mastodon/app/services/follow_service.rb

class FollowService < BaseService
  # Follow a remote user, notify remote user about the follow
  # @param [Account] source_account From which to follow
  # @param [String] uri User URI to follow in the form of username@domain
  def call(source_account, uri)
    target_account = follow_remote_account_service.(uri)

    return nil if target_account.nil? || target_account.id == source_account.id

    follow = source_account.follow!(target_account)

    if target_account.local?
      NotificationMailer.follow(target_account, source_account).deliver_later
    else
      subscribe_service.(target_account)
      NotificationWorker.perform_async(follow.stream_entry.id, target_account.id)
    end

    merge_into_timeline(target_account, source_account)
    source_account.ping!(account_url(source_account, format: 'atom'), [Rails.configuration.x.hub_url])
    follow
  end

  private

  def merge_into_timeline(from_account, into_account)
    timeline_key = FeedManager.instance.key(:home, into_account.id)

    from_account.statuses.find_each do |status|
      redis.zadd(timeline_key, status.id, status.id)
    end

    FeedManager.instance.trim(:home, into_account.id)
    FeedManager.instance.broadcast(into_account.id, type: 'merge')
  end

  def redis
    $redis
  end

  def follow_remote_account_service
    @follow_remote_account_service ||= FollowRemoteAccountService.new
  end

  def subscribe_service
    @subscribe_service ||= SubscribeService.new
  end
end