Scope: make key more random

[davidrans]

I think it is actually possible there's cache collisions happening here. The Scope key was only an 8 character hex value.

If the number of things we're caching under one cache scope is in the hundreds of thousands or millions I think there could be multiple collisions:
https://en.wikipedia.org/wiki/Birthday_problem#Probability_table

qa_req 0 -- Nothing really to QA here.

[davidrans]

@sterlinghirsh also suggested adding the time() to the key, as another way of making collisions less likely.

Something like:

time() . substr(md5(microtime() . $this->scopeName), 0, 8);

I'm good with either, I just didn't want to do both and make this prefix unnecessarily long, since memcached does have a max cache key length.

[jarstelfox]

CR 🌵

[danielbeardsley]

CR 👍

[sctice-ifixit]

I think it is actually possible there's cache collisions happening here. The Scope key was only an 8 character hex value.

If this is the case, shouldn't we be able to pretty easily validate that we get the same key for two different wikiids? I'd feel a lot more confident that we've identified the problem if we could demonstrate it.

[davidrans]

I'd feel a lot more confident that we've identified the problem if we could demonstrate it.

Agreed. I tried and could not manage to demonstrate it.

[danielbeardsley]

Collisions are easy to make if you boil it down (assuming 100k wikis):

[danielbeardsley]

Here's that one-liner:

php -r 'for($x=0; $x<100000; $x++) {echo substr(md5(microtime() . "wiki-scope-$x"), 0, 8) . "\n";}' | sort | uniq -c | sort -nr | head

[sctice-ifixit]

Collisions are easy to make if you boil it down (assuming 100k wikis):

I already know that if I generate enough random 8-character hexadecimal strings I'll get collisions, but I'm interested in validating the actual application behavior. That is, does the application actually end up generating keys with the same properties as your model? Why are the errors we see biased toward the tr and zh view languages? Why haven't we been able to see an actual collision for the wikis that end up generating the errors we're seeing?

[danielbeardsley]

That is, does the application actually end up generating keys with the same properties as your model?

I think that part is trivially proven given the code in this class. How often it happens in practice is dependent on how long keys stay in memcache and how many wikis we access.

Why are the errors we see biased toward the tr and zh view languages?

Woah, TIL! That indicates it's caused by something else.

Why haven't we been able to see an actual collision for the wikis that end up generating the errors we're seeing?

I'm not sure what this means. Collisions are fleeting, not permanent (the values are stored in cache).

[sctice-ifixit]

I'm not sure what this means. Collisions are fleeting, not permanent (the values are stored in cache).

Go to https://zh.ifixit.com/Device/Nikon_D80 (wikiid 7827), and note that you're shown a user profile (wikiid 262147).

php > var_dump(WikiLib::getWikiById(7827, 'zh')->wikiid);
int(262147)

So long as this behavior is reproducible, if it's caused by a cache collision, I expect to be able to get the same key when trying to look up both of those wikis from the cache. So far, I haven't been able to do that.

[davidrans]

I never busted the cache after pushing this fix. I did that a few minutes ago (at the risk of blowing away our reproduction cases) to see if we get any new instances of the error.

[danielbeardsley]

So far, I haven't been able to do that.

That's a great experiment. David and I played with it a bunch on a live machine and noticed that the cache prefix values were still short because they hadn't been expired since we deployed the change to scoping.

Also, if two scopes (A and B) point to the same prefix value (and thus are writing to the same keys) B can move on, update its prefix and the bug may remain because A still points at the values that B wrote, even though B points somewhere else now.

[sctice-ifixit]

Also, if two scopes (A and B) point to the same prefix value (and thus are writing to the same keys) B can move on, update its prefix and the bug may remain because A still points at the values that B wrote, even though B points somewhere else now.

Agreed. I thought it would be unlikely that, having checked 3 collisions, the wiki that caused the collision would have a new prefix value in all cases, but maybe I was unlucky, or maybe it's not unlikely at all.