If you want invalid states unrepresentable, and time as a primary key... How do you deal with time regularly becoming non-linear within the realm of computing?

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josephg 5 days ago | parent | next [-]

The general answer is to accept that time isn’t linear. In a collaborative editing environment, every event happens after some set of other events based on what has been observed locally on that peer. This creates a directed acyclic graph of events (like git).

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shakna 5 days ago | parent [-]

That requires a different primary key than the time then, no?

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josephg 4 days ago | parent [-]

It requires a different primary key than an autoincrementing integer. One popular choice is to use a tuple of (peer_guid, incrementing integer). Or a randomly generated GUID, or a hash of the associated data.

Then each event is associated with zero or more "parent events".

- An event has 0 parents if it is the first change

- An event has 1 parent if it simply came after that event in sequence

- And if an event merges 2 or more branches in history, it says it comes after all of those events

You can also think about it like a set. If I know about events {A, B, C} and generate event D, then D happens-after {A, B, C}. (Written {A,B,C} -> D). But if A->B, then I only need to explicitly record that {B,C} -> D because the relationship is transitive. A -> B -> D implies A -> D.

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shakna 4 days ago | parent [-]

And the moment you need to merge, unrepresentable states become possible.

There are techniques to make it less painful, but it will still be possible.

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tempodox 4 days ago | parent [-]

You mean, like attempting to merge contradictory states? You will need some resolution stategy then, but in general that would be application-specific, and sometimes it may not exist.

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shakna 4 days ago | parent [-]

Okay... But we're now back to invalid states being possible. Tagging with time isn't enough.

	▲	josephg 4 days ago \| parent [-]
		It isn’t enough for what? What are you trying to do? There may be a way to solve whatever problem you have, but without specifics it’s impossible to tell.

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johnecheck 5 days ago | parent | prev | next [-]

It might be nice if our universe conformed to our intuitions about time steadily marching forward at the same rate everywhere.

Einstein just had to come along and screw everything up.

Causality is the key.

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throwawaymaths 5 days ago | parent | prev [-]

logical clocks

	▲	Phelinofist 5 days ago \| parent [-]
		Even with a hybrid logical clock they are not that useful in case of a network partition AND clock drift, no?