https://www.hiredintech.com/classrooms/system-design/lesson/72

user‘s browser / mobile App => Load Balancer => AppSrv1, AppSrv2, AppSrv3, ... => DB

Load Balancer:

　　User => DNS load balancing, CDN => Software Load Balancer (nginx) => App Servers (Apache, Tomcat, Django..) => DB Cache => DB.

Database Sharding:

　　application level (driven by config or some implicit rule e.g. tweet id‘s some bits)
　　library level
　　existing product, 

Database Schema:

　　Users, Tweets, Followers, Favorites

Indexes on each table

Design Twitter