Understand the Problem

In this chapter, you are asked to design YouTube. The solution to this question can be applied to other interview questions like designing a video sharing platform such as Netflix and Hulu. Figure 1 shows the YouTube homepage.

YouTube looks simple: content creators upload videos and viewers click play. Is it really that simple? Not really. There are lots of complex technologies underneath the simplicity. Let us look at some impressive statistics, demographics, and fun facts of YouTube in 2020 ^{1 2}.

• Total number of monthly active users: 2 billion.

• Number of videos watched per day: 5 billion.

• 73% of US adults use YouTube.

• 50 million creators on YouTube

• YouTube’s Ad revenue was $15.1 billion for the full year 2019, up 36% from 2018.

• YouTube is responsible for 37% of all mobile internet traffic.

• YouTube is available in 80 different languages.

From these statistics, we know YouTube is enormous, global and makes a lot of money.

Step 1 – Understand the problem and establish design scope#

As revealed in Figure 1, besides watching a video, you can do a lot more on YouTube. For example, comment, share, or like a video, save a video to playlists, subscribe to a channel, etc. It is impossible to design everything within a 45- or 60-minute interview. Thus, it is important to ask questions to narrow down the scope.

In the chapter, we focus on designing a video streaming service with the following features:

• Ability to upload videos fast

• Smooth video streaming

• Ability to change video quality

• Low infrastructure cost

• High availability, scalability, and reliability requirements

• Clients supported: mobile apps, web browser, and smart TV

Back of the envelope estimation#

The following estimations are based on many assumptions, so it is important to communicate with the interviewer to make sure she is on the same page.

• Assume the product has 5 million daily active users (DAU).

• Users watch 5 videos per day.

• 10% of users upload 1 video per day.

• Assume the average video size is 300 MB.

• Total daily storage space needed: 5 million * 10% * 300 MB = 150TB

• CDN cost.

• When cloud CDN serves a video, you are charged for data transferred out of the CDN.

• Let us use Amazon’s CDN CloudFront for cost estimation (Figure 2) ³. Assume 100% of traffic is served from the United States. The average cost per GB is $0.02. For simplicity, we only calculate the cost of video streaming.

• 5 million * 5 videos * 0.3GB * 0.02=0.02 = 0.02=150,000 per day.

From the rough cost estimation, we know serving videos from the CDN costs lots of money. Even though cloud providers are willing to lower the CDN costs significantly for big customers, the cost is still substantial. We will discuss ways to reduce CDN costs in deep dive.