mastodon/app/services/fan_out_on_write_service.rb

class FanOutOnWriteService < BaseService
  MAX_FEED_SIZE = 800

  # Push a status into home and mentions feeds
  # @param [Status] status
  def call(status)
    replied_to_user = status.reply? ? status.thread.account : nil

    # Deliver to local self
    push(:home, status.account.id, status) if status.account.local?

    # Deliver to local followers
    status.account.followers.each do |follower|
      next if (status.reply? && !follower.following?(replied_to_user)) || !follower.local?
      push(:home, follower.id, status)
    end

    # Deliver to local mentioned
    status.mentions.each do |mentioned_account|
      next unless mentioned_account.local?
      push(:mentions, mentioned_account.id, status)
    end
  end

  private

  def push(type, receiver_id, status)
    redis.zadd(key(type, receiver_id), status.created_at.to_i, status.id)
    trim(type, receiver_id)
  end

  def trim(type, receiver_id)
    return unless redis.zcard(key(type, receiver_id)) > MAX_FEED_SIZE

    last = redis.zrevrange(key(type, receiver_id), MAX_FEED_SIZE - 1, MAX_FEED_SIZE - 1)
    redis.zremrangebyscore(key(type, receiver_id), '-inf', "(#{last.last}")
  end

  def key(type, id)
    "feed:#{type}:#{id}"
  end

  def redis
    $redis
  end
end