Distrubute statuses as a fan-out-on-write system, with optional precomputing

app/services/fan_out_on_write_service.rb

@@ -0,0 +1,46 @@
+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