SpaceCadet

ACID is really just an arbitrary set of requirements for databases that made sense way back in the day when things were much simpler. ACID starts to hold you back when you want to scale out, because to have consistency you have to wait for your transaction to percolate through all the nodes of your system, and it doesn’t allow for things like a replicating node to be temporarily offline or lagging behind. Turns out though that not everything needs to be strictly ACID. For example, there are many cases where it doesn’t matter that a reader node has stale data for a second or two.

The thing MongoDB does is that instead of being dogmatically ACID all the time it allows you to decide exactly how ACID your transactions and your reads need to be, through the writeConcern and readConcern parameters. If you want it to be completely ACID, you can, but it comes at a cost.

Traditionally, ACID is where relational databases shine.

Relational databases shine with ACID on single-node systems when they’re not trying to solve the scale-out problem that MongoDB is trying to solve, but when they are trying to do that, they actually do much worse.

For example: most RDBMS systems have some kind of replication system, where you can replicate your transactions to one or more backup nodes either for failover or to use as a read-only node.

Now if you consider that whole system, replicas included, as “the database”, none of them are ACID, and I don’t know of any RDMBS-es that has mechanisms to automatically recover from a crashed primary without data loss, or that can handle the “split brain” problem.

thinking of purging it all and starting over.

Don’t do that. You’ll learn nothing.

Administrators can disable this, so I think the larger point is: if a tech literate person receives a zip file, they understand that it is in fact a compressed archive that can contain one or more files and directories, and that you need an archive tool to extract the contents, whereas a tech illiterate person doesn’t understand this and expects it to just be handled magically when they double click on it and are stumped when that doesn’t work.