Guide to Caching in Rails using Memcache 1

The post intends to cover the topics and tools that can help in implementing Memcache as caching store in Rails and debugging issues with it.

To help newbies grasp it from start, it also provides references to installing and validating the memcache install without Rails too.

“Memcached is an in-memory key-value store for small chunks of arbitrary data (strings, objects) from results of database calls, API calls, or page rendering”

Installing Memcache

  • Mac OS X Snow Leopard
    • brew install memcached
  • Ubuntu
    • sudo apt-get install memcached

More information about Installing Memcache can be found here

Starting Memcache

  • memcached -vv

-v option controls verbosity to STDOUT/STDERR. Multiple -v’s increase verbosity. Memcache Settings can be inspected in detail using echo "stats settings" | nc localhost 11211. Get the thirst quenched more at http://linux.die.net/man/1/memcached or man memcached for all options.

Connecting to Memcache Server through Telnet

  • telnet localhost 11211 can be used to connect locally to run/inspect memcache data store
  • Its recommended to follow Using Memcache Commands to get a full hang of memcache commands
  • For the sake of completeness, here are three main commands (store/retrieve/remove key/values)
    • Write to Memcache (Value need to be entered in new line)
      • set abc 0 60 5 # where abc is the key, 60 is duration in seconds and 5 is length of data
      • hello # data to store in abc key
    • Read from Memcache
      • get abc
    • Delete from Memcache
      • delete abc

Implementing and Using in Rails

Installing Dalli (Memcache Client)

Configure Memcache as cache store in Rails

  • Set perform_caching and specify cache_store in the environment file (e.g. RAILS_ROOT/config/environments/production.rb)
    • config.action_controller.perform_caching = true
    • config.cache_store = :dalli_store, { :namespace => “my_project”, :expires_in => 1.day, :socket_timeout => 3, :compress => true }
      • Detailed explanation of Dalli Client options can be found here

Getting Ready for the Fight/Debugging

  • IRB as always helps us get started:
    • require ‘rubygems’
    • require ‘dalli’
    • CACHE = Dalli::Client.new(‘127.0.0.1’, { :namespace => “my_project”, :expires_in => 3600, :socket_timeout => 3, :compress => true }) # Options are self-explanatory
    • CACHE.set(‘key’, ‘value’, 180) # last option is expiry time in seconds
    • CACHE.get(‘key’) 2

  • Memcache does not provide any command to list all keys present but at times seeing the keys and their expiry time can be a life saviour. Here is a Ruby Script to list all memcache keys

    Sample Output:

    id expires bytes cache_key
    1 2014-01-22 19:05:09 +0530 5 my_project:key

Using Memcache to store expensive calculations and avoid recalculations

  • We can store expensive calculations in memcache and retrieve it next time if expensive calculation expected to return the same result else recalculate and store again.
  • Rails provides helpers like Rails.cache.read to read cache, Rails.cache.write to write in cache and Rails.cache.fetch to return result if present in cache else write in cache and return the result
  • Code Snippet to pass a block to evaluate if key not present in cache, else return the value of key

      # File: application_controller.rb
    
      # Always Calculate if caching is disabled
      # Calculate the result if key not present and store in Memcache
      # Return calculated result from Memcache if key is present
    
      def data_cache(key, time=2.minutes)
        return yield if caching_disabled?
        output = Rails.cache.fetch(key, {expires_in: time}) do
          yield
        end
        return output
      rescue
         # Execute the block if any error with Memcache
    
         return yield
      end
    
      def caching_disabled?
        ActionController::Base.perform_caching.blank?
      end
    
  • Using data_cache helper created in last step in application_controller.rb

      # File: posts_controller.rb
    
      respond_to :json
    
      def index
          # Create cache to expire after 3 minutes
          all_posts_for_a_category = data_cache("#{@category.name}", 3.minutes) do
              @category.posts.all
          end
          respond_with(posts: all_posts_for_a_category)
      end
    
      # Post#index - Before Filter
      def find_category
          @category = Category.where(id: params[:category_id]).first
          respond_with({message: 'Category Not Found'}, status: 404) if @category.blank?
      end
    

Clearing Cache

Auto-Expire after a specified time

  • Memcache accepts an expiration time 3 option with the key to write and only returns the stored result if key has not expired yet, else returns nothing.
    • Rails.cache.write(key, value, {expires_in: time})
    • Rails.cache.fetch(key, {expires_in: time}) { # Block having expensive calculations }

