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

app/services/precompute_feed_service.rb

@@ -0,0 +1,35 @@
+class PrecomputeFeedService < BaseService
+  MAX_FEED_SIZE = 800
+
+  # Fill up a user's home/mentions feed from DB and return it
+  # @param [Symbol] type :home or :mentions
+  # @param [Account] account
+  # @return [Array]
+  def call(type, account)
+    statuses = send(type.to_s, account).order('created_at desc').limit(MAX_FEED_SIZE)
+    statuses.each { |status| push(type, account.id, status) }
+    statuses
+  end
+
+  private
+
+  def push(type, receiver_id, status)
+    redis.zadd(key(type, receiver_id), status.created_at.to_i, status.id)
+  end
+
+  def home(account)
+    Status.where(account: [account] + account.following)
+  end
+
+  def mentions(account)
+    Status.where(id: Mention.where(account: account).pluck(:status_id))
+  end
+
+  def key(type, id)
+    "feed:#{type}:#{id}"
+  end
+
+  def redis
+    $redis
+  end
+end