API-only application setup in Ruby on Rails - Performance & Optimization

Performance: API-only application setup

MEDIUM IMPACT

This setup affects initial page load speed and server response time by minimizing unnecessary frontend assets and middleware.

Setting up a Rails backend to serve only JSON APIs without frontend views

Ruby on Rails

rails new myapp --api
# API-only mode disables views, asset pipeline, and reduces middleware

Smaller server response size and faster JSON API responses by removing unused frontend code.

📈 Performance Gainreduces response size by 80-90 KB, improves LCP by faster backend response

Setting up a Rails backend to serve only JSON APIs without frontend views

Ruby on Rails

rails new myapp
# default full-stack setup with views, assets, and middleware
# serves HTML views and includes asset pipeline

Includes unnecessary middleware and frontend assets that increase server response size and slow down API response.

📉 Performance Costadds 100+ KB to server response, blocks rendering with asset compilation

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Full-stack Rails app serving HTML views	Many DOM nodes generated by views	Multiple reflows due to large DOM	High paint cost from complex UI	[X] Bad
Rails API-only app serving JSON	No DOM generated by server	No reflows triggered by server	No paint cost from server response	[OK] Good

Rendering Pipeline

In API-only mode, the server skips view rendering and asset compilation, directly sending JSON responses. This reduces backend processing and network payload, speeding up the critical rendering path on the client.

→Server Processing

→Network Transfer

→Client Rendering

⚠️ BottleneckServer Processing due to view rendering and asset compilation in full-stack mode

Core Web Vital Affected

LCP

This setup affects initial page load speed and server response time by minimizing unnecessary frontend assets and middleware.

Optimization Tips

1Use --api flag when creating Rails app to minimize middleware and assets.

2Avoid serving HTML views if only JSON APIs are needed to reduce payload size.

3Check network response size and time to verify API-only performance gains.

Performance Quiz - 3 Questions

What is the main performance benefit of using Rails API-only mode?

A Improves client-side rendering by adding more JavaScript

B Automatically caches frontend assets for faster loading

C Smaller server response size by excluding views and assets

D Enables server-side rendering of React components

DevTools: Network

How to check: Open DevTools > Network tab, reload API request, and inspect response size and time

What to look for: Look for smaller response payload and faster time to first byte (TTFB) in API-only mode

Practice

1. What is the main purpose of creating a Rails application with the --api flag?

easy

A. To generate a full Rails app with HTML views and assets

B. To enable automatic frontend scaffolding

C. To create a lightweight backend that serves JSON responses only

D. To create a desktop application using Rails

Solution

1. Step 1: Understand the --api flag purpose

   The --api flag creates a Rails app optimized for API use, focusing on JSON responses without HTML views.

2. Step 2: Compare with full Rails app

   Full Rails apps include views and assets, but API-only apps skip these for lightweight backend.

3. Final Answer:

   To create a lightweight backend that serves JSON responses only -> Option C

4. Quick Check:

   API-only app = JSON backend [OK]

Hint: API-only means JSON backend, no HTML views [OK]

Common Mistakes:

• Thinking API-only apps generate HTML views
• Confusing API-only with full Rails app
• Assuming API-only creates frontend code

2. Which command correctly creates a new Rails API-only application named my_api?

easy

A. rails new my_api --full

B. rails new my_api --api

C. rails generate api my_api

D. rails create my_api --api-only

Solution

1. Step 1: Recall the Rails new command syntax

   The correct way to create an API-only app is using rails new app_name --api.

2. Step 2: Match the command with the app name

   Using rails new my_api --api matches the requirement exactly.

3. Final Answer:

   rails new my_api --api -> Option B

4. Quick Check:

   rails new + app name + --api = correct command [OK]

Hint: Use 'rails new app_name --api' to create API apps [OK]

Common Mistakes:

• Using --full instead of --api
• Using 'generate' instead of 'new'
• Incorrect flag like --api-only

3. Given this Rails API-only controller code, what will be the JSON response when accessing /api/greetings?

class Api::GreetingsController < ApplicationController
  def index
    render json: { message: 'Hello, API!' }
  end
end

medium

A. Redirect to root path

B. Hello, API!

C. Error: Missing view template

D. {"message":"Hello, API!"}

Solution

1. Step 1: Analyze the controller action

   The index action uses render json: to send a JSON response with a message.

2. Step 2: Understand API-only behavior

   API-only apps respond with JSON, so the output is the JSON string of the hash.

3. Final Answer:

   {"message":"Hello, API!"} -> Option D

4. Quick Check:

   render json = JSON output [OK]

Hint: render json sends JSON, not HTML [OK]

Common Mistakes:

• Expecting HTML output in API-only app
• Assuming missing view causes error
• Thinking it redirects automatically

4. You created a Rails API-only app but get an error: ActionController::UnknownFormat when requesting JSON. What is the likely cause?

medium

A. The controller action does not specify render json:

B. The app was created without the --api flag

C. The route is missing in routes.rb

D. The database is not migrated

Solution

1. Step 1: Understand the error meaning

   ActionController::UnknownFormat means the controller does not know how to respond to the requested format.

2. Step 2: Check controller response method

   If the action lacks render json:, Rails can't respond with JSON, causing this error.

3. Final Answer:

   The controller action does not specify render json: -> Option A

4. Quick Check:

   UnknownFormat = missing render json [OK]

Hint: Always use render json in API controllers [OK]

Common Mistakes:

• Assuming missing --api flag causes this error
• Thinking missing route causes UnknownFormat
• Blaming database migration for format errors

5. You want to add authentication to your Rails API-only app using token-based headers. Which setup step is essential to support this in an API-only app?

hard

A. Add middleware for session and cookies support manually

B. Generate views for login and logout pages

C. Use rails new without --api to enable sessions

D. Disable JSON responses to allow HTML forms

Solution

1. Step 1: Understand API-only app default middleware

   API-only apps exclude middleware for sessions and cookies by default to stay lightweight.

2. Step 2: Add middleware manually for token auth

   To support token-based authentication using headers, you often need session or cookie middleware added manually.

3. Final Answer:

   Add middleware for session and cookies support manually -> Option A

4. Quick Check:

   API-only needs manual middleware for sessions [OK]

Hint: Add session middleware manually for token auth [OK]

Common Mistakes:

• Expecting views for login in API-only app
• Recreating full app without --api flag
• Disabling JSON responses breaks API design