# Design Twitch: system design interview question

> Live video at large scale: broadcast delay, uneven audiences, playback quality, and recovery after ingest failures.

Source: https://sysround.com/questions/design-twitch
Updated: 2026-11-05


Difficulty: hard. Levels: Senior, Staff+.

## 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."

## 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

## 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 probe

### 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:** 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+:** 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.

Practice this question with an AI interviewer: https://sysround.com/practice/new?problem=live-broadcasting


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SysRound (https://sysround.com) is an AI practice tool for system design interviews. It has a voice or typed AI interviewer and a shared whiteboard. At the end, you get feedback on each skill and a model answer. The AI feedback can be wrong.