Design YouTube

Question 1

Who is watching?

Accepted Answer

Playback is the product. The player belongs on the board.

Question 2

What do the apps connect to?

Accepted Answer

A gateway in front of the services is where auth and routing live, and what lets upload and playback scale on completely different curves.

Question 3

Where do the video files themselves live?

Accepted Answer

Video bytes do not go in a database and they do not go on your service disks. Object storage is built for large immutable blobs and is the only thing that stays affordable at this volume.

Question 4

A viewer in another continent presses play. Where do the bytes come from?

Accepted Answer

Streaming every view out of one region is slow for the viewer and ruinous in egress. A CDN puts the segments near the viewer and takes almost all of the read traffic off your infrastructure — at 58,000 views a second that is not an optimisation, it is the design.

Question 5

Someone uploads a two-hour 4K file. What happens on that request?

Accepted Answer

Transcoding into every resolution and bitrate takes minutes to hours — it cannot happen inside the upload. The request stores the file and enqueues the work; workers behind the queue do it, and the video becomes watchable when they finish.

Question 6

Where do the title, owner and playback URLs live?

Accepted Answer

Everything about a video except its bytes is small, relational and queried constantly. That is a database, and it is a different store from the one holding the video.

Question 7

Your client reads the metadata store directly. Who decides what it may see?

Accepted Answer

Private videos, unlisted videos, region blocks and takedowns are all decisions made in a service. A client with a direct line to the store makes none of them.

Question 8

How many views a second are you serving?

Accepted Answer

5,000,000,000 / 86,400 seconds is about 58,000 views a second. That is the number that says the read path has to be a CDN and not your servers.

Question 9

And how many uploads a second?

Accepted Answer

500 hours a minute at 10 minutes a video is 3,000 videos a minute — about 50 a second. Tiny next to the reads, which is why the write path can afford to be slow and asynchronous.

Blueprint — system design