perf improvements for traffic intensity

TrafficHelper's hot-path intensity does a single select for a single row and
loads the value rather than instantiate an ActiveRecord object.

Updating the intensity has moved entirely to the cron job to prevent dogpiling,
and does three upserts instead of select + upsert.

No more hassling around with different keys for intensity when we only use the
latest value.

app/helpers/traffic_helper.rb

@@ -29,19 +29,15 @@ module TrafficHelper
     result.to_a.first
   end
 
-  def self.cached_traffic_range
-    low, high = nil, nil
-    low = Keystore.readthrough_cache('traffic:low') do
-      low, high = traffic_range
-      Keystore.put('traffic:high', high)
-      low
-    end
-    high ||= Keystore.value_for('traffic:high')
-    [low, high]
+  def self.cache_traffic!
+    low, high = self.traffic_range
+    Keystore.put('traffic:low', low)
+    Keystore.put('traffic:high', high)
+    Keystore.put('traffic:intensity', current_intensity(low, high))
   end
 
   def self.current_activity
-    start_at = Time.now.utc - 15.minutes
+    start_at = PERIOD_LENGTH.minutes.ago
     result = ActiveRecord::Base.connection.execute <<-SQL
       select
         (SELECT count(1) AS n_votes   FROM votes    WHERE updated_at >= '#{start_at}') +
@@ -51,20 +47,13 @@ module TrafficHelper
     result.to_a.first.first
   end
 
-  def self.current_intensity
-    low, high = cached_traffic_range
+  def self.current_intensity(low, high)
     return 0.5 if low.nil? || high.nil? || high == low
     activity = [low, current_activity, high].sort[1]
     [0, ((activity - low)*1.0/(high - low) * 100).round, 100].sort[1]
   end
 
-  def self.current_period_key
-    "traffic:at:#{(Time.now.utc.to_i/CACHE_FOR.minutes).floor}"
-  end
-
   def self.cached_current_intensity
-    Keystore.readthrough_cache(current_period_key) do
-      current_intensity
-    end
+    Keystore.value_for('traffic:intensity') || 0.5
   end
 end

app/models/keystore.rb

@@ -6,11 +6,11 @@ class Keystore < ApplicationRecord
   validates :key, presence: true, length: { maximum: MAX_KEY_LENGTH }
 
   def self.get(key)
-    self.find_by(:key => key)
+    self.find_by(key: key)
   end
 
   def self.value_for(key)
-    self.find_by(:key => key).try(:value)
+    self.where(key: 'traffic:intensity').limit(1).pluck(:value).first
   end
 
   def self.put(key, value)
@@ -76,6 +76,14 @@ class Keystore < ApplicationRecord
     end
   end
 
+  def self.decrement_value_for(key, amount = -1)
+    self.increment_value_for(key, amount)
+  end
+
+  def self.decremented_value_for(key, amount = -1)
+    self.incremented_value_for(key, amount)
+  end
+
   # deliberately no lock/transaction as TrafficHelper is on the hot path of every request
   def self.readthrough_cache(key, &blk)
     if (found = value_for(key))
@@ -87,14 +95,6 @@ class Keystore < ApplicationRecord
     end
   end
 
-  def self.decrement_value_for(key, amount = -1)
-    self.increment_value_for(key, amount)
-  end
-
-  def self.decremented_value_for(key, amount = -1)
-    self.incremented_value_for(key, amount)
-  end
-
   def self.validate_input_key(key)
     exception = ActiveRecord::ValueTooLong.new("#{MAX_KEY_LENGTH}" \
       " characters is the maximum allowed for key")

script/traffic_range

@@ -9,11 +9,5 @@ require File.expand_path('../../config/boot', __FILE__)
 require APP_PATH
 Rails.application.require_environment!
 
-# Delete old period keys
-Keystore
-  .where('`key` like "traffic:at:%"')
-  .where.not('`key` = ?', TrafficHelper.current_period_key)
-  .delete_all
-
-# calculate and cache new low and high activity range for traffic
-TrafficHelper.cached_traffic_range
+# calculate and cache activity range for traffic
+TrafficHelper.cache_traffic!