Add cold-start follow recommendations (#15945)

app/workers/scheduler/follow_recommendations_scheduler.rb

@@ -0,0 +1,61 @@
+# frozen_string_literal: true
+
+class Scheduler::FollowRecommendationsScheduler
+  include Sidekiq::Worker
+  include Redisable
+
+  sidekiq_options retry: 0
+
+  # The maximum number of accounts that can be requested in one page from the
+  # API is 80, and the suggestions API does not allow pagination. This number
+  # leaves some room for accounts being filtered during live access
+  SET_SIZE = 100
+
+  def perform
+    # Maintaining a materialized view speeds-up subsequent queries significantly
+    AccountSummary.refresh
+
+    fallback_recommendations = FollowRecommendation.safe.filtered.limit(SET_SIZE).index_by(&:account_id)
+
+    I18n.available_locales.each do |locale|
+      recommendations = begin
+        if AccountSummary.safe.filtered.localized(locale).exists? # We can skip the work if no accounts with that language exist
+          FollowRecommendation.safe.filtered.localized(locale).limit(SET_SIZE).index_by(&:account_id)
+        else
+          {}
+        end
+      end
+
+      # Use language-agnostic results if there are not enough language-specific ones
+      missing = SET_SIZE - recommendations.keys.size
+
+      if missing.positive?
+        added = 0
+
+        # Avoid duplicate results
+        fallback_recommendations.each_value do |recommendation|
+          next if recommendations.key?(recommendation.account_id)
+
+          recommendations[recommendation.account_id] = recommendation
+          added += 1
+
+          break if added >= missing
+        end
+      end
+
+      redis.pipelined do
+        redis.del(key(locale))
+
+        recommendations.each_value do |recommendation|
+          redis.zadd(key(locale), recommendation.rank, recommendation.account_id)
+        end
+      end
+    end
+  end
+
+  private
+
+  def key(locale)
+    "follow_recommendations:#{locale}"
+  end
+end