class ActiveSupport::Cache::MemoryStore

MemoryStore is thread-safe.
of the maximum size by removing the least recently used entries.
a cleanup will occur which tries to prune the cache down to three quarters
initializer (default is 32Mb). When the cache exceeds the allotted size,
This cache has a bounded size specified by the :size options to the
appropriate cache in that scenario.
to share cache data with each other and this may not be the most
then this means that Rails server process instances won’t be able
(which is the case if you’re using mongrel_cluster or Phusion Passenger),
same process. If you’re running multiple Ruby on Rails server processes
A cache store implementation which stores everything into memory in the

def cached_size(key, entry) 

def cached_size(key, entry)
  key.to_s.bytesize + entry.size + PER_ENTRY_OVERHEAD
end

def cleanup(options = nil)

Preemptively iterates through all stored keys and removes the ones which have expired. 
def cleanup(options = nil)
  options = merged_options(options)
  instrument(:cleanup, :size => @data.size) do
    keys = synchronize{ @data.keys }
    keys.each do |key|
      entry = @data[key]
      delete_entry(key, options) if entry && entry.expired?
    end
  end
end

def clear(options = nil) 

def clear(options = nil)
  synchronize do
    @data.clear
    @key_access.clear
    @cache_size = 0
  end
end

def decrement(name, amount = 1, options = nil)

Decrement an integer value in the cache. 
def decrement(name, amount = 1, options = nil)
  synchronize do
    options = merged_options(options)
    if num = read(name, options)
      num = num.to_i - amount
      write(name, num, options)
      num
    else
      nil
    end
  end
end

def delete_entry(key, options) # :nodoc:

:nodoc: 
def delete_entry(key, options) # :nodoc:
  synchronize do
    @key_access.delete(key)
    entry = @data.delete(key)
    @cache_size -= cached_size(key, entry) if entry
    !!entry
  end
end

def delete_matched(matcher, options = nil) 

def delete_matched(matcher, options = nil)
  options = merged_options(options)
  instrument(:delete_matched, matcher.inspect) do
    matcher = key_matcher(matcher, options)
    keys = synchronize { @data.keys }
    keys.each do |key|
      delete_entry(key, options) if key.match(matcher)
    end
  end
end

def increment(name, amount = 1, options = nil)

Increment an integer value in the cache. 
def increment(name, amount = 1, options = nil)
  synchronize do
    options = merged_options(options)
    if num = read(name, options)
      num = num.to_i + amount
      write(name, num, options)
      num
    else
      nil
    end
  end
end

def initialize(options = nil) 

def initialize(options = nil)
  options ||= {}
  super(options)
  @data = {}
  @key_access = {}
  @max_size = options[:size] || 32.megabytes
  @max_prune_time = options[:max_prune_time] || 2
  @cache_size = 0
  @monitor = Monitor.new
  @pruning = false
end

def inspect # :nodoc:

:nodoc: 
def inspect # :nodoc:
  "<##{self.class.name} entries=#{@data.size}, size=#{@cache_size}, options=#{@options.inspect}>"
end

def prune(target_size, max_time = nil)

recently accessed entries.
To ensure entries fit within the specified memory prune the cache by removing the least 
def prune(target_size, max_time = nil)
  return if pruning?
  @pruning = true
  begin
    start_time = Time.now
    cleanup
    instrument(:prune, target_size, :from => @cache_size) do
      keys = synchronize{ @key_access.keys.sort{|a,b| @key_access[a].to_f <=> @key_access[b].to_f} }
      keys.each do |key|
        delete_entry(key, options)
        return if @cache_size <= target_size || (max_time && Time.now - start_time > max_time)
      end
    end
  ensure
    @pruning = false
  end
end

def pruning?

Returns true if the cache is currently being pruned. 
def pruning?
  @pruning
end

def read_entry(key, options) # :nodoc:

:nodoc: 
def read_entry(key, options) # :nodoc:
  entry = @data[key]
  synchronize do
    if entry
      @key_access[key] = Time.now.to_f
    else
      @key_access.delete(key)
    end
  end
  entry
end

def synchronize(&block) # :nodoc:

:nodoc:
is not thread safe.
Synchronize calls to the cache. This should be called wherever the underlying cache implementation 
def synchronize(&block) # :nodoc:
  @monitor.synchronize(&block)
end

def write_entry(key, entry, options) # :nodoc:

:nodoc: 
def write_entry(key, entry, options) # :nodoc:
  entry.dup_value!
  synchronize do
    old_entry = @data[key]
    return false if @data.key?(key) && options[:unless_exist]
    if old_entry
      @cache_size -= (old_entry.size - entry.size)
    else
      @cache_size += cached_size(key, entry)
    end
    @key_access[key] = Time.now.to_f
    @data[key] = entry
    prune(@max_size * 0.75, @max_prune_time) if @cache_size > @max_size
    true
  end
end