Meet the Global LINERs
Engineering
Seungwook Han / Backend Server Engineer (distributed storage) / LINE Plus
Distributed storage plays a critical role in ensuring LINE Messenger remains a secure and reliable platform. It allows messages to be delivered seamlessly, providing users with an uninterrupted experience. Handling petabytes of data means we are constantly working to enhance both availability and scalability. We sat down with Seungwook Han, one of the engineers building and maintaining this backbone. Though a bit nervous at first, he soon spoke with passion and pride about distributed storage development. Here’s his story.
A quick three-line summary
- As a backend engineer, Seungwook is responsible for the stability and scalability of LINE’s messaging platform through distributed storage.
- His work spans data consistency guarantees, cache layer optimization, and automated operational tooling - all critical to sustaining messaging services during failures.
- Successful backend engineering at global scale requires meticulous design, data-driven decision-making, and clear cross-team collaboration.
About Work
Q Please introduce yourself briefly.
Hello! My name is Seungwook Han, and I'm part of the Messaging Server Engineering team. I began my career during my alternative military service after completing my master's degree. What drew me to LINE was the opportunity to directly design and build systems at global scale, something both exciting and technically challenging.
Q How did you feel when you were asked to do this interview?
I was honestly surprised. Developers are often stereotyped as being quiet or introverted, so I wasn't sure how I'd do in an interview. But since our team supports LINE's core messaging service, I realized this was a great chance to share our work, and that gave me the confidence to participate.
Q What does your team do, and what's your role in it?
Our team manages caching and storage for the messaging server platform. We own the storage layer of the messaging application servers, where we contribute to both feature development and reliability improvements. In broad strokes, our work falls into three categories: 1. Service continuity - making sure users can continue sending and receiving messages even if a data center goes down. 2. Access control and security - managing permissions and building automation tools to streamline operations. 3. Storage layer optimization - addressing technical debt, implementing new designs or caching strategies to ensure data consistency and scalability. My main focus is the third area - data caching.
Q What does a typical day look like for you?
I start by listing all tasks, then pick three top priorities. I check email, wiki updates, and Slack for urgent issues, respond quickly, review company-wide announcements, and check my calendar. Regular activities include reviewing teammates' PRs, commenting on wiki documents, and preparing discussion points for scheduled meetings.
Q What's on your To-Do list today?
This interview was first on the list. Afterward, I'll review cache algorithm logic flow with project members. Finally, I plan to do an experiment before production to measure how much traffic shifts back to storage when a cache server fails.
Thoughts on competencies
Q What’s the most important quality for a distributed storage developer?
Attention to detail. Even small changes can ripple through the entire system. For example, a momentary network instability can generate a flood of error logs across messaging servers. If code is wrong, you can usually roll it back. But if user data is lost, rollback won’t fix it. Mixing correct and corrupted data makes recovery extremely difficult. Because mistakes here have wide and lasting effects, thoroughness is non-negotiable.
Q What do you value most when collaborating with other teams?
Our main collaborators are application server developers—we treat them as our direct “users.” Communication is relatively easy since we share technical context, but I make sure to ground discussions in data. Service metrics and server performance logs help us reason about trade-offs objectively.
Q Anything you keep in mind when working with global colleagues?
We communicate in English, and since few of us are native speakers, clarity matters more than fluency. Written messages can sometimes sound blunt, so I use emojis or add background context to keep things friendly. I always aim to convey “we’re on the same team” while keeping the technical content precise. Even when giving negative feedback, I try to provide alternatives instead of just saying “No.”
Challenges and Growth
Q Which projects have been the most fun for you?
Two stand out. The first was database access optimization. Originally, fetching 100 profiles meant 100 sequential API calls. By adding parallel read functionality, we dramatically reduced response time. Seeing the improvement in real metrics was rewarding. The second was building an API rate limiter. Abusers were making hundreds of API calls per second. By enforcing per-user limits, those patterns vanished. It felt like catching someone in a game of tag! It was fun, and impactful for system stability.
Q What’s your proudest accomplishment so far?
I recently worked on improving recovery processes for messaging services during data center outages. We introduced a “Fire Drill” practice: simulate failures, assign roles, and check whether messaging can be restored within 90 minutes. It required coordination across many teams and took over six months, but seeing it succeed in practice was deeply satisfying.
Q What does “challenge” mean to you?
Challenge often feels uncomfortable, sometimes like being a beginner again. Not all challenges succeed, and some end in failure. But every attempt brings learning and humility. For me, challenge is a necessary process for growth.
Special experiences at LINE
Q What do you enjoy most about working at LINE?
The bar is high, so projects are demanding but rewarding. My teammates are skilled and proactive, and we constantly share knowledge and trends. I also appreciate the open culture where everyone can freely share ideas and take ownership. When issues arise, the whole team jumps in to solve them together. I find it meaningful to grow technically while also supporting my teammates.
Q Does your team have any unique practices?
Yes, we hold “Help Meetings.” These are sessions where anyone can ask for help on tough issues. The team collectively prioritizes and tackles the problem. Just this morning, we identified a caching issue that needed collective input, so I’m planning to share it with the team later today at a Help Meeting.
Wrap-up
Q Any final advice for those applying for this role?
LINE is a global service aiming for near-perfect reliability. That requires persistence and curiosity, but it also means every engineer directly impacts millions of users. Storage development isn’t just about infrastructure; it directly impacts user experience. Even though our work isn’t always visible, it shapes seamless communication for millions of users. We’re looking for backend engineers who bring precision, accountability, and curiosity, people eager to take on the challenge of keeping LINE’s messaging seamless worldwide.
