implement token bucket rate limiting

app/models/token_bucket.rb

@@ -0,0 +1,40 @@
+class TokenBucket < ActiveRecord::Base
+  self.primary_key = "user_id"
+  belongs_to :user
+
+  def self.prune!
+    where("last_touched_at < ?", 1.day.ago).delete_all
+  end
+
+  def self.create_default(user)
+    TokenBucket.create(user_id: user.id, token_count: user.api_burst_limit, last_touched_at: Time.now)
+  end
+
+  def accept?
+    token_count >= 1
+  end
+
+  def add!
+    TokenBucket.where(user_id: user_id).update_all(["token_count = least(token_count + (? * extract(epoch from now() - last_touched_at)), ?), last_touched_at = now()", user.api_regen_multiplier, user.api_burst_limit])
+
+    # estimate the token count to avoid reloading
+    self.token_count += (Time.now - last_touched_at)
+    self.token_count = user.api_burst_limit if token_count > user.api_burst_limit
+  end
+
+  def consume!
+    TokenBucket.where(user_id: user_id).update_all("token_count = greatest(0, token_count - 1)")
+    self.token_count -= 1
+  end
+
+  def throttled?
+    add!
+
+    if accept?
+      consume!
+      return false
+    else
+      return true
+    end
+  end
+end