indicator.rb

# app/models/indicator.rb

##
# Example usage:
# indicator = Indicator::UserActivity.new(user_id)
#
# When a new activity is added to a user's feed:
# indicator.increment
#
# When a user views their unseen activity:
# indicator.clear
#
# Does the user have any unseen activity?
# indicator.set?
#
# How many unseen activities does the user have? (for the iOS app)
# indicator.value

module Indicator
  class Base # Abstract base class
    
    # Store at most this many entries per hash.
    # Allows ~7x memory compression with small CPU trade-off.
    # Benchmark: http://git.io/Yo_lYA
    # Docs: http://redis.io/topics/memory-optimization
    MAX_HASH_ENTRIES = 512

    attr_accessor :obj_id

    def initialize(obj)
      self.obj_id = obj.respond_to?(:id) ? obj.id : obj.to_i
    end
    
    # Increment ids in bulk
    def self.increment(ids)
      redis.pipelined do
        Array(ids).each {|id| new(id).increment }
      end
    end

    def key_prefix
      raise NotImplementedError
    end

    def hash_key
      "#{key_prefix}:#{obj_id/MAX_HASH_ENTRIES}"
    end

    def clear
      @value = nil
      redis.hdel(hash_key, obj_id)
    end

    def increment
      @value = nil
      redis.hincrby(hash_key, obj_id, 1)
    end

    # Directly set the value. Only used for backfilling
    def set(value)
      @value = nil
      redis.hset(hash_key, obj_id, value)
    end

    def value
      @value ||= redis.hget(hash_key, obj_id).to_i
    rescue ::Redis::BaseError
      # Return 0 if anything goes wrong
      # Prevent errors from bubbling up to views
      0
    end

    def set?
      value > 0
    end

  end

  # Concrete classes must implement the `key_prefix` method
  class UserActivity < Base
    def key_prefix; 'indicators/user_activity'; end
  end

end

LICENSE

Copyright 2014 Kickstarter, Inc.

Released under an MIT License.

user.rb

# app/models/user.rb
class User < ActiveRecord::Base

  def activity_indicator
    @activity_indicator ||= Indicator::UserActivity.new(id)
  end

end