David Xu
Verified Expert in Engineering
Software Developer
Denver, CO, United States
Toptal member since October 20, 2016
David has taken several mobile apps from ideation to millions of users worldwide as the CTO of WhereTo, Chief Architect of Castle Global, Inc., and technical lead in other companies. He has been programming since he was nine and has won medals in various competitive programming competitions, from the Australian Informatics Olympiad to ACM-ICPC. David has invaluable experience in all technical areas, from architecture and design to engineering and DevOps.
Portfolio
Experience
Availability
Preferred Environment
GitHub, Git, Visual Studio Code (VS Code), MacOS, WebStorm
The most amazing...
...thing I've done was scale the Kiwi Q&A to 50 million users, all while building new features for the platform and fixing bugs.
Work Experience
Chief Technology Officer
WhereTo, Inc.
- Acted as the CTO for WhereTo, a SaaS technology company that empowers the business traveler or manager to get the best travel deal, enhances their booking experience, and incentivizes their business travel.
- Partnered with top Fortune 100 companies to solve their frustrations with traditional corporate booking tools by utilizing our proprietary algorithm to provide the most optimal choices of flights, hotels, and car rentals.
- Oversaw all aspects of technology, infrastructure, and R&D at WhereTo.
Senior Android Developer (Kotlin)
Tessie Technology
- Built Bluetooth controls to control Tesla vehicles and phone key integration.
- Developed the Wear OS app using Jetpack Compose and Kotlin.
- Created Android widgets for the Android application using Kotlin.
- Reverse-engineered the new Hermes protocol for direct end-to-end encrypted communication to vehicles.
Android Engineer | Project Manager
TruckSpy Inc.
- Led a team that built an electronic logging device application with a deadline of only five weeks.
- Implemented Bluetooth (BLE) protocols for both J1939 and J1708.
- Replaced an entire legacy application with the one that we built in-house.
- Managed a project and a team of senior engineers.
Senior Engineer | Consultant
Superius d.o.o.
- Turned a fragmented repository into a monorepo with many build targets for different markets.
- Upgraded React and React Native to the latest versions across many major version updates.
- Created documentation for the existing team to add new shared modules to the monorepo in a clean, modular way.
Senior Engineer
ReMatter, Inc.
- Dockerized the application to make it easier to onboard new engineers.
- Improved test runtime significantly, from 30 minutes to 5 minutes.
- Fixed logging to correlate logs with APM tracing correctly.
Developer
Overtime
- Helped build a video editor for the Overtime app to add effects.
- Built a commenting system for the overtime mobile app.
- Built a leaderboard system for the Overtime mobile app.
Back-end Developer (SuiteScript)
GovDocs, Inc
- Built a REST API on top of NetSuite to provide all sorts of information to customers.
- Architected the build system to allow the use of typed JavaScript to build around SuiteScript faster.
- Worked with the front-end engineer to implement the API and helped other back-end engineers get up to speed and build efficiently.
Chief Technology Officer
FlipTix, Inc
- Managed and was responsible for all technical development, infrastructure, management, and R&D.
- Architected the entire app with its back end.
- Built the server in Node.js and deployed it on AWS.
- Wrote the initial code for the iOS app in Swift.
- Built the entire microservice architecture and architected all protocols.
iOS Developer
SiteLite
- Built extra features for the SiteLite app, an app allowing users to position and preview lights.
- Implemented APIs and the UI using UIKit and AFNetworking.
- Fixed a lot of bugs left over from the previous developer.
Full-stack Android Developer
MudFlap App
- Built an MVP of the Mudflap app, an app that shows vital information to truckers on a map.
- Worked with the front-end engineer to implement APIs I developed. Scraped a lot of information from various sources and merged them to create a giant database of POIs (Points of Interest).
- Worked with the product and design team to implement an MVP for the Mudflap Android app.
Chief Architect
Hive AI, Inc. (previously Castle Global, Inc.)
- Built the commenting system for Kiwi using Node.js, PostgreSQL, and Redis.
- Developed the AMA (Ask Me Anything) system for Kiwi, using Node.js, PostgreSQL, and Redis.
