Nate Wiger vs Technology Code always works better after a few beers

Real-time Leaderboards with ElastiCache and Redis Objects

With the launch of AWS ElastiCache for Redis this week, I realized my redis-objects gem could use a few more examples. Paste this code into your game’s Ruby backend for real-time leaderboards with Redis.

Redis Sorted Sets are the ideal data type for leaderboards. This is a data structure that guarantees uniqueness of members, plus keeps members sorted in real time. Yep that’s pretty much exactly what we want. The Redis sorted set commands to populate a leaderboard would be:

ZADD leaderboard 556  "Andy"
ZADD leaderboard 819  "Barry"
ZADD leaderboard 105  "Carl"
ZADD leaderboard 1312 "Derek"

This would create a leaderboard set with members auto-sorted based on their score. To get a leaderboard sorted with highest score as highest ranked, do:

ZREVRANGE leaderboard 0 -1
1) "Derek"
2) "Barry"
3) "Andy"
4) "Carl"

This returns the set’s members sorted in reverse (descending) order. Refer to the Redis docs for ZREVRANGE for more details.

Wasn’t this a Ruby post?

Back to redis-objects. Let’s start with a direct Ruby translation of the above:

require 'redis-objects'
Redis.current = Redis.new(host: 'localhost')

lb = Redis::SortedSet.new('leaderboard')
lb["Andy"]  = 556
lb["Barry"] = 819
lb["Carl"]  = 105
lb["Derek"] = 1312

puts lb.revrange(0, -1)  # ["Derek", "Barry", "Andy", "Carl"]

And… we’re done. Ship it.

Throw that on Rails

Ok, so our game probably has a bit more too it. Let’s assume there’s a User database table, with a score column, created like so:

class CreateUsers < ActiveRecord::Migration
  def up
    create_table :users do |t|
      t.string  :name
      t.integer :score
    end
  end
end

We can integrate a sorted set leaderboard with our User model in two lines:

class User < ActiveRecord::Base
  include Redis::Objects
  sorted_set :leaderboard, global: true
end

Since we’re going to have just a single leaderboard (rather than one per user), we use the global flag. This will create a User.leaderboard sorted set that we can then access anywhere:

puts User.leaderboard.members

(Important: This doesn’t have to be ActiveRecord — you could use Mongoid or DataMapper or Sequel or Dynamoid or any other DB model.)

We’ll add a hook to update our leaderboard when we get a new high score. Since we now have a database table, we’ll index our sorted set by our ID, since it’s guaranteed to be unique:

class User < ActiveRecord::Base
  include Redis::Objects
  sorted_set :leaderboard, global: true

  after_update :update_leaderboard
  def update_leaderboard
    self.class.leaderboard[id] = score
  end
end

Save a few records:

User.create!(name: "Andy",  score: 556)
User.create!(name: "Barry", score: 819)
User.create!(name: "Carl",  score: 105)
User.create!(name: "Derek", score: 1312)

Fetch the leaderboard:

@user_ids = User.leaderboard.revrange(0, -1)
puts @user_ids  # [4, 2, 1, 3]

And now we have a Redis leaderboard sorted in real time, auto-updated any time we get a new high score.

But MySQL has ORDER BY

The skeptical reader may wonder why not just sort in MySQL, or whatever the kewl new database flavor of the week is. Outside of offloading our main database, things get more interesting when we want to know our own rank:

class User < ActiveRecord::Base
  # ... other stuff remains ...

  def my_rank
    self.class.leaderboard.revrank(id) + 1
  end
end

Then:

@user = User.find(1) # Andy
puts @user.my_rank   # 3

Getting a numeric rank for a row in MySQL would require adding a new “rank” column, and then running a job that re-ranks the entire table. Doing this in real time means clobbering MySQL with a global re-rank every time anyone’s score changes. This makes MySQL unhappy, especially with lots of users.

Kids are calling so that’s all for now. Enjoy!

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