Confluent rejection, system design round felt like talking to a wall

Confluent rejection, system design round felt like talking to a wall

Just heard back from Confluent and it's a rejection, still processing it a bit.

For context, around 4 YOE, mostly backend, so I felt decent about the coding side going in. Phone screen was the tail -n problem, printing the last N lines of a file without loading everything into memory. Had to talk through buffer vs pointer offset tradeoffs, felt like a solid conversation and the interviewer was engaged the whole time.

Onsite was 3 rounds. First coding round was the sensor health thing, checking if a server was actually alive based on ping timestamps, with a follow up on optimizing with binary search that I fumbled but recovered from. Second coding round was combination sum, backtracking, standard enough.

Then system design, the temporary email system, 100M users, figuring out how to generate usernames that don't look like spam and check duplicates fast. This is where it fell apart. My interviewer barely said anything the whole round, just let me talk and nod occasionally, so I had no idea if I was on the right track or way off in the weeds.

Got the email today saying they went with other candidates. Kind of expected it given how quiet that last round felt, but it still stings. Anyone else had a system design round go completely silent like that, or was I just unlucky with the interviewer?