- Led the development of Plaza's website, Plaza.tl.
- Managed and led the back-end team for Kiwi's site. Kiwi.qa.
- Extended the functionality of the moderation system I built previously; worked with data science team to add machine learning for automated pornographic, violent, and medical content detection.
- Architected and built the version 2 of Kiwi's REST API.
- Built the Chatous spam detection system using Golang and Redis.
- Created the public questions feature for Kiwi using Node.js, PostgreSQL, and Redis.
- Developed the topics feature for Kiwi (used Node.js, PostgreSQL, and Redis).
- Rearchitected the feed, enabling more than one type of feed element to be displayed, and allowing feeds to be filtered (used Node.js, PostgreSQL, and Redis).
- Developed the media questions feature for Kiwi, enabling rich media questions (used Node.js, PostgreSQL, Redis, and AWS S3).
- Built the question curation (moderation) back-end for Kiwi, helping keep spammy questions off the site (used Node.js, PostgreSQL, Redis, React, Webpack, and Babel).
- Made the moderation module for Plaza, preventing pornographic, violent, and medical content from being posted on the marketplace (used Node.js, PostgreSQL, Redis, React, Webpack, and Babel).
- Integrated Plaza with email, enabling email based marketing to function (used Haraka and Node.js).
- Built the mutual friends count feature for Plaza, enabling safer transactions (used Node.js, PostgreSQL, and Redis).
- Rewrote the notifications system for Kiwi, adding new types of notifications for user engagement (used Node.js, Redis, and Google ProtoBuf).
- Built the follower recommendation system for Kiwi (used Cassandra, Redis, Scala, and MySQL).
- Set up the Jenkins integration for the CI pipeline.
Back-end Engineer
Castle Global, Inc. (previously Chatous, Inc.)
- Built the moderation system, keeping pornographic, violent, and medical content off the service.
- Developed the analytics system, handling more than 500 million data points daily.
- Built the spatial search database server, powering nearby posts.
- Created the entire back end and initial front end for Plaza.tl.
- Introduced integration tests to Kiwi.
- Scaled Kiwi from five million users to more than 55 million users.
- Created various server monitoring tools.
- Rewrote the photo resizer in Go and made it 10x faster.
Founder | CEO | CTO
Aura Labs Pty Ltd
- Built Aura, a proximity-based social network used to discover what's happening around you.
- Developed iOS and Android apps.
- Built the back-end server and deployed it on AWS using Elastic Beanstalk.
iOS Engineer
BlondGorilla Pty Ltd
- Built the mobile app, Danny Green's Fighting Fit, which was an app developed for Danny Green. It was used to instruct people on how to become good at fighting through streaming video and social integration.
- Integrated Facebook and Twitter.
- Developed the embedded video player.
- Built the serialization mechanism for feeds.
- Worked with a designer to implement the app.
Mobile Engineer
Theme Solutions Pty Ltd
- Developed Shareability, an app used to help parents connect with their children's schools.
- Built and white-labeled the app for over 20 schools.
- Created custom features as requested by schools.
- Published more than 20 apps to the App Store and Google Play.
Lead Engineer
Holobyte Pty Ltd
- Worked as the lead developer of a mobile game for iPhone and iPad called Droodl.
- Developed the app in Objective-C, using Cocos2D.
- Built the back-end using PHP, deployed on AWS using Elastic Beanstalk.
- Developed the custom OpenGL-ES renderer to render drawings.
- Integrated everything with DynamoDB.
Developer
CREFCO (via an Online Freelance Agency)
- Developed the system for automatic credit pulls.
- Built the outlook integration for credit checking.
- Deployed it on Windows Server (in house).
- Developed the automated emailing system for credit checks.
Web Developer
TrimEla (via an Online Freelance Agency)
- Built the eCommerce platform for TrimEla, an online retailer for fashion goods.
- Wrote website code using jQuery, CSS, and HTML.
- Optimized the PHP back-end code.
- Added features for international purchasers.
- Built coupon features and implemented them.
