[bdamm]

There are many systems which can work just fine in an eventually-consistent manner. A database of people (customers, users, etc) is a classic example of such a thing.

In general I think consistency is over valued. There are plenty of cases where it is important. Lots of people are brainwashed in college to think that all data must be consistent all the time, and that's just not necessary.

[logicallee]

>Lots of people are brainwashed in college to think that all data must be consistent all the time, and that's just not necessary.

I knew it! So a foolish consistency is the hobgoblin of little minds.

[olau]

While I agree strict consistency is probably overkill in many if not most situations, the problem with not having consistency is that it potentially makes the application logic much more complicated.

Take the database of customers - so if you don't have consistency, what happens in case someone changes the company address and another person simultaneously requests a delivery of something. Do you risk ending up with half of the old address and half of the new one on the parcel?

Note you can certainly have this problem in a consistent system too, e.g. if you make a UI without a save button where the address is changed one field at a time.

Concurrency is just intrinsically hard.

[nostrademons]

Note that the real world operates like this too: before computers, if someone changes their address and simultaneously sends a package, the package probably will end up at the wrong address. We have a number of mechanisms in place to mitigate this when it occurs (address forwarding, return-to-sender, customer support, credit card chargebacks), but they still don't always work, and sometimes packages just get lost.

The real world solution to this is the acceptance that yes, sometimes bad things happen for no reason at all. I suspect that the computer world will eventually move to this as well, with consumers becoming more tolerant of machines that simply give the wrong answer some of the time, as long as they give the wrong answer less frequently as a human would.

[chousuke]

"Lack of consistency" here doesn't have to imply lack of atomicity; In the case of your example, an eventually consistent system could return either the old address or the new address at some point in time X, but never a mixture of both. In your situation, the parcel could end up getting delivered to the old address, but there would be no corruption.

[threeseed]

Couldn't agree more. Most people don't know that many critical systems e.g. banking are eventually consistent and rely on other methods.

The concepts of CQRS and Event Sourcing really need to be taught at universities.