A more efficient way to store data in the DB than the default approach of doing a raw Base64 on the Marshal-ed version of a session hash.

session.rb

# Add this to your Gemfile:
#
#   # Session store built on top of ActiveRecord
#   gem 'activerecord-session_store', github: 'rails/activerecord-session_store'
#   # C-based parser backend for multi_json
#   gem 'oj'
#   # Unified API for parsing JSON
#   gem 'multi_json'
#
class Session < ActiveRecord::SessionStore::Session
  def self.find_by_session_id(session_id)
    self.where(session_id: session_id).first
  end

  def self.cleanup(threshold=1.week.ago)
    where(['updated_at <= ?', threshold]).delete_all
  end

  # The default marshaling behavior does a base64 encode of the output of Marshal.dump.
  # There are two problems: the stored data uses up quite a lot of space
  # Here are the numbers from a test session with quite some data in it:
  #
  # * The default Session.marshal would have needed 2973 bytes.
  # * Now we need only 785 bytes to store 1616 bytes of json (before base64, gzipped json is 579 bytes)
  #
  # This saves us 46% storage space + less time to read them from disk + less time to transfer them across the wire.
  #
  # A benchmark (10k operations with GC disabled) resulted in this:
  #
  #  Custom Marshal:    0.065ms
  #  Custom Unmarshal:  0.062ms
  #  Default Marshal:   0.094ms
  #  Default Unmarshal: 0.069ms
  #
  # A 31% time savings for marshaling and 10% save for unmarshaling is nice.
  #
  # Since Mysql 5.0.5 we can have varchars larger than 256 bytes (max is 16k), so we only use as many bytes for storage as we need.
  # We could have skipped our own base64 encoding step to gain better storage compression and to save even more time processing the data.
  # Unfortunately Rails does not support the varbinary type by default. If we feel we need the speed boost, we can always build it later.
  def self.marshal(data)
    return unless data
    json                   = MultiJson.dump(data)
    compressed_json        = ActiveSupport::Gzip.compress(json)
    base64_compressed_json = Base64.encode64(compressed_json)
    base64_compressed_json
  end

  def self.unmarshal(base64_compressed_json)
    return unless base64_compressed_json
    compressed_json = Base64.decode64(base64_compressed_json)
    json            = ActiveSupport::Gzip.decompress(compressed_json)
    data            = MultiJson.load(json)
    data
  end
end