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The View Is Clear from Here

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Aaron Patterson

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@tenderlove

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I have stickers of my cat! (please say "hello")

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Ruby Core Team

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Rails Core Team

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G GitHub

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Le Git

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git push -f

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git checkout -b

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git cherry-pick

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No content

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Happy GPS Rollover Day! April 6, 23:59:42 UTC I guess this is actually tomorrow……

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GPS Uses 10 bits!

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Single Quotes Are 2x Faster Than Double Quotes

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You Have To Hit The Shift Key To Type "

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Logic AND OR NOT A

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Гематоген

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"атоген"

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gem 'атоген' Gemfile

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The View Is Clear from Here

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WARNING!@ There Will Be Code

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There Will Be Ruby

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There Will Be Rails

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Software Engineering Practices

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Rails is MVC

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MVC Model View Controller

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MVC Model View Controller Request

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MVC Model View Controller Request Action Controller Action View Active Record

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Action View

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Action View Finds Templates Compiles Templates Executes Views

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Finding Templates

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View Templates

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ERB

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Developed by @m_seki

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ERB Template

New User

<%= render_form %> <%= link_to 'Back' %> new.html.erb

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ERB Compiler require "erb" # Compile the template template = ERB.new File.read "test.html.erb" # Print the template source puts template.src # Evaluate the template puts template.result(binding) compile.rb

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Compiled Template _erbout = +"" _erbout << "

New User

\n\n".freeze _erbout << render_form.to_s _erbout << "\n\n".freeze _erbout << link_to("Back").to_s _erbout << "\n".freeze _erbout

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Modified Compile Script require "erb" def render_form "" end def link_to(text) "#{text}" end # Compile the template template = ERB.new File.read "test.html.erb" # Evaluate the template puts template.result(binding)

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Compiler Output $ ruby compiler.rb

New User

Back

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ERB Translations ERB Source Translated Ruby <%= something %> _erbout << something.to_s <% something_else %> something_else

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Capturing Blocks
<%= capture do %> Hello! <% end %>
require "erb" def capture yield end # Compile the template template = ERB.new File.read "test.html.erb" puts template.result(binding)

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What is the output? Hello!
Hello!
ERROR!

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Compiled Source _erbout = +'' _erbout << "
\n ".freeze _erbout << ( capture do ).to_s _erbout << "\n Hello!\n ".freeze end _erbout << "\n
\n".freeze _erbout
<%= capture do %> Hello! <% end %>

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How does this work in Rails?

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ERB Compiler BLOCK_EXPR = /\s*((\s+|\))do|\{)(\s*\|[^|]*\|)?\s*\Z/ def add_expression(indicator, code) flush_newline_if_pending(src) if (indicator == "==") || @escape src << "@output_buffer.safe_expr_append=" else src << "@output_buffer.append=" end if BLOCK_EXPR.match?(code) src << " " << code else src << "(" << code << ");" end end

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ERB in Rails

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Different ERB Flavors Original ERB ERubis ERubi

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ERB Performance

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Rendering Speed require "erb" require "benchmark/ips" class AaronView TEMPLATES = { } TEMPLATES["index"] = <<-eerb

Hello <%= name %>

eerb def name; "Aaron"; end def render(template) ERB.new(TEMPLATES[template]).result(binding) end end view = AaronView.new Benchmark.ips do |x| x.report("render") { view.render("index") } end $ ruby rendering_speed.rb Warming up -------------------------------------- render 2.696k i/100ms Calculating ------------------------------------- render 27.117k (± 5.9%) i/s - 137.496k in 5.092633s

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Cache Compilation class AaronView TEMPLATES = { } TEMPLATES["index"] = <<-eerb

Hello <%= name %>

eerb def name; "Aaron"; end def render(template) ERB.new(TEMPLATES[template]).result(binding) end COMPILED_TEMPLATES = { } def render_compiled(template) erb = COMPILED_TEMPLATES[template] ||= ERB.new(TEMPLATES[template]) erb.result(binding) end end view = AaronView.new Benchmark.ips do |x| x.report("render") { view.render("index") } x.report("render compiled") { view.render_compiled("index") } x.compare! end $ ruby rendering_speed.rb Warming up -------------------------------------- render 2.709k i/100ms render compiled 6.293k i/100ms Calculating ------------------------------------- render 27.115k (± 2.6%) i/s - 138.159k in 5.098881s render compiled 63.645k (± 3.9%) i/s - 320.943k in 5.050544s Comparison: render compiled: 63645.5 i/s render: 27114.8 i/s - 2.35x slower

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Define a Method class AaronView TEMPLATES = { } TEMPLATES["index"] = <<-eerb

