class RandomTopicSelector BACKFILL_SIZE = 3000 BACKFILL_LOW_WATER_MARK = 500 def self.backfill(category = nil) exclude = category&.topic_id options = { per_page: category ? category.num_featured_topics : 3, visible: true, no_definitions: true } options[:except_topic_ids] = [category.topic_id] if exclude if category options[:category] = category.id # NOTE: at the moment this site setting scopes tightly to a category (excluding subcats) # this is done so we don't populate a junk cache if SiteSetting.limit_suggested_to_category options[:no_subcategories] = true end # don't leak private categories into the "everything" group options[:guardian] = Guardian.new(CategoryFeaturedTopic.fake_admin) end query = TopicQuery.new(nil, options) results = query.latest_results.order('RANDOM()') .where(closed: false, archived: false) .where("topics.created_at > ?", SiteSetting.suggested_topics_max_days_old.days.ago) .limit(BACKFILL_SIZE) .reorder('RANDOM()') .pluck(:id) key = cache_key(category) if results.present? $redis.multi do $redis.rpush(key, results) $redis.expire(key, 2.days) end end results end def self.next(count, category = nil) key = cache_key(category) results = [] return results if count < 1 results = $redis.multi do $redis.lrange(key, 0, count - 1) $redis.ltrim(key, count, -1) end if !results.is_a?(Array) # Redis is in readonly mode results = $redis.lrange(key, 0, count - 1) else results = results[0] end results.map!(&:to_i) left = count - results.length backfilled = false if left > 0 ids = backfill(category) backfilled = true results += ids[0...count] results.uniq! results = results[0...count] end if !backfilled && $redis.llen(key) < BACKFILL_LOW_WATER_MARK Scheduler::Defer.later("backfill") do backfill(category) end end results end def self.cache_key(category = nil) "random_topic_cache_#{category&.id}" end end