[Facebook] Eventually Consistency

Facebook is using eventually consistency to update posts on the walls or comments of pictures, etc. Here’s the definition of eventually consistency on Wikipeida

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Eventual consistency is one of the consistency models used in the domain of parallel programming, for example in distributed shared memory, distributed transactions, and optimistic replication^[1][2].

The term itself suggests the following definition: Given a sufficiently long period of time, over which no updates are sent, we can expect that during this period, all updates will, eventually, propagate through the system and all the replicas will be consistent. While some authors use that definition (e.g., the Vogels citation above), others prefer a stronger definition that requires good things to happen even in the presence of continuing updates, reconfigurations, or failures. In the Terry et. al. work referenced above, eventual consistency means that for a given accepted update and a given replica eventually either the update reaches the replica or the replica retires from service.

In database terminology, this is known as BASE (Basically Available, Soft state, Eventual consistency), as opposed to the database concept of ACID ^[3].

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This model is apparently in contrast to immediate consistency, which updates any single new change. It’s a good way to save the frequency of notifications, storage space, and make it easier to get to know the overall status of a post. However I’m not sure Facebook is doing it well. Sometimes I won’t see a post after 12 hours of the initial one that I totally miss the chance to congratulate people on their engagements or something. Like I said before usually it’s not a big deal but it’s just really annoying.