Rails%203%20In%20Action

458 CHAPTER 16 Basic performance enhancements
1. Request comes in for
GET /projects/1/tickets/2
with If-None-Match header set to "ef2"
2. Trickles down...
3. ... to the controller
Browser
Web Server
Application
Controller
Even though this goes through the same series of events both times, what happens in
the controller is the clincher: by returning a 304 Not Modified, you can respond with
a lightweight response and get the user’s browser to render the page, rather than having
your application do it again.
For your ticket page, you’re going to want your application to send back this status
only when your ticket hasn’t been updated. When a ticket’s information such as the
title or description is updated, or when a comment is posted to the ticket, you’d want
to send back a proper response rather than the 304 Not Modified header. It’s this
timestamp that you’re going to be using to determine if a page is either fresh or stale.
A fresh page is one that’s been recently updated, with a stale page being one that
hasn’t been.
You’ve got a column in your tickets table that you can use to determine if a
ticket’s been updated: the updated_at column. Each time a ticket’s updated through
your application, this field will be set to the timestamp automatically. But, when a
comment is posted to the ticket, the updated_at field for the ticket will remain the
same.
To fix this problem, you can configure the Comment model to touch the ticket
object it’s related to, which will update its updated_at timestamp. The way you do this
is with an option on the belongs_to association in Comment called touch. Let’s change
the belongs_to :ticket line currently in app/models/comment.rb to this:
belongs_to :ticket, :touch => true
6. Browser knows to
use its local cache
5. Bubbles up
through the stack
4. Controller recognizes ETag,
sends back a
ed status
Figure 16.11
304 Not Modified
response
Whenever a comment is updated, created, or even destroyed, the related ticket’s
updated_at attribute will be updated. With the touch option, you can now confidently
use this attribute to provide a reliable timestamp for your new form of caching. This
particular form of caching uses a new method in your controllers called fresh_when.