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Comment by klysm | Hacker News Reader

In my experience, most developers don't even consider isolation level in the first place and just take whatever the default is. Any race conditions are met with an 'oh that's weird', and then they move on.

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I wish I could argue with you, but the highly successful early years of MongoDB proves your point nicely.

Mongodb did not have those concerns because work on a single document is atomic and there are no joins.

This depends on what you mean by "atomic". Prior to 5.0, MongoDB's defaults were to use a sub-majority write concern. This allowed all kinds of interesting atomicity violations, even on single documents. For instance, you could write a value, some clients might read it, and then your write would be silently lost as if it never happened. Clients could disagree on whether the write happened or not. A single client could observe, then un-observe the write. It gets weird. :-)

This is actually an evolved model, as the system has to handle data loss as a primitive upfront.

Similar to how Rust forces you to reason about allocators.

Nah they're super different. Rust failing at compile time means you fix the bug before you ship it, which is helpful. MongoDB failing in production means either you fix the bug via testing before shipping, or you discover the bug after shipping, which is at best extra work and at worst disastrous.

Are you saying data loss is a positive feature of MongoDB because it forces systems that use it become more robust to deal with the data loss?

Yes

But this is not a design problem, all clustered DB work like that this is more a configuration issue.

Um, no they don't?

When you have a multi master setup, usually they all work the same, see Cassandra:

https://docs.datastax.com/en/cassandra-oss/3.0/cassandra/dml...

https://opensource.docs.scylladb.com/stable/cql/consistency....

Exactly! That's why I just commented [1] that the default isolation level should be serializable.

[1] https://news.ycombinator.com/item?id=38696421

Completely agree, I've made that exact same argument on HN before.

It becomes so hard to reason about isolation issues that most things below serializable consistency will lead to you getting bitten in various ways, so I would argue most developers shouldn't even consider isolation levels. And that MySQL and some others provide too little guarantees for the average dev.

In my experience almost no developer considers consistency at all.

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