Simple Limiter based on individual keys instead of Redis hashes which are impossible to expire individually

limiter.rb

require 'redis'
require 'redis-namespace'

class Limiter
  def initialize(name:, threshold:, interval:, time_span: 600, bucket_span: 5)
    @name, @threshold, @interval, @time_span, @bucket_span = name, threshold, interval, time_span, bucket_span

    raise ArgumentError if @interval > @time_span || @interval < @bucket_span

    @redis ||= Redis::Namespace.new(:limiter, redis: $redis || Redis.new)

    @all_buckets_count = (@time_span / @bucket_span).floor
    @sliding_window_buckets_count = (@interval.to_f / @bucket_span).floor
  end

  def add(count: 1)
    key = [@name, get_bucket_index].join(":")

    @redis.multi do
      @redis.incrby(key, count)
      @redis.expire(key, @interval)
    end

    nil
  end

  def count
    current_bucket_index = get_bucket_index

    bucket_indices = @sliding_window_buckets_count.times.map do |i|
      (current_bucket_index - i) % @all_buckets_count
    end

    @redis.multi do
      bucket_indices.map do |i|
        key = [@name, i].join(":")
        @redis.get(key)
      end
    end.map(&:to_i).sum
  end

  def exec_within_threshold
    sleep @bucket_span while exceeded?
    yield
  end

  def exceeded?
    count >= @threshold
  end

  private

  def get_bucket_index
    ((Time.now.to_i % @all_buckets_count) / @bucket_span).floor
  end
end