Did the L5 system design round at Roblox last month. Here's the honest breakdown since I couldn't find much specific info when I was prepping.
The round was 60 minutes, one interviewer. They asked me to design a real-time leaderboard system that handles millions of concurrent users. Pretty classic at first glance but the Roblox angle matters: they pushed hard on the UGC platform context, so I had to think about multiple games each with their own leaderboard state, partitioned by game ID, with fan-out to players in session.
What they actually cared about: Consistency vs. availability tradeoffs. They wanted a concrete answer, not "it depends" hedging. How I'd handle hot partitions. Redis sorted sets came up naturally but they pushed on what happens when one game has 500k concurrent players. Read/write ratio reasoning. I said 20:1 read-heavy for leaderboard display and they seemed happy I had a rationale, not just a guess. Failure modes: what if the cache layer falls over, how does the game client degrade gracefully.
What mattered less: Perfect API design. They barely touched the REST/gRPC question. Exact capacity math down to the last TB. Ballpark estimates with reasoning were fine.
I'm L5-equivalent from a smaller company (staff title but headcount-adjusted). They placed me correctly in the process. One thing I noticed: the interviewer name-dropped their internal distributed key-value store a couple times. Don't panic if they mention internal systems you don't know. Just map it to an equivalent open-source tool and say "I'd assume semantics similar to X" and keep going.
The vibe was collaborative, not gotcha. I got stuck on the persistence layer for about 90 seconds and the interviewer gave a nudge without making it weird.
Total loop was phone screen, one-hour coding, system design, two behavioral rounds. Got the debrief call within a week. San Mateo office, hybrid (3 days on-site expectation for this role).