This seems simple for one stream, but scale that up to how many unique streams that Youtube is servicing at any given second. 10k?
Google doesn’t own all of the hardware involved in this video serving process. They push videos to their local CDNs, which then push the videos to the end users. If we’re configuring streams on the fly with advertisements, we need to push the ads to the CDNs pushing out the content. They may already be collocated, but they may not. We need to factor in additional processing which costs time and money.
I can see this becoming an extremely ugly problem when you’re working with a decentralized service model like Youtube. Nothing is ever easy since they don’t own everything.