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Comment by user5994461 | Hacker News Reader

> I think you're misunderstanding what the problem is here. I want global consistency (really I want two party consistency) to prevent the said user from witnessing the unkind thing I posted about them.

You don't want global consistency and you're saying it yourself.

What you want is to be presented with a view that is consistent with the actions you've done. [it's really basics101 for user experience.]

The internals of the system don't matter to you. It only matter to us, as the builders. We can make it so that it is internally 'inconsistent'[0] yet it always presents a 'consistent'[0] view to the user.

[0] I don't think the word consistent describes well the situation. Nothing is ever strictly perfectly consistent.

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You seem to be assuming that I'm a UX designer. I'm not, I'm a backend engineer. Just a note though: I wouldn't say something like "The internals of the system don't matter to you. It only matter to us, as the builders." to a UX designer, since that's super condescending.

Sequential consistency is basically the most lax model you can get away with in this case (and have it generalize to other similar operations). This is a very strong consistency model and weakening linearizability to this basically provides no advantage for availability. There is a proof that anything stronger than causal consistency cannot provide "total availability", which I'd argue is somewhat mythical anyways (except when you're colocated with where data is being stored).

My point here is that this is clearly a case where many of the strongest ordering guarantees are required to get the desired behavior, and subsequently availability must be sacrificed.

I was making the distinction on being the user VS being the builder of the system. The user doesn't need to know or understand the internals.

> consistency... consistency... consistency

What if I am not aiming for consistency in the first place?

We both understand that "total availability" is mythical. So why not target partial availability in the first place?

I'm not saying you should not have availability. On the contrary, many strongly consistent algorithms actually have very good availability characteristics. The whole point of being distributed is often to increase availability. Algorithms like raft and multi-paxos can provide availability so long as a majority of nodes remain reachable to the client, which I think is pretty acceptable in many cases. This is sort of why the availability vs consistency dichotomy is a false dilemma. Sure, having linearizability means you cannot have total availability and vice versa, but generally you can have very good availability and preserve strongly consistent semantics. What I disagree with is that trading strong consistency to achieve total availability is useful in many domains. To be fair, there are some, and for write-heavy workloads that tend to materialize a readable view in the future or for things like immutable caches, it can be extremely useful to weaken consistency, I just have often found that for the bulk of business logic, weaker consistency semantics make unacceptable tradeoffs to users.

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