the new API servers got globally balanced adding server instances and collapsing the deprecated ones as required.

Ad-tech in high scale

Full-stack developer at Sulvo

May 2017

Architected and implemented the next-generation app that is serving +1.2 billion transactions per month. Part of the innovation team making features and dealing with the challenges that add value to this platform used by premium users in the Ad industry. Full-stack with JavaScript (ES6) and DevOps using CloudFlare, CI, Docker, GCloud and AWS.

Native mobile and full-stack development

AppStore iOS Swift App and Admin webapp

January 2017

Published a native iOS application. For this particular app it was required Swift for the iOS app and the following frameworks: MapKit, FBSDKLoginKit, SwiftyJSON, ToastSwiftFramawork, Async and Bolts. And for its admin web app, it was used Meteor with SemanticUI.

Native mobile development

CS193P, Stanford University

March 2016

Learned the basics of Swift by learning to develop apps on iOS. This allowed me to release an app in the AppStore which used MapKit, FacebookSDK, JSON, multithreading and low-latency sound.

Encouraging the startup culture among smalltalkers

Smalltalk for Startups

November 2014

A talk at the Smalltalks 2014 meetup to link the Startup opportunities with Smalltalk opportunities. Also, flow's public presentation and demo.

Open-sourced full-stack framework


July 2014

Published flow as an open-source project with a mission. Flow's mission is to provide consultants, startups and software houses with a competitive Smalltalk full-stack framework that allows them to quickly deliver a demo with all the modern html5.

Open-sourced persistence framework


March 2014

Published Mapless as an open-source project. Persistence with very low maintenance. For real. No instVars. No getters and no setters - just models that can be saved. Mapless is a persistence framework with zero Object-Relational-Impedance-Mismatch on your data.

Lots of frontend and some full-stack development

Full-stack developer at StarterSquad

January 2014

At StarterSquad worked in various teams and projects with many stacks. Among them Angular, Meteor, ExpressJS, MongoDB, PostgreSQL, RESTful APIs, Swift and flow.

Design Thinking

Design Thinking Action Lab

July 2013

I gained valuable insights through my experience with Leticia Britos Cavagnaro during Stanford's Design Thinking Action Lab. Initially intended to enhance the startup creation and innovation stages, I discovered its significance extended beyond that scope. It emerged as a crucial asset, particularly in guiding the Customer Development and Prototyping phases.

Hospitality Industry


February 2013

Lead conversion for back-office processing with seaside web app. Integrated the API of a highly customized CRM with a Seaside based web app and the admin dashboard to process hundreds of leads for the hospitality industry in this region.



Jan 2012

I was involved in leading, organizing, fundraising, creating partnerships and coordinating volunteers and providers to make the first TEDx event in the incredible Iguassu Falls. The event was sold-out.

Being a startup founder


July 2009

Airflowing was a web application that provided management to service companies. With an original UI design and simplicity in mind, it allowed the operations team to coordinate jobs, tasks and finances across different teammates. It was a simple yet end-to-end solution for small businesses. It ran on Pharo Smalltalk, jQuery, Aggregates, OmniBase object persistence achieving scalable and subsecond server-side rendering speeds. Shutdown in 2015.

Feature out of a Berkeley's paper

Implementing Cheshire II

July 2008

Developed a product for medical practice management including functions that were quite an innovation at the time like efficient video capture of various medical instruments used in different exams. Also organizing the medical records storage with probabilistic indexing based on U.C. Berkeley's Cheshire II catalog design, a feature that allowed sub-second non-boolean searches and results presented in order of relevance. All that developed using Dolphin Smalltalk.

Client-server architecture in the aftermarket industry

Ford parts e-commerce

July 2006

As an independent consultant, developed an e-commerce application for Cromosol S.A., a leader in the Argentinean cars and trucks aftermarket. The application had a very simple to use user interface, designed to help Cromosol’s clients to see always updated prices with their stock availability and send parts requests.

Road toll system

Toll system

July 2001

Project Leader at TEING, an industrial control systems company. The developed product was a Smalltalk based system to manage and control toll processing in a Road Concession Company. The software-controlled custom hardware devices like barriers, alarms, beacons, fiscal printers, hubs, axle counters and more. In those industrial environments, getting parts of the system offline due to thunderbolts is a common occurrence, so the system also had high-performance requirements with several fault-tolerant strategies that allowed the system to operate in real-time and also with the server offline. All that running on Dolphin Smalltalk.

Pictures I like

I think these ideas are seriously cool


This is the privilege of working with great teammates

During my time as a Core IP Engineer at Telna I worked as part of a small yet deeply talented team of consummate technical professionals, and without hesitation consider Sebastian as having held up his part to count among that number.

Anytime we had to deal with insanely tight build deadlines and tough problems, he was there with us.

Seb could be always be depended on to help us work his angle of whatever challenges we faced through to their resolution - even under sometimes extremely stressful conditions.

Over my 25-year career, he is one of those few people I can honestly say I miss working with everyday. He would be a valued and highly regarded member of any team he finds himself on.

Chris Aspeling
Head of Architecture and Development @ The Starter Pack Company

Sebastian is the kind of Software Engineer you don't find often. He is very organized in his work, writes amazing code and really delivers.

I learned a lot from him while working together, especially in architecting and writing highly readable code. His great attention to details makes him someone you can completely trust for tasks that require high precision.

He is also a true team player with excellent communication skills, which makes him a very valuable asset to any team.

Ariel Fuggini
Helping publishers maximize revenue through technology and innovation

It is my pleasure to recommend Sebastian Sastre for the position of software developer / project manager / CTO. I worked as Sebastian’s project manager within StarterSquad for the last 5 years, during which time we took on many medium and large software projects.

Sebastian is a great developer and a wonderful teammate. During the many projects I had the opportunity to work with him, he always added a creative angle to the technical solution, was easy to work with and responsible. He is well-liked by our clients and respected by his colleagues.

Sebastian has a passion for coding and new technologies, even writing / contributing to software frameworks, giving lectures at various software community events.

His energy is limitless and his enthusiasm serves to motivate everyone on his team. I’m confident that you will be very pleased with his work, should you choose to involve him with your company / projects. Please feel free to contact me if you have questions, either by email: vlad@startersquad.com or by phone: 0040 742 301 902.


Vlad Ioanitescu
Project Manager
StarterSquad BV
COO at Seamless Ahead

Connecting on social media

We can continue the conversation on any of these

I'm confident I can add a lot of value to your team