Experience
Kiwi | Q&A
Plaza
Chatous
https://chatous.comGold | Australian Informatics Olympiad 2010
Bronze | ACM-ICPC Regional 2011
Renoria
https://www.youtube.com/user/RenoriaDevMelon
https://www.businesstravelnews.com/Management/In-Ripe-SME-Market-Corporate-Traveler-Rolls-Out-MelonEducation
Incomplete Bachelor's Degree in Computer Science
University of Western Australia - Perth, Western Australia
Skills
Libraries/APIs
React.js, jQuery, Node.js, REST API, API Development, React.js, React.js, WebRTC, Google Maps Development, Stripe Development, Google API, X (formerly Twitter) API, Facebook API, RxSwift, Puppeteer, Twilio Development, Stripe Connect, Stripe Development, OpenAPI, FFmpeg, Core Graphics, OpenGL, TensorFlow, FlightAware XML3 Data APIs, API
Tools
Git, GitHub, React.js, TestFlight, GIS, Webpack, RabbitMQ, Android, Amazon Elastic Container Service (ECS), ChatGPT, Sublime Text, Atom, Babel, PostCSS, Jenkins, WebStorm, CircleCI
Languages
PHP, C, Kotlin, Objective-C, SQL, ES7, Java, CSS, HTML, JavaScript, HTML5, TypeScript, GraphQL, Stored Procedure, Go, Swift, Python, Scala, Lua, C#, C++, XML, Rust, SuiteScript 1
Frameworks
Laravel, Express.js, Koa, Redux, React Native, JSON Web Tokens (JWT), Jest, Flux, Next.js, UIKit, Core Location, Cocos2d, Spark, Flutter UI
Paradigms
MVVM, Model View Controller (MVC), Object-oriented Programming, Concurrent Programming, Mobile App Design, Back-end Developers, Event-driven Programming, REST, Management, Application Architecture, DevOps, Agile Development, Load Testing, Continuous Integration (CI), RESTful Development, Agile Development, Functional Programming, Real-time Messaging Protocol (RTMP), Mobile App Design
Platforms
AWS, Linux, Docker, Android, iOS, Amazon EC2, Apache Kafka, Azure Design, Cloud Engineering, Mobile App Design, Linux, Ubuntu, Twilio Development, AWS Lambda, Bluetooth Low Energy (LE), MacOS, Visual Studio Development, HubSpot Development
Storage
JSON, Amazon S3, Redis, PostgreSQL, MySQL, Database, NoSQL, GIS, SQL, MongoDB, Realm, SQLite, AWS, HBase, Cassandra
Industry Expertise
Travel & Leisure
Other
Ajax, Software Development, Front-end Development, Mixed Reality (MR), Freelance Design, APIs, Google Play Development, Mobile App Design, App Store Development, Full-stack, CI/CD Pipelines, Technical Leadership, API Integration, Back-end Developers, Front-end, System Security, Responsive Web Development, Software Architecture, Architecture, Enterprise Architecture, Leadership, Web Scraping, Scraping, Maps, Dashboard, Team Leadership, Interim CTOs, Artificial Intelligence, Machine Learning, Cross-functional Team Leadership, Teamwork, Cloud Architecture, Networking, Apollo, AWS RDS, Design Consulting, Data Visualization, Compilers, Code Generators, Estimations, Marketplace Design, Web Development, NetSuite, Site Reliability, WebSockets, Quality Assurance Engineering, Debugging, Full-stack, Large-scale Projects, Authentication, Monorepos, Memory Leaks, AWS Cloud, Technical Architecture, Data Scraping, Integration, Technical Design, Payment Gateways, Streaming, Broadcasting, PayPal Development, RHEL, Google V8, Amazon Route 53, Haproxy, Amazon API, Dating Apps, Fintech Development, NLP, Generative Pre-trained Transformers (GPT), Chatbot Development, GPT-4, PCI Compliance, RTMP, Urban Airship, Image Processing, Travelport, Hotels & Lodging, SOAP, Mobile Architecture, Bluetooth, Database, UI Development, Product Management
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