Hello <%= name %>

eerb def name; "Aaron"; end def render(template) ERB.new(TEMPLATES[template]).result(binding) end COMPILED_TEMPLATES = { } def render_compiled(template) erb = COMPILED_TEMPLATES[template] ||= ERB.new(TEMPLATES[template]) erb.result(binding) end METHODS_DEFINED = { } def render_with_method(template) unless METHODS_DEFINED.key? template erb = ERB.new(TEMPLATES[template]) self.class.class_eval "def render_#{template}; #{erb.src}; end" METHODS_DEFINED[template] = true end send "render_#{template}" end end $ ruby rendering_speed.rb Warming up -------------------------------------- render 2.750k i/100ms render compiled 6.462k i/100ms render method 114.420k i/100ms Calculating ------------------------------------- render 28.389k (± 2.7%) i/s - 143.000k in 5.041158s render compiled 65.860k (± 2.4%) i/s - 329.562k in 5.006962s render method 1.477M (± 1.7%) i/s - 7.437M in 5.038304s Comparison: render method: 1476591.3 i/s render compiled: 65860.1 i/s - 22.42x slower render: 28388.6 i/s - 52.01x slower

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Method Generation require "erb" erb_source = <<-erb

Hello <%= name %>

erb erb = ERB.new erb_source template = "index" self.class.class_eval "def render_#{template}; #{erb.src}; end" send "render_#{template}" def render_index _erbout = +'' _erbout << "\n \n

Hello ".freeze _erbout << name.to_s _erbout << "

\n\n".freeze _erbout end Template Source Compile Source Define a m ethod Render the Tem plate Compiled Source

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Templates Are Translated to Methods

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Rails Template Methods

This is an inner template <% puts methods.grep(/_app/) %>

Template in Rails Console Output _app_views_users_index_html_erb___4485674087862414886_70282093274980 _app_views_users__first_ja_html_erb__657974686689484881_70282053324000 _app_views_users__second_html_erb___3508242123539422948_70282053523460

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Memory Size def render_index _erbout = +'' _erbout << "\n \n

Hello ".freeze _erbout << name.to_s _erbout << "

\n\n".freeze _erbout end require "objspace" iseq = RubyVM::InstructionSequence.of( method(:render_index)) p ObjectSpace.memsize_of iseq $ ruby test.rb 1264 Measuring Memory Console Output

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Template Handling In Rails

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Instance Variable Visibility

This is template Foo!

<%= @some_ivar %>

This is template Bar!

<%= @some_ivar %> _foo.html.erb _bar.html.erb

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Compiled Templates class ActionView::Base def render_foo _erbout = +'' _erbout << "

This is template Foo!

\n".freeze _erbout << @some_ivar.to_s _erbout << "\n".freeze _erbout end def render_bar _erbout = +'' _erbout << "

This is template Barr!

\n".freeze _erbout << @some_ivar.to_s _erbout << "\n".freeze _erbout end end _foo.html.erb _bar.html.erb Same Instance Same Instance

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Don’t use instance variables in your templates

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"Don’t use instance variables in your templates" - Aaron

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Instance Variable Visibility

This is template Foo!

<%= @some_ivar %>

This is template Bar!

<%= @some_ivar %> _foo.html.erb _bar.html.erb

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Local Variables

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Rendering With Locals <%= render "content", locals: { name: "Gorby" } %>

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Compiling With Locals

Hello, <%= name %>

def render_content _erbout = +'' _erbout << "

\n Hello, ".freeze _erbout << name.to_s _erbout << "\n

\n".freeze _erbout end content.html.erb Compiled Method Method Call

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Preamble With Locals

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Compiling With Locals

Hello, <%= name %>

def render_content(locals) # Locals preamble name = locals[:name] _erbout = +'' _erbout << "

\n Hello, ".freeze _erbout << name.to_s _erbout << "\n

\n".freeze _erbout end content.html.erb Compiled Method

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Templates Require Context <%= render "content", locals: { name: "Gorby" } %> <%= render "content" %>

Hello, <%= name %>

main.html.erb content.html.erb Method call or local variable? local variable method call

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Templates Can’t Be Compiled In Advance

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Compiled Too Many Times

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Too Many Compilations <%= render "content", locals: { name: "Gorby" } %> <%= render "content", locals: { name: "Gorby", friend: "Aaron" } %> def render_content_1(locals) # Locals preamble name = locals[:name] _erbout = +'' _erbout << "

\n Hello, ".freeze _erbout << name.to_s _erbout << "\n

\n".freeze _erbout end def render_content_2(locals) # Locals preamble name = locals[:name] friend = locals[:friend] _erbout = +'' _erbout << "

\n Hello, ".freeze _erbout << name.to_s _erbout << "\n

\n".freeze _erbout end main.html.erb Compiled "content" Unique Preambles

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"This is nice, but what should I do?"

