There are more reasons than listed why we should use server in between client and the encoder. The application code on the server can write logic to write the metadata to database. That is not the job of encoder.
Course: Grokking Modern System Design Interview for Engineers & Managers - Learn Interactively
Lesson: Design of YouTube
Hi Pramod.
Your observation is correct, that the encoder does not write metadata to the database. We have stated some reasons for having servers in between the client and the encoder in the high-level design and the detailed design components sections. We explicitly mention in the high-level design that the server is responsible for storing metadata in the databases, not the encoder.
Our goal was to cover the essential aspects needed to meet the functional requirements of YouTube. Discussing these low-level intricacies would drastically increase the length of YouTube’s design, and therefore, we consider them out of scope for this course.
If you have any other queries, feel free to reach out to us.
Thank you.