Re: 掛掉了 - Flickr

※ 引述《ericapricorn (為彥)》之銘言：
: 剛剛上站　
: Flickr已經回覆正常囉
: ※ 引述《enohs (在光圈先決之外)》之銘言：
: : 7:45PM PST Update: We've got what we hope is a suitable solution to the
: : problem. Double checking things now!
: : 8:30PM PST Update: We're back again. And again, a SHIFT + REFRESH should
: : clear up any remaining image weridness. Please accept our apology for this
: : extended outage.
: : 大家再等等吧

今天的結論
http://blog.flickr.com/flickrblog/2007/02/crapola.html

Tonight's problems - an explanation

[Flickr is now back up, but this is still probably a useful explanation for
many people.]

While the site is still down and everyone else is working on it, I thought
it'd be a good time to give a more thorough explanation of what is going on.
Earlier tonight, people started seeing strange photos in place of their own
about 1/7th of the time.

This was the result of our caching servers returning random photos each time
they got asked. The caching servers (called "photocaches") are a thin layer
of servers which sit between your browser and our primary storage. They store
the most recently requested photos in a way that's quick to access in order
to speed up serving the photos you see on the site.

To explain the problem, a little background on how Flickr works is required:

Flickr serves hundreds of millions of photos each day (on the highest traffic
days, just over a billion photos are served). Because relative to other
computers components like memory (RAM) or processors (CPUs), reading from
disks is relatively slow -- and randomly accessing hundreds of terabytes of
storage is both slow and a strain on the primary storage servers -- it
wouldn't be possible to run Flickr without this caching layer.

Each photo has a unique address (or URL). This is what your browser uses to
request a particular photo. It knows the address from the web page which is
produced by the "application layer" (the "program" or software that runs
Flickr") based on data stored in the database.

The database knows whose photos are whose, what permissions everyone has,
what comments have been left and by whom, etc. In contast, the storage and
the caches are "dumb": they just store the 1s and 0s that represent your
photos.

Tonight's problem was a result a few of the photocaches going berzerk and
instead of returning the correct image file when a particular photo was being
requested, it just returning some random image that happened to be in the
cache. The result was web pages which had some correct photos, and some
random ones. And the random ones would change when you reloaded the page.

This is not a permenant problem: the primary storage, the database and the
software that runs Flickr is all fine. The problem was with the internal
directory of a few photocaching servers - the bit that keeps track of which
image files correspond with which photo URLs (and therefore items in the
database).

To be clear, we regard this as a serious problem, but it is something that
goes away as soon as we restart the malfunctioning servers (tonight we found
that the servers were going insane again shortly after restarting, but we
have isolated the problem and believe we have a permanent fix).

We want everyone to understand that there are no permanent problems with any
data, we have not been "hacked" and you don't need to do anything in order to
have your photos return to normal (though you might need to do a "hard
refresh" in order to clear your web browser's internal image cache where the
wrong photos might still be stored). In particular, you do NOT need to
delete, replace or reupload any photos.

We shamefacedly apologize for the inconvenience and the scare. We understand
that it probably seems very, very strange and we know that many people got
the impression that their photos were lost forever. But they should all be
back now, safe and sound. And everyone who works on Flickr's engineering and
technical operations teams are working double time to ensure that it never
happens again. Thanks for your understanding and patience!

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