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Always pass the same locals to individual templates.

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The render function

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Finds a Template Compiles the Template Calculates a Method Name Calls the Method

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Find a Template (from the cache) Compiled? Compile the Template Call the method Calculate a Method Name

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Finding a Template

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Template Rendering Depends on "Requested Format"

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Users Controller class UsersController < ApplicationController # GET /users # GET /users.json def index @users = User.all render "index" end end $ tree app/views/users app/views/users !"" index.html.erb #"" index.xml.erb Controller app/views/users/*

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Template Source

XML template

Users

index.xml.erb index.html.erb

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Requests and Rendering Request Response Template Rendered curl -H 'Accept: application/xml' http://localhost:3000/users

XML template

index.xml.erb curl http://localhost:3000/users

Users

index.html.erb curl -H 'Accept: text/html' http:// localhost:3000/users

Users

index.html.erb

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Template Cache Keys • Local variables • Format • Locale • Variant (iPhone, etc)

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Strange Render Behavior

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Users Controller class UsersController < ApplicationController # GET /users # GET /users.json def index @users = User.all render "index" end end $ tree app/views/users app/views/users !"" _my_template.png.erb !"" _my_template.xml.erb #"" index.html.erb Controller app/views/users/*

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Template Contents

Users

<%= render "my_template" %> I guess this is a png? index.html.erb _my_template.png.erb XML is cool! _my_template.xml.erb

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Requests and Rendering Request Response Template Rendered curl -H 'Accept: application/xml' http://localhost:3000/users Missing XML template Error None curl http://localhost:3000/users

Users

I guess this is a png? index.html.erb _my_template.png.erb curl -H 'Accept: text/html' http:// localhost:3000/users Missing HTML partial Error None Browser Missing HTML partial Error None

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Error!!

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We Can’t Predict

Users

<%= render "my_template" %>

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respond_to Controller class UsersController < ApplicationController # GET /users # GET /users.json def index @users = User.all render "index" end end class UsersController < ApplicationController # GET /users # GET /users.json def index @users = User.all respond_to do |format| format.html do render "index" end end end end "Bare" render respond_to render

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Requests and Rendering Request Response Template Rendered curl -H 'Accept: application/xml' http://localhost:3000/users Missing XML template Error None curl http://localhost:3000/users Missing HTML partial Error None curl -H 'Accept: text/html' http:// localhost:3000/users Missing HTML partial Error None Browser Missing HTML partial Error None

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Context Dependent

Users

<%= render "my_template" %> class UsersController < ApplicationController # GET /users # GET /users.json def index @users = User.all render "index" end end class UsersController < ApplicationController # GET /users # GET /users.json def index @users = User.all respond_to do |format| format.html do render "index" end end end end index.html.erb

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Templates are not "Context Free"

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Prediction and Consistency

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Find a Template (from the cache) Compiled? Compile the Template Call the method Calculate a Method Name Cheap! Cheap! Expensive!

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What Method Will This Call?

Users

<%= render "my_template" %>

Users

<%= render_my_template_00011123abf %> index.html.erb translated template Calls a unique method. But what method?

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Find a Template (from the cache) Compiled? Compile the Template Call the method Calculate a Method Name Cheap! Cheap! Expensive!

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Slower Production Boot (but could be eliminated by BootSnap)

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Lower Memory

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Faster Runtime No More "Cache Check" Penalty

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DETOUR!

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Three Templates
    <%= render partial: "customer", collection: customers %>
    <%= render partial: "customer", collection: customers, cached: true %>
_col.html.erb _cached_col.html.erb
  • Hello: <%= customer.name %>
    • _customer.html.erb

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    Benchmark https://github.com/rails/rails/issues/35257

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    Results $ be ruby render_benchmark.rb -- create_table(:customers, {:force=>true}) -> 0.0108s <#ActiveSupport::Cache::MemoryStore entries=1000, size=333893, options={}> Warming up -------------------------------------- collection render: no cache 6.000 i/100ms collection render: with cache 1.000 i/100ms Calculating ------------------------------------- collection render: no cache 65.319 (± 7.7%) i/s - 330.000 in 5.080651s collection render: with cache 11.504 (± 8.7%) i/s - 58.000 in 5.071030s Comparison: collection render: no cache: 65.3 i/s collection render: with cache: 11.5 i/s - 5.68x slower

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    I was convinced the cache didn’t work

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    I was convinced the cache didn’t work

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    I was convinced the cache didn’t work

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    I was convinced the cache didn’t work

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    But, there were entries in the cache

