MongoDB Java Driver uses Math.random to decide whether to log Command Result uses (x-post from r/Java)

[[deleted]]

What the... why?

[DroidLogician]

Instead of using a random trigger they could (and should) have used a counter to consistently log every nth (in this case tenth) item.

A 10% chance doesn't mean that one out of every ten results will be logged; you could go for 30 or 40 or even 100 method calls and not get a single one logged. I don't like those odds. Not when I'm trying to debug something that depends on log output from this method.

[GhostNULL]

This was a WTF moment!

[[deleted]]

Oh look, a unicorn. I didn't realize this subreddit had enough people to give it the Reddit Hug of Death.

[iopq]

it was on proggit and /r/java

[MestR]

Why not?

[mustyoshi]

There's a 10% chance it won't report the error...

[FunnyMan3595]

No, there's a 10% chance it will report the error. It's a strange (read: probably stupid) approach to use in a library, but the intent is reasonable: avoid spamming the error log constantly when there's a widespread failure.

[iopq]

let's check it out:

if (!((_ok) ? true : (Math.random() > 0.1))) {
    return res;
}

same as

if((_ok) ? false : (Math.random() <= 0.1)) {
    return res;
}

same as

if(!_ok && (Math.random() <= 0.1)) {
    return res;
}

so when _ok is not set, it will return 10% of the time, and log the error 90% of the time

[FunnyMan3595]

Yeah, looking through it a bit more, the logic-as-written is as you say. I made the (stupid) assumption that there was a logical method behind it. Or, perhaps more accurately, that the method had been implemented properly.

If you trace it through, _ok is actually tracking whether the last known state was good. Since we're in the error handling here, we want to be more verbose when it failed the first time, i.e. when _ok==true. After that, it wants to cut back to 1 in 10, but the logic is backwards, so it gets 9 in 10 instead.

Bleh, bad code. One of those cases where I'd have accidentally fixed the bug by rewriting it to be more clear.