30 lines rate limited

rate_limiter.rb

# USAGE:
#   rate_limiter = RateLimiter.new('throttle_key', limit: 5, ttl: 1.hour)
#   rate_limiter.throttled { not_too_often }
#
class RateLimiter
  attr_reader :topic, :limit, :ttl, :cache

  def initialize(topic:, limit:, ttl:, cache: default_cache)
    @topic, @limit, @ttl, @cache = topic, limit, ttl, cache
  end

  def throttled
    raise LimitReached if entries.size > limit
    update_entries
    yield
  end

  private

  def entries
    Array(cache.get(topic)).map do |epoch|
      Time.at(epoch)
    end.reject do |entry|
      (entry + ttl).past?
    end
  end

  def update_entries
    updated_entries = entries << Time.now
    cache.write(entries.map(&:to_i), ttl)
  end

  def default_cache
    Rails.cache
  end
end