Facebook News Feed

Facebook Timeline: Fanout read

Fanout method

Fanout write = Megafeed

Pro:

Con:

If you have N friends, your post will be repeatedly wrote N times. It's time and space comsuming.

Fanout read = Multifeed

Facebook chose Fanout read (Multifeed).

Write amplification makes the storage needs expensive in Megafeed
- Write amplification: suppose you have 300 followers, you sending a new post will result in 300 writes to the system. This changes the problem from a tractable dataset that can be saved in memory to a big disk storage problem.
Developing with read-time aggregation is flexible
- The developer can simply aggregate the data, apply their algorithms, and see the output right away. It would take longer with fanout write.
Memory and network easier to engineer around
- News feed launches in 2005 with fanout write. At that time memory and network weren't that good. The fanout read approach started 2007. Now the hardware is more powerful to support fanout read.
Never have huge fan-out write to do, only bounded (<10k) fan-out read
- Justin Bieber problem: with fanout write, when JB sends a new post, we need to write to a million different places. And because facebook's friend relationship is bidirectional, JB's news feed pool will be write a million times if each of his friends sends a new post. But nobody would read one million posts.

In-memory (mostly) databases
Do ~40 requests per feed query
- 10 billion querys per day to the aggregator. So ~400 billion requests per day, about 4.6 million QPS.
About 50% of the total LOC.

Must store a number of users on each leaf
Once we find a user, we want to scan his/her activity in time order
Want to have an easy way of adding new activity without locking
Most natural data structure is a hashtable to a linked list (map from userId to posts)

If FB is using fanout read for news feed generation, how does it handle new posts?

Last updated 3 years ago

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Fanout method

Fanout write = Megafeed

Pro:

just one thing to read

we can do pre-rank.

Con:

If you have N friends, your post will be repeatedly wrote N times. It's time and space comsuming.

Fanout read = Multifeed

Facebook chose Fanout read (Multifeed).

Write amplification makes the storage needs expensive in Megafeed

Write amplification: suppose you have 300 followers, you sending a new post will result in 300 writes to the system. This changes the problem from a tractable dataset that can be saved in memory to a big disk storage problem.

Developing with read-time aggregation is flexible

The developer can simply aggregate the data, apply their algorithms, and see the output right away. It would take longer with fanout write.

Memory and network easier to engineer around

News feed launches in 2005 with fanout write. At that time memory and network weren't that good. The fanout read approach started 2007. Now the hardware is more powerful to support fanout read.

Never have huge fan-out write to do, only bounded (<10k) fan-out read

Justin Bieber problem: with fanout write, when JB sends a new post, we need to write to a million different places. And because facebook's friend relationship is bidirectional, JB's news feed pool will be write a million times if each of his friends sends a new post. But nobody would read one million posts.

Leaf Nodes

In-memory (mostly) databases

Do ~40 requests per feed query