Faking Regex based Expiry

  • Unfortunately, Memcache does not allow listing of all keys so, apparently, there is no way to partially expire a subset of keys. Memcache only provides a delete method for the specified key restricting us to, beforehand, know the key to delete.
  • But, there is a nice trick to partially expire a subset of keys Faking Regex Based Cache keys in Rails 4.
    • To achieve it, we have to focus more on keyword subset and defining subset is the Nirvana to crack the problem.
    • Introduce a dynamic component in all the keys which we want to treat as subset
    • Have a way to control the change of dynamic component
    • When dynamic component changes, a new key is created and stored in Memcache which would be referred for data from now.
    • The old key still remains in memcache and has still not expired but it stores the stale data and fresh data is hold by new key which we are referring in all of our interactions.
    • Memcache would eventually kick the old key with stale data out (uses LRU algorithm to allocate space for new data)
    • In the snippet below, we have added dynamic component last_cache_refreshed_at in Category which can be selectively changed to expire cache of particular categories instead of all

        # File: posts_controller.rb
      
        def index
            # Update Category#last_cache_refreshed_at anytime to force the block evaulation
            # again and storing the new key, hence expiring the last key by effectively not
            # refering it for data and consulting new key for fresh data
            # Time converted to integer to avoid sanitization hassle
      
            all_posts_for_a_category = data_cache("#{@category.name}-#{@category.last_cache_refreshed_at.to_i}", 3.minutes) do
                @category.posts.all
            end
            respond_with(posts: all_posts_for_a_category)
        end
      

Writing Test Cases

  • Enable Caching with Rspec metadata

      # File: spec/spec_helper.rb
    
      # describe "Action", caching: true {}
      # Above blocks would toggle the caching behaviour
      # default caching behavior is OFF
      # Use `bundle exec rspec -t caching:true spec` to run only caching examples
    
      config.around(:each, :caching) do |example|
        caching = ActionController::Base.perform_caching
        ActionController::Base.perform_caching = example.metadata[:caching]
        example.run
        ActionController::Base.perform_caching = caching
      end
    
      # Using `bundle exec rspec spec` would exclude caching:true examples
      # to run from test suite
    
      config.filter_run_excluding :caching => true
    
  • Caching Test Example (Dependency: Memcache should be running)

      # File: spec/controllers/posts_controller_spec.rb
    
      # Use `bundle exec rspec -t caching:true spec` to run only caching examples
    
      context 'Caching Enabled', caching: true do
        before(:each) do
          # last_cache_refreshed_at would ideally be handled through before_create callback
          # by setting current time as value to it
          # Assumption: Current Time in seconds = 1271325600
    
          @category = Category.new(name: 'Ruby', last_cache_refreshed_at: Time.current)
          @category.save.should be_true
          @post = @categroy.posts.build(content: 'Expiring Cache is tough !!!')
          @post.save.should be_true
        end
    
        after(:each) do
          @post.delete
          @category.delete
        end
    
        it "should return posts" do
          @category.last_cache_refreshed_at.should eq(1271325600)
    
          # Check Memcache doesnot have key
          Rails.cache.fetch("Ruby-1271325600").should be_nil
          get :index, {category_id: 1}
          response.should be_ok
          response.body.should == {success: true, posts: @all_posts}.to_json
    
          # As caching is enabled, key should be created in memcache
          Rails.cache.fetch("Ruby-1271325600").should_not be_nil
    
          # Invoke an action to refer a different key in memcache for same action
          # Assumption: Current Time in seconds = 1456782340
          @category.update_attributes(last_cache_refreshed_at: Time.current).should be_true
          @category.last_cache_refreshed_at.should_not eq(1271325600)
    
          # Old key is still present in memcache as still not expired
          Rails.cache.fetch("Ruby-1271325600").should_not be_nil
          get :index, {category_id: 1}
          response.should be_ok
    
          # New key is generated even though old key has still not expired
          # after the specified time, old key would expire
          Rails.cache.fetch("Ruby-1456782340").should_not be_nil
        end
      end
    

Gotchas

  • Using touch command to update expiry cache time (Fixed in Memcache 1.4.14)
  • Check Memcache Version using telnet client
    • Execute stats
    • Look for STAT version 1.4.13

  1. Caching Expensive Api Requests Results

  2. If Object is saved in Memcache, Cache.get(‘key’) might raise

    Dalli::UnmarshalError: Unable to unmarshal value: undefined class/module <model_name>

    Use require_dependency "app/models/.rb"

  3. Expiry Time of 0 makes the key stays for indefinite time only limited by Memcache clearing it out in case of cleaning up

  4. Scoped Expiration AND Reload Namespace using a Proc

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