Caching in Rails
Caching in Rails is kind of like C++, if you know what you're doing it's possible to make a small elegant solution, but if you get it wrong then god help you when you try to work out what's going on.
There are plenty of tutorials online and sections in books that go into the basics of caching and some of the finer points, but here are some particular problems I encountered.
Moving Parameters from Session
Most tutorials hammer in that Rails caching only works based on the URL, so session variables and parameters are completely forgotten. The original version of my site stored the user's preferred language in the session and retrieved the language data based on that.
By changing the language to be specified by the URL (e.g. benhumphreys.co.uk/en/...) instead of the session, I solved the caching problem. Then it was just a matter of updating the
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# routes.rb map.home ':language/', :controller => 'entries', :requirements => { :language => /(en|ja)/} map.en 'en', :controller => 'entries', :language => 'en' map.ja 'ja', :controller => 'entries', :language => 'ja' map.connect ':language/:controller/:action/:id', :requirements => { :language => /(en|ja)/ } |
In the same way as when you specify a link_to with only the action Rails assumes you're in the same controller/path, Rails assumes you're in the same language/path if you don't specify the language. So link_tos automatically point to the controller in the right language.
Get Your Paths Right
If like me you have your admin section in a subdirectory double-check that the path in your expire_page commands have a preceding backslash or you'll be trying to expire stuff in the same admin directory (thanks to rsl in #rubyonrails for helping me decypher this).
Also, as far as I know you have to specify every part of the route. When using link_to, Rails can use the current location to work out unspecified parts of the route. However as sweepers can be called from any location, you must specify all parts of the route (lines 21 & 24).
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class EntrySweeper < ActionController::Caching::Sweeper observe Entry # QUESTION: Does anyone know how to roll these 3 into one method? def after_create(entry) expire(entry) end def after_update(entry) expire(entry) end def after_destroy(entry) expire(entry) end private def expire(entry) # expire single page expire_page(:controller => '/entries', :action => 'view', :language => entry.language, :type => entry.type, :slug => entry.slug) # expire list page for type expire_page(:controller => '/entries', :action => 'list', :language => language, :type => entry.type) # expire general list expire_page(:language => language, :controller => '/entries', :action => 'list') # expire rss feed expire_page(:language => language, :controller => '/entries', :action => 'rss') end end |
Use the Logs!
If you ever don't know what's going on in your Rails app, be sure to check the logs. The same applies for when you're implementing caching. Make sure you turn on caching in config/environment/development.rb, and use the logs to work out what pages are being expired and whether the paths are correct.
Expired page: /en/articles/caching-in-rails.html (0.00367)
Expired page: /en/entries/list.html (0.00037)
Expired page: /en/rss.xml (0.00046)
I'm not sure how this compares with other people's experience, but developing on a iBook the time taken to expire existing pages (the first entry) and the time taken to expire non-existant pages (usually indicative of a bug) are noticably different and can help you debug. Not rocket science but I thought I'd point it out.
General Tips
Here are some general points to remember when implementing caching, some of them seem obvious but they might save you some time trying to solve unusual bugs:
render vs redirect_to
Cache files are created from the URL, irrespective of how the URL was reached. Remember that the following code will stay at the original URL.
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list render :action => 'list' |
Whereas the following changes the URL.
redirect_to :action => 'list' |
Multiple Paths to Enlightenment
If you have many ways of reaching the same content, e.g view/id and permalink, you need to be especially careful.
Final Word
I'm still a long way from knowing how to do caching on a large scale and I've yet to do detailed fragment caching, so I'd appreciate any corrections anyone has on the points I've made.
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