Homo-Adminus Blog

Having a reverse-proxy web cache as one of the major infrastructure elements brings many benefits for large web applications: it reduces your application servers load, reduces average response times on your site, etc. But there is one problem every developer experiences when works with such a cache – cached content invalidation.

It is a complex problem that usually consists of two smaller ones: individual cache elements invalidation (you need to keep an eye on your data changes and invalidate cached pages when related data changes) and full cache purges (sometimes your site layout or page templates change and you need to purge all the cached pages to make sure users will get new visual elements of layout changes). In this post I’d like to look at a few techniques we use at Scribd to solve cache invalidation problems.

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It’s been a while since I’ve posted my first post about the way we do document pages caching in Scribd and this approach has definitely proven to be really effective since then. In the second post of this series I’d like to explain how we handle our complex document URLs and logged in users in the caching architecture.

First of all, let’s take a look at a typical Scribd’s document URL: http://www.scribd.com/doc/1/Improved-Statistical-Test.

As we can see, it consists of a document-specific part (/doc/1) and a non-unique human-readable slug part (/Improved-Statistical-Test). When a user comes to the site with a wrong slug in the document URL, we need to make sure we send the user to the correct URL with a permanent HTTP 301 redirect. So, obviously we can’t simply send our requests to the squid because it’d cause few problems:

• When we change document’s title, we’d create a new cached item and would not be able to redirect users from the old URL to the new one
• When we change a title, we’d pollute cache with additional document page copies.

One more problem that makes the situation even worse – we have 3 different kinds of users on the site:

1. Logged in users – active web site users that are logged in and should see their name at the top of the page, should see all kinds of customized parts of the page, etc (especially when a page is their own document).
2. Anonymous users – all users that are not logged in and visit the site with a flash-enabled browser
3. Bots – all kinds of crawlers that can’t read flash content and need to see a plain text document version

All three kinds of users should see their own document page versions whether the page is cached or not.

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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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