MongoDB found a bug in the retry mechanism which they think is responsible for the issues we found in 4.2.6--a fix is scheduled for 4.2.8!

MongoDB rewrote their Jepsen page, but it still goes to great lengths to avoid talking about write loss or aborted reads, both of which are the default behavior:

I keep thinking about their VLDB paper which says ~80% of writes to MongoDB's hosted service don't set a write concern, and 99.6% of reads don't set a read concern.


If users are really aware of, and OK with, with write loss by default (presumably because the probability of failure is small or the impact is low) then it should be fine to talk about it. If users *aren't* aware of this behavior, but most are subject to it by accepting defaults, then of *course* you should educate people about it!

Or, you know, choose safer defaults. That's an option!

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I called this out in the report as well, but the write concern documentation still doesn't say anything about rollbacks/write loss:

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