Pitfalls of Rails fragment caching with memcached

Fragment caching is a powerful technique for improving performance of your web application. Rails site describes in detail how to apply this technique.

Rails are providing developers with really excellent abstractions, but it’s always good to know what’s under the hood and how it all works.

There are a few things that might potentially cause bugs in your code, or waste your time (speaking from my own experience). So here goes:

1. Beware of globally keyed fragments

Let’s take example from Rails tutorial:

<% cache do %>
  All available products:
  <% Product.all.each do |p| %>
    <%= link_to p.name, product_url(p) %>
  <% end %>
<% end %>

Now if you need to deal with a multi-language site you might want to make cache fragment language dependent. What might seem a convenient solution:

<%- cache([user.locale.to_s]) do -%>

will turn into a source of very interesting problems. While calling the cache method without parameters will automatically create a controller/action specific cache key, calling it with a key will make this fragment a globally keyed fragment. Cache key in the first case is going to look like “views/localhost:3000/controller-name”, and in the other case “views/en” – this is not as unique identifier any more.

While automatic cache key naming provided by rails is very convenient, it is very easy to run into a problem with duplicate cache key names used in different places.

2. Another pitfall of automatic cache key naming is that you shall never assume that when creating a cache with global key you can later find it using e.g. telnet interface to memcache. Example – add

<%- cache('unique_cache_key') do -%>
<%- end -%>

in your view and then try to read directly from memcache:

$ telnet localhost 11211
GET unique_cache_key

At the same time

GET views/unique_cache_key

will work. It’s easy to make this mistake trying to check or delete cache keys directly from memcache when using Rails cache methods.

3. delete_matched is not supported by memcached (see rails/activesupport/lib/active_support/cache/mem_cache_store.rb)

In practice that means that if you’re using memcached as Rails cache engine and trying to delete or expire fragment cache using standard Rails methods and regexp – you’ll fail.


will fail miserably. Ideal solution is not to use explicit cache expiration, but rather create cache keys in such a way that doesn’t require expiration. Alternatively it’s possible to use extensions implementing delete_matched for memcached (haven’t tried it myself though).

Tip: one very useful tool for checking memcached is peep by Evan Weaver – allows you to peek into the cache and see what’s really cached and how it is used.


Notes from Gothenburg – Nordic Ruby 2011 conference

Here are my notes from Nordic Ruby conference in Göteborg, Sweden.

I’d like to say big thanks to the organisers of the conference (especially CJ @cjkihlbom) – everything went really smooth, even though there’s been 150 people attending this year compared to 90 last year.
Some points that I’d really like to highlight are:

  • a lot of time to meet people and discuss: 30 minutes talks followed by 30 minutes breaks, no q&a – those who had questions had an opportunity to talk to the speakers during the breaks
  • venue was great (of course, the boat 🙂 – there was enough space for everyone to move around, but at the same time it was compact enough not to get lost also everyone had an opportunity to have lunch and dinner together
  • “job board” a huge white board where anyone can post information about open positions in their companies – it got filled withing firts few hours – job market is really hot
  • lightning talks that any participant can give – 5 minute talks in the end of the day – it was really great
  • real coffee 🙂 espresso, latte, cappuccino, americano – you name it – professional baristas were at your service
  • 5K Nordic Ruby run organised on the second day’s morning

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vim is the best editor, also for RoR development

vim is a natural choice when you’re starting a new programming project (if you’re emacs or textmate adept – you can stop reading now 🙂 If you’re starting a Ruby on Rails project there are a couple of scripts/configurations you might want to install to make development with vim an even more pleasant experience.

1. Rails.vim by Tim Pope

To install just copy autoload/rails.vim, plugin/rails.vim, and doc/rails.txt to corresponding directories in ~/.vim

Vim scripts section has a full description of the functionality. Some highlights:

  • Easy navigation between models, controllers and views with :Rmodel, :Rview, :Rcontroller commands
  • Syntax highlighting
  • CTRL-X CTRL-U for autocompletion
  • :Rtree for project tree (see item 2)

2. NERD Tree by Marty Grenfell

Another must-have. Provides you with an easy way to navigate your project tree. Rails.vim nicely integrates with this one.

3. Colour schemas

If you want to save your eyes – use dark background when developing. Personally I prefer one of the standard vim themes – Desert , but Ocean Deep is also very good.

Copy theme file to ~/.vim/colors and then use

:colorscheme oceandeep

command to apply it.


Installing Flash player browser plugin in 64-bit Ubuntu 11.04

I got a new hard drive for my laptop and decided to make a leap of faith and move to 64-bit version of Ubuntu, since I had to install a fresh system anyway.

In case you didn’t know – Adobe doesn’t have stable Flash player version for 64-bit Linux. Adobe Labs offer preview release codenamed “Square” for 64-bit platforms. It can’t be installed via standard Ubuntu repositories, so get ready to get your hands dirty in the terminal.

