System Requirements

Functional requirements:

1. The user can post and share tweets
2. The user can like/favorite tweets
3. The user can see home timeline
4. The user can see other user's timeline

Non-functional requirements

1. Availability: Each request should get a response without error, without the guarantee that the data is the most recent
2. Consistency: Eventual consistency is chosen
3. Partition tolerance: The system should still operate even if some message are dropped due to the network between nodes
4. Low-latency: The user can see their timeline within 500ms

Capacity Estimation

Assumption:

200 million DAU, each user post 3 tweets per day = 600 million tweet per day.

Each tweet with 140 bytes as content and 30bytes as metadata, and 20% of them contains photo 20KB, and 10% of contains 2Mb video.

Each user read 5 times hometimeline and 5 times other user's timeline, each timeline contains 20 tweets.

Data storage:

so the total size will be: 600m * (170bytes + 20kb * 30% + 2Mb * 10%) = 180TB per day

Bandwidth: 200 million * (5 + 5) * 20 * (140 bytes + 10 % * 2Mb + 20% * 20kb) / 86400 = 120 GB/s

API Design

1. createTweet(userToken, String tweetcontent) -> response status code
2. hometimeline(userToken, int pagesize, optional int pageOffset: indicating current page location) -> tweets list
3. user timeline(userToken, int userId, int pagesize, optional int pageOffset) -> tweets list
4. likeOrUnlikeTweet(userToken, int tweetId, boolean likeOrDislike) -> response status code

Database design

I'd choose MongoDB as our database, because:

1. The tweets data are 180TB per day, a lot of data.
2. The low latency is our requirement.
3. We have horizontal scalability needs.

Database design:

Tweet:

TweetID: Integer, primary key

content: Varchar(140)

Metadata: Varchar(30)

....

User:

userId: Integer, primary key

email: varchar(30)

isHotUser: Boolean

Follower:

followerUserId: Integer

FolloweeUserId: Integer

FollowingDate: Timestamp

High level design

Request Flow

Can see from the high level diagram