multi_part_rails_cache.rb

class Memcached::MultiPartRailsCache
  MAX_KEY_LENGTH = 255 # Actually overshooting what the client will let you send through
                       # but this means our chunk sizes will just a few bytes of headroom
                       # as we split
  PART_SIZE = 1.megabyte - MAX_KEY_LENGTH
  
  def initialize(rails_cache)
    @rails_cache = rails_cache
  end
  
  def get(key, raw = true) # raw is discarded
    parts = (@rails_cache.get(key) or []).map do |part_key|
      @rails_cache.get(part_key, true) or break
    end
    
    parts.blank? ? nil : parts.join
  end
  
  def set(key, value, expiry = 0, raw = true) # raw is discarded
    parts = split(value)
    keys = parts.map.with_index do |p, i|
      "%s[%i/%i]" % [ key, i + 1, parts.size ]
    end # Key[1/3] ...
    
    parts.zip(keys).each do |part, part_key|
      @rails_cache.set(part_key, part, expiry, true)
    end
    @rails_cache.set(key, keys, expiry)
  end
  
  def delete(key)
    @rails_cache.delete(key)
  end
  
  private
  def split data
    total_parts = (data.size/PART_SIZE.to_f).ceil
    Array.new(total_parts) do |n|
      data[n*PART_SIZE...(n+1)*PART_SIZE]
    end
  end
end