To install 64-bit Flash player plugin do the following:

1. Download the latest version of the plugin at http://labs.adobe.com/downloads/flashplayer10_square.html (currently it is v.10.2 preview 3 from November 30th, 2010)
2. Go to your downloads directory and extract the plugin binary

tar xvzf flashplayer10_2_p3_64bit_linux_111710.tar.gz

3. Create a directory for browser plugins in your users home directory

mkdir -p ~/.mozilla/plugins

4. Move extracted in step 2 Flash player plugin binary to its new location

mv libflashplayer.so ~/.mozilla/plugins

5. Close all browser windows and restart the browser.

6. In Firefox or Chrome go to about:plugins to verify that there’s Shockwave Flash plugin available



Increasing Ruby interpreter performance by adjusting garbage collector settings

According to Evan Weaver from Twitter it is possible for a typical production Rails app on Ruby 1.8 to recover 20% to 40% of user CPU by simply adjusting Ruby garbage collector settings. In August I set out on a quest to verify that statement on HeiaHeia servers. Results have really exceeded my expectations. Time to execute application tests locally decreased by 46%. On production servers CPU utilisation decreased by almost 40%.


Call to undefined function: imagecreatefromjpeg()

While installing new Joomla modules I came across this PHP error (yep, still have to deal with PHP occasionally). I had PHP compiled from source on Ubuntu 10.04 as per earlier instructions. Quick check of phpinfo() indicated that while gd module was compiled in, it didn’t have JPEG support:

GD Support         enabled
GD Version         bundled (2.0.34 compatible)
GIF Read Support   enabled
GIF Create Support enabled
PNG Support        enabled
WBMP Support       enabled
XBM Support        enabled

Making sure that JPEG libraries are installed

sudo aptitude install libjpeg libjpeg-dev

and reconfiguring PHP with –with-jpeg-dir flag (the rest of the compilation process remains the same as here)

./configure --enable-fastcgi --enable-fpm --with-mcrypt --with-zlib 
--enable-mbstring --with-openssl --with-mysql --with-mysql-sock 
--with-gd --without-sqlite --disable-pdo --with-jpeg-dir=/usr/lib

and then restarting nginx

sudo /etc/init.d/nginx restart

helped to solve the problem.


Running Rails applications using Nginx with Passenger on Ubuntu Server

If you’re planning to run Rails applications on Nginx using Phusion Passenger, and do it on Ubuntu Linux, here’s what needs to be done.

Even though there’s Ubuntu nginx package available (which works perfectly when you’re running PHP apps using FCGI), if you want to take into use Phusion Passenger, you’ll need to recompile Nginx from sources.

Instructions below were verified on Ubuntu 10.04 (Lucid Lynx) Server Edition.

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Moving Joomla, WordPress and other PHP/FastCGI apps to Nginx

Have you moved your site from Apache to Nginx and now your FastCGI (php-cgi/spawn-fcgi) processes die/hang/crash periodically and your users see “HTTP 502 Bad gateway” or “HTTP 504 Gateway timeout” instead of a website?

I have faced this problem and found a relatively simple and robust solution. Here’s how I did it on Ubuntu 9.10 (Karmic Koala) and 10.04 (Lucid Lynx) server edition.

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Setting up your own git server on Ubuntu

This will create a new user ‘gitosis’ and prepare a structure for repositories in /srv/gitosis. Now let’s initialize a gitosis-admin repo – it is used for managing repositories and access

Of course there’s always an option to use github. And if you’re working on an open source project, or want to concentrate on coding and not system administration, github is a lot better option than setting up and managing your own git server (I’ve been so impressed by @defunkt‘s presentation on #frozenrails, that started recommending github to everyone 🙂 But if you already pay for a virtual machine somewhere (like Linode), then setting up your own git server might be a viable option, especially that it is sooo easy.

The following instructions have been verified on Ubuntu 10.04 Lucid Lynx, but should work at least on Ubuntu 9.04 and 9.10 just as well.

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Informal notes from #frozenrails 2010

Thanks to organisers from Kisko Labs and the HHLinuxClub on Friday, May 7th, 2010 Finland got its first  Rails conference.

Conference has drawn very interesting speakers and  international crowd – from Finland (naturally), Sweden, Poland, Germany, Russia and other countries. I made a few notes from selected talks on the conference.

Chris Wanstrath / GitHub (@defunkt)

  • Slides: http://www.slideshare.net/err/inside-github
  • Linus Trovalds Google tech talk about git – where Linus tell you that you’re stpid if you’re not using git
  • rack-mobile-detect – is used by GitHub, super useful if you’re planning to create mobile optimized version
  • GitHub uses Unicorn as an application server – personally I’m not sure if that’s better than Apache + Passenger. Chris tells that Unicorn is cool, because does fair load balancing on Linux kernel level, also Rails are loaded only once – and then required number of processes are forked – and this is very fast, a lot faster than loading rails separately for each Mongrel. And when one of the processes dies – there’s no need to re-load Rails, but just fork another process.
  • GitHub users BERT to forward requests to one of their six servers – BERT to Erlang is the same as JSON to JavaScript
  • GitHub doesn’t use delayed_job anymore since they needed several queues with different priorities – so far they use resque, but are considering developing a real queue management system

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