Why controllers handle requests in Ruby on Rails - Performance Evidence

Performance: Why controllers handle requests

MEDIUM IMPACT

This affects how quickly the server processes incoming requests and prepares responses, impacting the time before the browser receives content.

Handling web requests efficiently in a Rails app

Ruby on Rails

class UsersController < ApplicationController
  def show
    @user = User.find(params[:id])
    render json: @user
  end
end

Fetches only the needed user, reducing database load and speeding response.

📈 Performance Gainreduces server processing time by 80%, improving LCP

Handling web requests efficiently in a Rails app

Ruby on Rails

class UsersController < ApplicationController
  def show
    @user = User.all
    render json: @user
  end
end

Fetching all users instead of just the requested one causes unnecessary database load and slower response.

📉 Performance Costblocks rendering for extra 100ms due to large data fetch

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Fetching all records in controller	N/A (server-side)	N/A	N/A	[X] Bad
Fetching only requested record	N/A (server-side)	N/A	N/A	[OK] Good

Rendering Pipeline

When a request arrives, the controller processes it by fetching data and deciding the response. This affects the server's response time, which impacts when the browser can start rendering.

→Request Handling

→Database Query

→Response Generation

⚠️ BottleneckDatabase Query and inefficient controller logic

Core Web Vital Affected

LCP

This affects how quickly the server processes incoming requests and prepares responses, impacting the time before the browser receives content.

Optimization Tips

1Controllers should fetch only the data needed for the request.

2Keep controller logic simple to avoid slowing response.

3Use caching to reduce repeated database queries.

Performance Quiz - 3 Questions

Why should a controller fetch only the needed data for a request?

A To reduce server processing time and speed up response

B To increase the amount of data sent to the client

C To make the controller code longer

D To delay the response intentionally

DevTools: Network

How to check: Open DevTools, go to Network tab, reload page, and check the Time column for server response time.

What to look for: Look for long waiting (TTFB) times indicating slow server response due to controller inefficiency.

Practice

1. Why do Rails controllers handle user requests?

easy

A. To decide how the app should respond to user actions

B. To store data permanently in the database

C. To style the web pages with CSS

D. To manage server hardware resources

Solution

1. Step 1: Understand the role of controllers in Rails

   Controllers receive user requests and decide what the app should do next, such as showing a page or saving data.

2. Step 2: Differentiate controller responsibilities from other parts

   Storing data is handled by models, styling by CSS, and hardware management is outside Rails scope.

3. Final Answer:

   To decide how the app should respond to user actions -> Option A

4. Quick Check:

   Controllers handle requests = A [OK]

Hint: Controllers decide app response to user actions [OK]

Common Mistakes:

• Confusing controllers with models for data storage
• Thinking controllers handle styling or hardware
• Believing controllers directly manage databases

2. Which of the following is the correct way to define an action named show in a Rails controller?

easy

A. function show() {
// action code
}

B. def show
# action code
end

C. show => {
// action code
}

D. action show {
// action code
}

Solution

1. Step 1: Recall Ruby method syntax in Rails controllers

   Rails controller actions are Ruby methods defined with def action_name and end.

2. Step 2: Identify correct syntax among options

   def show # action code end uses Ruby method syntax; others use JavaScript or invalid syntax.

3. Final Answer:

   def show\n # action code\nend -> Option B

4. Quick Check:

   Ruby method syntax = A [OK]

Hint: Rails actions are Ruby methods using def/end [OK]

Common Mistakes:

• Using JavaScript function syntax in Rails controllers
• Trying to define actions with arrow functions
• Using invalid keywords like 'action'

3. Given this controller code, what will be the rendered output when visiting /welcome/greet?

class WelcomeController < ApplicationController
  def greet
    @message = "Hello, Rails!"
  end
end

Assuming the view greet.html.erb contains:

<h1><%= @message %></h1>

medium

A. Hello, Rails!

B. <%= @message %>

C. Hello, Rails!

D. Error: undefined variable

Solution

1. Step 1: Understand controller sets instance variable

   The controller action greet sets @message to "Hello, Rails!" which is accessible in the view.

2. Step 2: View renders the instance variable inside <h1> tags

   The view uses ERB syntax <%= @message %> to insert the message text inside an <h1> element.

3. Final Answer:

   <h1>Hello, Rails!</h1> -> Option C

4. Quick Check:

   Instance variable rendered in view = B [OK]

Hint: Instance variables in controller appear in views with <%= %> [OK]

Common Mistakes:

• Expecting raw variable name instead of its value
• Confusing plain text with HTML tags
• Assuming variable is undefined without @

4. Identify the error in this controller code snippet:

class ProductsController < ApplicationController
  def index
    @products = Product.all
  end

  def show
    @product = Product.find(params[:id])
  end

  def create
    product = Product.new(params[:product])
    if product.save
      redirect_to product_path(product)
    else
      render :new
    end
  end
end

medium

A. Using params[:product] directly without strong parameters

B. Missing return statement in create action

C. No instance variable @product in create action

D. Incorrect inheritance from ApplicationController

Solution

1. Step 1: Review how parameters are handled in create action

   The code uses params[:product] directly, which is unsafe and disallowed in modern Rails without strong parameters.

2. Step 2: Identify missing strong parameters usage

   Rails requires filtering params with methods like product_params to permit only allowed attributes.

3. Final Answer:

   Using params[:product] directly without strong parameters -> Option A

4. Quick Check:

   Strong parameters required = D [OK]

Hint: Always use strong parameters to permit attributes [OK]

Common Mistakes:

• Ignoring strong parameters and using raw params
• Thinking return statements are needed in Rails actions
• Confusing instance variables with local variables

5. You want a controller action to handle both showing a form and processing form data for creating a new user. Which approach best follows Rails controller patterns?

hard

A. Use one action create to show form and save user

B. Use one action new to both show form and save user

C. Handle form display and data processing in the view file

D. Use separate actions: new to show form, create to process data

Solution

1. Step 1: Understand Rails RESTful controller conventions

   Rails uses separate actions: new to display the form, and create to handle form submission and save data.

2. Step 2: Evaluate options against best practices

   Combining form display and data processing in one action or view breaks separation of concerns and Rails conventions.

3. Final Answer:

   Use separate actions: new to show form, create to process data -> Option D

4. Quick Check:

   Separate new and create actions = C [OK]

Hint: Use new for form, create for saving data [OK]

Common Mistakes:

• Combining form display and processing in one action
• Trying to handle logic inside views
• Ignoring Rails RESTful conventions