-
-
Save hinnerk/1992792 to your computer and use it in GitHub Desktop.
| import time | |
| def test_time(): | |
| collisions = 0 # number of collisions | |
| dist = {} # distinct values | |
| microsecond = 0.000001 | |
| count = 0 | |
| while count < 1000000: | |
| count += 1 | |
| t1 = time.time() | |
| time.sleep(microsecond) | |
| t2 = time.time() | |
| s1 = "%016.05f" % t1 | |
| s2 = "%016.05f" % t2 | |
| if s1 == s2: | |
| collisions += 1 | |
| else: | |
| diff = t2 - t1 | |
| dist[diff] = dist.get(diff, 0) + 1 | |
| percent = collisions / (count / 100.0) | |
| print "Collisions:\t%s (%s%%)" % (collisions, percent) | |
| print "Min value:\t%s" % min(dist.iterkeys()) | |
| print "Max value:\t%s" % max(dist.iterkeys()) | |
| print "Dist values:\t%s" % len(dist) | |
| if __name__ == '__main__': | |
| test_time() |
Cool on the disqus thing, we should probably move over to that. :)
What exactly happens depends somehwat upon the number of replicas Swift is configured for. If we assume the standard 3 replicas, you could theoretically end up with 3 different "answers" to the question: What is the data in this object? But, since all three were at the exact same collision time, there's really no way to know which is most correct anyway.
Many systems use a content hash to resolve these conflicts at sync, but early in Swift's development it was decided that highly concurrent single object writes weren't a use case for Swift needing such extra resolution steps.
For Swift, it would keep those multiple versions until the hardware on one failed, in which case it would get one of the other versions, etc. until there was just one version again.
Sorry for the missing way to comment, I've just now enabled disqus for the blog.
Regarding Swift: It's quite possible that I've drawn a wrong conclusion from my cursory glance on the code. Let me elaborate with an example: Assuming somehow two (or more) different entries get stored under the same id and timestamp (which due to screwed clocks may happen even without any concurrency). Does Swift preserve all entries?