Hello, Kim Yo-hwan :) Thank you for your feedback! I think the explanation of non-stop (replica set) and horizontal scaling (sharded cluster) was a bit lacking. Although it is a great advantage of MongoDB, I thought it would go beyond the scope of the lecture if I covered it in too much detail. So for database infrastructure such as non-stop and horizontal scaling, I focused on the bold concepts so that you would think, "Oh, there is something like this!", "This is possible!" I will consider reinforcing the lecture content a bit more in the future!