How the interviewer opens it
“Design a live broadcast platform with channels and subscriptions. Broadcasters send live video, and large audiences watch it with little delay.”
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
- Channels and subscriptions
- Audience scale
- Video quality
- Request mix
- Delay targets
- Availability and consistency
- Durability
- Clients and geography
- Failure and recovery
- Excluded features
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 a channel and control its broadcast
- Watch a live channel with automatic video quality changes
- Subscribe to and unsubscribe from channels
- List subscribed channels and their live state
Usually out of scope
- Paid subscriptions
- Chat and advertisements
- Video recommendations
- Recordings and video on demand
- Several broadcasters on one channel
What interviewers ask about
- Broadcast delay and playback quality
- Small delays accumulate across capture, encoding, delivery, and playback. Smaller buffers reduce delay but increase pauses on weak networks.
- A large audience for one channel
- One channel can consume half the delivery capacity. Requests for each new video part arrive before delivery sites have a copy.
- Broadcast interruptions
- Failures can leave old and new broadcast sessions active at once. Delayed messages can overwrite the state of a newer session.
Common mistakes
- Treats subscriptions as paid access without asking.
- Designs channel metadata in detail but leaves the video path unexplained.
- Serves each viewer directly from the ingest server.
- Ignores the combined delay from video processing, caches, and player buffers.
- Assumes every CDN supports low-latency delivery with default settings.
- Allows an old broadcast session to overwrite the state or media of a new session.
The bar by level
- Senior (L5/E5)
- Budgets broadcast delay and calculates delivery bandwidth. Explains playlist freshness and cache misses. Handles duplicate sessions, late stop requests, and the bounded recovery target.
- Staff (L6+/E6+)
- Quantifies source load during audience surges and failures. Defines session authority across regions. Budgets failure detection and player recovery. Compares costs for stricter delay targets.