How to design the push and pull of News Feed service for twitter/facebook/ig/RSS system?

we need to know what are the key points of the news feed system?
follow and unfollow
the new feed for everyone is different.

我们有两种模型来构建news feed: pull(主动) push(被动)

Pull model:
when user looking for his news feed, the system should get the latest 100 tweets of each of his friends and combined to 100 news feed(based on the timestamps)
algorithm: getNewFeed+ Merged K sort arrays.

getNewsFeed(request)
• followings = DB.getFollowings(user=request.user) //get the following
• news_feed = empty
• for follow in followings: //iterate the following
• tweets = DB.getTweets(follow.to_user, 100) //get the tweets of each one of them
• news_feed.merge(tweets) //merge them
• sort(news_feed) //merge k sort arrays them by time stamps
• return news_feed

time complexity analysis: suppose we have N following, so there will be the time of N DB reads+K merge time
the principle of Pull:

Push Model:
push happens when there are new tweets are being posted.
algorithm:

time complexity analysis:

• postTweet(request, tweet_info): //异步执行
• tweet = DB.insertTweet(request.user, tweet_info)
• AsyncService.fanoutTweet(request.user, tweet)
• return success

• AsyncService::fanoutTweet(user, tweet)
• followers = DB.getFollowers(user)
• for follower in followers:
• DB.insertNewsFeed(tweet, follower)

the principle of push:

如何选择pull还是push呢？
根据存在即合理原则，每一种都有实际应用：(主动型占优)
facebook: pull
ig: push+pull
twitter: pull
可以话说回来 如何选择呢？每一种都可以，但是要针对每一个的缺点要有解决方法 这个再系统设计4S中的最后一步Scale中解决。
具体请参见本博主的《System Design Divide and Conquer, use Twitter Design as an example》

特殊的情况：

1. 明星用户，适用push model的话 fanout的整个过程可能需要几个小时，既然Push不行 那么就换成pull? 可是哪有那么简单啊 想换就换！
   正确的思路：首先尝试在现有模型下做最小改动来进行优化 比如加几台用于做Push任务的机器。或者对长期的增长进行评估来判断是否值得转换整个模型。
   比较好的答案：针对普通用户 我们只Push 但是针对明星用户，我们不主动给push给所有的follower,而是当其关注着需要的时候 来到明星用户的timeline里面取i，并合并到newsFeed里面。
   2.明星用户 但是其粉丝数量出现摇摆 出现减小或者别的情况
   仍然Push

下面总结一下常见的什么时候用pull/push
push: 资源少，实时性要求低 用户发帖数量少 双向好友关系（即没有明星，比如朋友圈）
pull:资源充足 实时性要求高 用户大量发帖 有明星问题