系统设计就是不断问自己问题，并解决问题的过程。有很强的套路性，只要练几个就能把规律摸熟：

第一问：设计的系统是干啥用的 (use cases)？

• User posts a tweet, which will be pushed to followers
• User views a user's timeline
• User views homepage
• User search

第二问：设计的系统有哪些限制条件？

• Posting a tweet should be fast
• Fanning out a tweet should be fast, unless there are millions of followers
• Viewing the timeline should be fast
• Searching should be fast

第三问：设计系统时有哪些假设？

• 用户总数：100 million daily active user
• 日访问量：500 million write per day with average fan out 10 deliveries. 10 billion read per day.

第四问：基于假设的估计?

• tweet size: 10KB, 5TB per day
• 5700 write requests per second, 115K read requests per second, 57K deliveries on fanout per day.

第五问：如何做high level design?

• client -> web server -> write api / read api -> fanout service / timeline service -> cache -> sql db

第六问：如何设计关键部件？(基于user case)

• user posts a tweet -> write api -> stores user's tweet -> fanout service finds followers -> store tweet in follower's timeline -> notification service to notify followers (async)
• user views home timeline -> user service to get all user's followers -> fetch all tweets

第七问：如何scale?

• Load balancer
• Cache
• DB sharding