How the interviewer opens it
“Design a video conferencing service with meeting rooms and participant controls. Severe bandwidth limits may require audio-only calls. A regional failure may briefly interrupt media, but the meeting must recover.”
The prompt is vague on purpose. The first minutes are for you. Reduce the prompt to the core features and agree on the numbers before you draw.
What to clarify first
- Scale
- Room access and controls
- Media volume
- Read-to-write mix
- Latency
- Clients and networks
- Consistency and availability
- Regional failures
- Durability and privacy
- Scope limits
The interviewer has the answers. In a SysRound round, Alex gives these answers, as a real interviewer does.
Scope of a strong answer
Core requirements
- Create, join, and leave private meeting rooms.
- Send live audio and video in rooms with up to 100 participants.
- Let hosts mute and remove participants.
- Adapt media quality to bandwidth limits and restore calls after regional failures.
Usually out of scope
- Record calls or produce transcripts.
- Support broadcasts with thousands of viewers.
- Provide strict end-to-end encryption.
- Build payment or subscription services.
What interviewers ask about
- Media paths
- A room can contain 100 participants. A poor media path can exhaust a client's upload capacity or add too much delay.
- Audio and video quality
- Each participant has different network limits. Delayed feedback and sudden bitrate changes can damage audio before the system reduces video.
- Participant controls
- A host's mute or removal must change the media path. Concurrent controls and reconnects must not restore a participant's old rights.
- Regional failures
- A failed region can lose every active media session in that region. Reconnects can overload another region and revive old room state.
Common mistakes
- The design sends every video to every participant despite the nine-video display limit.
- The design treats an interactive call like a video file stream.
- The design uses one bitrate for every receiver.
- The server trusts the client to enforce host mute and removal.
- The design assumes DNS changes preserve active media sessions.
- The capacity estimate excludes TURN traffic and spare capacity for a regional failure.
The bar by level
- Mid-level (L4/E4)
- Separates control and media paths. Explains a working group call, connectivity fallback, basic bandwidth reduction, and server-enforced host controls. Recognizes that regional recovery requires client reconnects.
- Senior (L5/E5)
- Designs per-receiver media selection and congestion response. Handles repeated controls, concurrent changes, and removal during reconnect. Defines regional recovery with durable permissions and spare capacity.
- Staff (L6+/E6+)
- Quantifies network throughput, packet rates, TURN traffic, and recovery limits. Proves that isolated workers cannot retain authority indefinitely. Budgets the recovery target and explains cross-continent delay limits.