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    Profiled The Cache ================================== Mode: wall(1000) Samples: 4690 (0.00% miss rate) GC: 207 (4.41%) ================================== TOTAL (pct) SAMPLES (pct) FRAME 594 (12.7%) 594 (12.7%) ActiveModel::LazyAttributeHash#[] 387 (8.3%) 387 (8.3%) ActiveRecord::Transactions#update_attributes_from_transaction_state 1199 (25.6%) 246 (5.2%) ActiveSupport::Cache::Store#expanded_key 301 (6.4%) 218 (4.6%) AbstractController::Caching::Fragments#combined_fragment_cache_key 207 (4.4%) 207 (4.4%) (garbage collection) 932 (19.9%) 196 (4.2%) ActionView::CollectionCaching#collection_by_cache_keys 450 (9.6%) 181 (3.9%) ActiveSupport::Cache::Store#expanded_version

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    Cache Key Calculation Was More Expensive Than Template Execution

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    Cache Payoff
  • Hello: <%= customer.name %>
  • Hello: <%= customer.name %>
  • Hello: <%= customer.name %>
  • Hello: <%= customer.name %>
  • Hello: <%= customer.name %>
  • Hello: <%= customer.name %>
  • Hello: <%= customer.name %>
  • Hello: <%= customer.name %>
  • Hello: <%= customer.name %>
  • Hello: <%= customer.name %>
  • Hello: <%= customer.name %>
  • Hello: <%= customer.name %>
  • Hello: <%= customer.name %>
  • Hello: <%= customer.name %>
  • Hello: <%= customer.name %>
  • Hello: <%= customer.name %>
  • Hello: <%= customer.name %>
  • Hello: <%= customer.name %>
  • Hello: <%= customer.name %>
  • Hello: <%= customer.name %>
  • Hello: <%= customer.name %>
  • Hello: <%= customer.name %>
  • Hello: <%= customer.name %>
    • Old Template New Template

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    Benchmark Results $ be ruby render_benchmark.rb -- create_table(:customers, {:force=>true}) -> 0.0075s <#ActiveSupport::Cache::MemoryStore entries=1000, size=900893, options={}> Warming up -------------------------------------- collection render: no cache 4.000 i/100ms collection render: with cache 4.000 i/100ms Calculating ------------------------------------- collection render: no cache 41.066 (± 2.4%) i/s - 208.000 in 5.070051s collection render: with cache 43.192 (± 4.6%) i/s - 216.000 in 5.012803s Comparison: collection render: with cache: 43.2 i/s collection render: no cache: 41.1 i/s - same-ish: difference falls within error

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    "Cache" doesn’t mean "fast"

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    Making Render Fast

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    Make Render Usually Fast

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    "Same Format Assumption"

    Users

    <%= render "my_template" %> <%= render "my_template", format: :png %> $ tree app/views/users app/views/users !"" _my_template.html.erb !"" _my_template.png.erb !"" _my_template.xml.erb #"" index.html.erb index.html.erb templates

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    Ambiguous Render Problem

    Users

    <%= render "my_template" %> <%= render "my_template", format: :html %> $ tree app/views/users app/views/users !"" _my_template.png.erb !"" _my_template.xml.erb #"" index.html.erb index.html.erb templates Sometimes an exception Sometimes a PNG Always an exception

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    "Always Optimize"

    Users

    <%= render "my_template" %> <%= render "my_template", format: :html %>

    Users

    <%= render "my_template", format: :png %> <%= render "my_template", format: :html %> index.html.erb index.html.erb

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    "Same Format Assumption"

    Users

    <%= render "my_template" %> <%= render "my_template", format: :png %> $ tree app/views/users app/views/users !"" _my_template.html.erb !"" _my_template.png.erb !"" _my_template.xml.erb #"" index.html.erb index.html.erb templates

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    "Same Format Assumption"

    Users

    <%= render "my_template" %> <%= render "my_template", format: :png %>

    Users

    <%= render_my_template_html_00123abc %> <%= render_my_template_png_00123abc %> index.html.erb Optimized Template

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    Ambiguous Render Problem

    Users

    <%= render "my_template" %> <%= render "my_template", format: :html %> $ tree app/views/users app/views/users !"" _my_template.png.erb !"" _my_template.xml.erb #"" index.html.erb index.html.erb templates

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    Ambiguous Render Problem

    Users

    <%= render "my_template" %> <%= render "my_template", format: :html %>

    Users

    <%= render "my_template" %> <%= raise MissingTemplateError %> index.html.erb Optimized Template

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    Wrap It Up

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    Be "Context Free" Don’t depend on side effects

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    Be Consistent

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    Cache != Fast

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    Benchmark First!

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    Make Your Locals Match

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    Thank You!!!