Job creation and queuing in Ruby on Rails - Performance & Optimization

Performance: Job creation and queuing

HIGH IMPACT

This affects page load speed and user interaction responsiveness by offloading heavy tasks to background jobs.

Handling a slow task like sending emails during a web request

Ruby on Rails

def create
  UserMailer.welcome_email(@user).deliver_later
  render :show
end

Queues the email to be sent asynchronously, freeing the web request immediately.

📈 Performance GainNon-blocking request, improves INP by 200-500ms

Handling a slow task like sending emails during a web request

Ruby on Rails

def create
  UserMailer.welcome_email(@user).deliver_now
  render :show
end

Sending email synchronously blocks the web request, delaying response time.

📉 Performance CostBlocks rendering for 200-500ms depending on email server speed

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Synchronous job execution	Minimal	Multiple reflows due to blocking	High paint cost due to delayed rendering	[X] Bad
Asynchronous job queuing	Minimal	Single reflow	Low paint cost, fast rendering	[OK] Good

Rendering Pipeline

Synchronous job execution blocks the main thread during the Layout and Paint stages, delaying rendering. Asynchronous queuing defers work, allowing the browser to complete rendering faster.

→Layout

→Paint

→Composite

⚠️ BottleneckLayout and Paint stages due to blocking synchronous job execution

Core Web Vital Affected

INP

This affects page load speed and user interaction responsiveness by offloading heavy tasks to background jobs.

Optimization Tips

1Never run slow jobs synchronously during web requests.

2Use perform_later to queue jobs asynchronously.

3Monitor job queue length to avoid delays in background processing.

Performance Quiz - 3 Questions

What is the main performance benefit of queuing jobs asynchronously in Rails?

A It prevents blocking the web request, improving input responsiveness.

B It reduces the number of database queries.

C It decreases the size of the JavaScript bundle.

D It improves CSS selector matching speed.

DevTools: Performance

How to check: Record a performance profile during a web request that triggers job execution. Look for long tasks blocking the main thread.

What to look for: Long blocking tasks in the Main thread timeline indicate synchronous job execution slowing rendering.

Practice

1. What is the main purpose of creating a job in Rails using ApplicationJob?

easy

A. To run tasks in the background without blocking the main app flow

B. To create database tables automatically

C. To handle user authentication

D. To style the user interface

Solution

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

   Jobs are designed to run tasks asynchronously, so they don't slow down the main app.

2. Step 2: Identify the correct purpose

   Running tasks in the background matches the job's purpose, unlike database creation or UI styling.

3. Final Answer:

   To run tasks in the background without blocking the main app flow -> Option A

4. Quick Check:

   Jobs = background tasks [OK]

Hint: Jobs run tasks later to keep app fast [OK]

Common Mistakes:

• Thinking jobs create database tables
• Confusing jobs with UI styling
• Assuming jobs handle user login

2. Which of the following is the correct way to enqueue a job named SendEmailJob to run later?

easy

A. SendEmailJob.perform_later

B. SendEmailJob.run_later

C. SendEmailJob.enqueue

D. SendEmailJob.perform_now

Solution

1. Step 1: Recall the method to queue jobs in Rails

   The method perform_later adds the job to the queue for later execution.

2. Step 2: Compare options

   perform_now runs immediately, enqueue and run_later are not valid Rails job methods.

3. Final Answer:

   SendEmailJob.perform_later -> Option A

4. Quick Check:

   Use perform_later to queue jobs [OK]

Hint: Use perform_later to queue jobs [OK]

Common Mistakes:

• Using perform_now instead of perform_later
• Using non-existent methods like enqueue or run_later
• Confusing immediate execution with queuing

3. Given this job definition:

class CleanupJob < ApplicationJob
  def perform
    puts 'Cleaning up!'
  end
end

What will happen when you call CleanupJob.perform_later?

medium

A. Prints 'Cleaning up!' immediately

B. Adds the job to the queue and prints 'Cleaning up!' later

C. Raises an error because perform_later is undefined

D. Does nothing because perform is empty

Solution

1. Step 1: Understand perform_later behavior

   Calling perform_later queues the job to run asynchronously later, not immediately.

2. Step 2: Check the perform method

   The perform method prints 'Cleaning up!' when the job runs, so this will happen later after queuing.

3. Final Answer:

   Adds the job to the queue and prints 'Cleaning up!' later -> Option B

4. Quick Check:

   perform_later queues job, runs perform later [OK]

Hint: perform_later queues job, runs perform method later [OK]

Common Mistakes:

• Thinking perform_later runs perform immediately
• Assuming perform_later is undefined
• Believing perform method is empty and does nothing

4. Identify the error in this job code:

class NotifyUserJob < ApplicationJob
  def perform(user_id)
    User.find(user_id).notify
  end
end

NotifyUserJob.perform_later

medium

A. Cannot inherit from ApplicationJob

B. Wrong method name; should be perform_now

C. User model does not exist

D. Missing argument when calling perform_later

Solution

1. Step 1: Check perform method parameters

   The perform method requires one argument: user_id.

2. Step 2: Check perform_later call

   Calling perform_later without arguments causes an error because user_id is missing.

3. Final Answer:

   Missing argument when calling perform_later -> Option D

4. Quick Check:

   perform_later needs all perform arguments [OK]

Hint: Always pass perform arguments to perform_later [OK]

Common Mistakes:

• Calling perform_later without required arguments
• Confusing perform_later with perform_now
• Assuming inheritance from ApplicationJob is invalid

5. You want to create a job that sends a welcome email only if the user has verified their email. Which is the best way to implement this in perform?

hard

A. Raise an error if user is not verified inside perform

B. Send email unconditionally in perform and handle verification elsewhere

C. Check user.verified? inside perform and send email only if true

D. Call perform_now instead of perform_later

Solution

1. Step 1: Understand conditional logic in jobs

   Jobs can include logic to decide whether to perform actions, like checking if user is verified.

2. Step 2: Evaluate options

   Check user.verified? inside perform and send email only if true correctly checks verification inside perform before sending email, ensuring only verified users get emails.

3. Final Answer:

   Check user.verified? inside perform and send email only if true -> Option C

4. Quick Check:

   Use condition inside perform for selective job actions [OK]

Hint: Put conditions inside perform to control job actions [OK]

Common Mistakes:

• Sending emails without checking verification
• Raising errors unnecessarily in perform
• Confusing perform_now with perform_later