Homo-Adminus Blog

Since the day one when I joined Scribd, I was thinking about the fact that 90+% of our traffic is going to the document view pages, which is a single action in our documents controller. I was wondering how could we improve this action responsiveness and make our users happier.

Few times I was creating a git branches and hacking this action trying to implement some sort of page-level caching to make things faster. But all the time results weren’t as good as I’d like them to be. So, branches were sitting there and waiting for a better idea.
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We were using memcache in our application for a long time and it helped a lot to reduce DB servers load on some huge queries. But there was a problem (sometimes called a “dog-pile effect”) - when some cached value was expired and we had a huge traffic, sometimes too many threads in our application were trying to calculate new value to cache it.

For example, if you have some simple but really bad query like

which could be really slow on a huge tables, and your cache expires, then ALL your clients calling a page with this counter will end up waiting for this counter to be updated. Sometimes there could be tens or even hundreds of such a queries running on your DB killing your server and breaking an entire application (number of application instances is constant, but more and more instances are locked waiting for a counter).

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