[PluralSight] - Rails 4.1 Performance Fundamentals - Module 4 - Browser Caching

Browser Caching

Apply for GET request only

Response: Last-modified (When the resource was last changed) Request: If-modified-since (Is the resource newer than the last time I saw it?)

Pros

Reduces response bandwidth You control when the browser uses its cache Reduces Server Load (but not automatically)

Cons

Browser sends a request every time

ETag / If-None-Match

Like Last-modified + If-modified-since - Instead of checking if the content is newer, it checks if the content is different

Response includes ETag Subsequent request includes If-None-Match (Is the resource different than the last time I saw it?)

Return HTTP Code 304-Not-Modified

Cache-Control

Additional caching info for the browser
Response includes Cache-Control
Semicolon-separated list of directives:
- max-age: allowed to be cached for this long

You’re basically telling the browser “Don’t even bother checking for a new version of this resource.” for some amount of time. (Using max-age, it will hit server until then)

Pros:

Doesn’t even hit the server when cache is still fresh.

Cons:

Browser may show out-of-date content

You can’t usually know when the resource will change in advance but what you can do is you can decide ‘I’m okay with letting users see stale data for a certain amount of time.’ if the trade-off is faster performance for users and lower load on the server,

—

How to use ETag in Rails

Rack::ETag and Rack::ConditionalGet are a wonderful pair of rack middleware.

Rack::ETag provides build-in, automatic ETag generation.
Generates ETag based on an MD5 digest of the response body.
Rack::ConditionalGet compares If-None-Match with ETag
- Converts 200 OK with body to 304 Not Modified without body. (-> Less bandwidth consume)
Saves network time with zero effort.
Does not save server time (renders the response to make and compare the digest)

`stales?` and `fresh_when`

`stales?`

Save server time by not rendering views (Generate an ETag without having to take the time to render the entire view)
Set ETag, Last-Modified, and Cache-Control headers
To set ETag, Rails calls ActiveSupport::Cache.expand_cache_key, then makes an MD5 digest of the result
- Cache key for a student looks like: 'students/25-20140630120340'
- Pass any object that responds to cache_key
- Also works for an array.
- Last-modified is updated_at timestamp by default. (If it is an array, you must tell it what value is Last-Modified)

def show
  @student = Student.find(params[:id])
  if stale?(last_modified: @student.updated_at, etag: @student)
    respond_to do |wants|
       # rnder different formats
     end
  end
end

`fresh_when`

def show
  @student = Student.find(params[:id])
  fresh_when last_modified: @student.updated_at,
                      etag: @student
end

Using `stale?` and `fresh_when` when in `new` and `edit`

def new
  @student = Student.new
  fresh_when last_modified: @student.updated_at,
    etag: [@student, form_authenticity_token]
end

def edit
  @student = Student.find(params[:id])
  fresh_when last_modified: @student.updated_at,
    etag: [@student, form_authenticity_token]
end

Using `stale?` and `fresh_when` with Pagination

def index
  @students = Student.order(:name).page(params[:page])
  count = Student.count
  max_updated_at = Student.maximum(:updated_at).try(:utc).try(:to_s, :number)
  fresh_when etag: "students/all-#{count}-#{max_updated_at}"
end

Propagate `updated_at` to Owning Objects

class Enrollment < ActiveRecord::Base
  belongs_to :course, touch: true
  belongs_to :student, touch: true 
end

viphat / til