This was my main job for almost 5 years. I managed two teams with 40+ engineers total, and we built many important things:

• Built the Zalo Mini App platform from the beginning. Now it is used by 1000+ businesses, which is a lot.
• Created 10+ multiplayer social games that many people play - we have millions of daily active users.
• Made sure everything works well with many users. When you have millions of users, problems happen, so we had to fix them quickly.
• Spent a lot of time helping the team learn and work together better. Good team and good process make good products.

This was a very important project - I built the authentication system that powers everything at Zalo:

• Built an authentication system that handles 20M+ users every day. When it stops working, everything stops, so reliability was very important.
• Led a team of 20+ engineers. Most of my time was spent helping them solve problems and learn new skills.
• Created the Zalo Developer Platform with OAuth integration so other developers could use our APIs.
• Wrote a lot of documentation because good documentation is important for developers.

I was the front-end lead for Zalo's web and desktop chat app:

• Built UI components that work smoothly and respond quickly. Real-time chat is difficult - messages cannot be slow.
• Set up architecture patterns and best practices that other teams started using. It is good when your work helps others.
• Spent a lot of time making performance better. When you have millions of users, every small improvement matters.
• Made the development process better so we could finish features faster without breaking things.

I worked on multiplayer mobile games here, mostly using Java:

• Built the main game mechanics and networking code that makes multiplayer work. It is harder than it seems.
• Designed the backend to handle many players at the same time. When many people play together, it can be difficult.
• Made performance better so the game works smoothly on different devices. Nobody likes a slow game.
• Made sure real-time features work in real-time. Fast response is very important in games.

I worked on web and mobile apps here, using the technology that worked best:

• Used Java, PHP, JavaScript - whatever was needed to finish the work. Sometimes you just need to finish quickly.
• Built responsive front-ends that worked well on different devices. Mobile was becoming popular at this time.
• Made sure the backend worked smoothly. When things break, users notice.
• Worked with other teams to make sure we built what people actually needed.

This was my first real developer job, and I was still in school:

• Built web apps with PHP and MySQL. Old technology, but it worked.
• Made interfaces that clients could use easily. Making things user-friendly was not always easy.
• Got real experience with full-stack development. Learning by building real things is the best way.
• Learned to solve problems quickly. In client work, deadlines are real and you need to find solutions fast.