Flowchart

• Pull-based

  • Use case

  • Flowchart

  • Cons

• Push-based

  • Use case

  • Cons

  • Flowchart

• Push and pull combined

  • Oscillation problems

Pull-based

Use case

• Single direction relationship

  • Star users

• High latency

Flowchart

Cons

• Get news feed: N DB reads + K way merge (KlogK, K is the number of friends)

  • Bottleneck is in N DB reads, although they could be integrated into one big DB query.

  • This result in high latency because it needs to wait until N DB reads finish

Push-based

Use case

• Bi-direction relationship

  • No star users: Users do not have a lot of followers

• Low latency

Cons

• When number of followers is really large, the number of asynchronous task will have high latency.

• Post a tweet: N followers, N DB writes. Executed asynchronously.

Flowchart

Push and pull combined

• For users with a lot of followers, use pull; For other users, use push.

• Define a threshold (number of followers)

  • Below threshold use push

  • Above threshold use pull

• For popular users, do not push. Followers fetch from their timeline and integrate into news feed.

Oscillation problems

• May miss updates.

• Solutions:

  • Star users: Pull not push

  • Half star user: Pull + Push

  • Normal user: Push