Ask HN: How does Twitter implement following and followers between users?

In 2010, Twitter was using a graph database, FlockDB, instead of a relational database to implement their social graph where relationships like following and followers are represented as edges between nodes that represent people. It's since been deprecated, and I'm not sure what type of database they're using today. Anyway, they have a nice write-up blog post [1] and FlockDB is open source [2].

[1]: https://blog.twitter.com/engineering/en_us/a/2010/introducin...

[2]: https://github.com/twitter-archive/flockdb

--- USER TABLE ---

| USER_ID |

| USERNAME |

| NB_FOLLOWERS |

| TWEETS -> List of TWEET_ID's |

| FOLLOWING -> List of USER_ID's |

| FOLLOWED_BY -> List of USER_ID's |

etc.

NB_FOLLOWERS, FOLLOWING and FOLLOWED_BY are updated when a USER follows/unfollows another USER.

User1 follows User2

User1 follows User3

User3 follows User1

User3 follows User5

...