ROR汇集---memcached

转自：http://nubyonrails.com/articles/memcached-basics-for-rails

 

Many speedysites use memcached to save the results of expensive database queriesand intense rendered templates. This is a basic introduction to usingmemcached with Rails. Thanks to Eric Hodel and Layton Wedgeworth whohave answered many questions.

Yes, there is a hash in the sky

Memcachedis a lightweight server process that stakes out a fixed amount ofmemory and makes it available as a quick access object cache.

Some of the things you can do with it are:

• Automatically cache a row from the database as an ActiveRecord object
• Explicitly render_to_string and save the results (works well for tag clouds)
• Manually store a complicated database query and retrieve it later

I thought there was some kind of fancy voodoo happening, but it turns out that it’s basically just a hash! You can put a key and value into the cache, and get it later. memcache-client also gives you a delete method.

Objects are serialized using Marshal, so it’s very fast.

@tags = Tag.find :all
Cache.put 'all_your_tags', @tags

Cache.put 'favorite_skateboarder', 'Tom Penny'
skateboarder = Cache.get 'favorite_skateboarder'

Cache.delete 'all_your_tags'

Installation on Mac OS X

Ifirst installed the memcached server using DarwinPorts but each querywas taking five seconds to answer. It turns out that this can be fixed.I wrote a shell script that builds memcached and dependencies with theproper fixes. Use at your own risk…

memcached Install Script for Mac OS X

Other platforms can find the source or an installer at the memcached site.

Install cached_model

The Robot Co-Op has made a few libraries available. cached_model makes it easy to cache single row queries from ActiveRecord tables. memcache-client is a pure Ruby client and is included with the installation of cached_model.

sudo gem install cached_model

cached_model requires Rails 1.1.2 or greater.

Start the memcached server

The memcached server takes several options but the defaults work well to start with. While learning, the -vv argument shows what’s happening as values are entered and retrieved.

# Start the server with the default of 64 MB RAM
memcached -vv

Use with ActiveRecord

Theeasiest way to experiment with memcached is to make a new Rails appwith a single database table. Add the following code to the bottom of config/environment.rb:

require 'cached_model'

memcache_options = {
  :c_threshold => 10_000,
  :compression => true,
  :debug => false,
  :namespace => 'my_rails_app',
  :readonly => false,
  :urlencode => false
}

CACHE = MemCache.new memcache_options
CACHE.servers = 'localhost:11211'

NOTE: You should move this to config/environments/production.rb for production.

You’re almost ready. Edit your model so it inherits from CachedModel instead of ActiveRecord::Base.

class Foo < CachedModel
end

Try it out

Finally, start ./script/console and watch the output of memcached -vv. It also helps to tail -f log/development.log.

Create a few records, findthem by id, and watch as they are automatically placed in the cache.Finding the same record repeatedly should hit the cache, but not thedatabase.

>> Foo.find 1
=> MemCache Get (0.134546)  active_record:Foo:1
=> SELECT * FROM foos WHERE (foos.id = 1) LIMIT 1
=> MemCache Set (0.024758)  active_record:Foo:1

>> Foo.find 1
=> MemCache Get (0.032337)  active_record:Foo:1

If you save a record that is already in the cache, CachedModel will automatically update it in the database and in the cache.

CachedModel automatically creates unique keys based on the model name and id number. You can cache arbitrary data with the provided Cache object, but you will need to plan a consistent key system.

>> all_foo = Foo.find :all

>> Cache.put 'Foo:all', all_foo
=> MemCache Set (0.082277)  Foo:all

>> Cache.get 'Foo:all'
=> [#<Foo:0x2642ad4 @attributes=...

A few things to remember

• Cache.get will return nilif the item is not in the cache. You can use this to determine whethersomething should be refetched and re-inserted into the cache.
• Records in CachedModel expire after 15 minutes (or you can adjust the expiration time by setting CachedModel.ttl).
• Manually inserted objects can be expired, too. Cache.put('foo', 'bar', 60) will cause the foo key to expire in sixty seconds.
• When the allotted memory fills up, older items will be deleted.

Deployment details

Production use of memcached deserves its own article. In the meantime…

• Memcached is only useful for a dedicated server or a VPS that has sufficient memory available.
• Try running an instance of memcached on each of your app servers. Running with 128 MB is sufficient for most applications.

Other Resources

• Official memcached site
• Robot Co-Op Libraries
• Improving performance on Mac OS X
• Other memcached installation notes

参考：

http://www.ridingtheclutch.com/2009/01/08/cache-anything-easily-with-rails-and-memcached.html

http://www.gisblogs.net/post/183.html