Background email delivery in Ruby on Rails - Performance & Optimization

Performance: Background email delivery

HIGH IMPACT

This affects page load speed and user interaction responsiveness by offloading email sending from the main request cycle.

Sending emails after user actions without blocking the UI

Ruby on Rails

def create
  UserMailer.welcome_email(@user).deliver_later
  redirect_to root_path
end

Email is queued and sent asynchronously, freeing the web request to complete immediately.

📈 Performance GainNon-blocking request, improves INP by hundreds of milliseconds

Sending emails after user actions without blocking the UI

Ruby on Rails

def create
  UserMailer.welcome_email(@user).deliver_now
  redirect_to root_path
end

Sending email synchronously blocks the web request until the email is sent, causing slow response times.

📉 Performance CostBlocks rendering for 500ms+ depending on email server response

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Synchronous email sending (deliver_now)	N/A	N/A	Blocks response, delays paint	[X] Bad
Asynchronous email sending (deliver_later)	N/A	N/A	Non-blocking, fast paint	[OK] Good

Rendering Pipeline

Synchronous email sending blocks the main thread during the request, delaying response and paint. Background delivery moves email sending to a separate job queue, allowing the browser to receive and render the page faster.

→Request Handling

→Response Time

→Interaction Responsiveness

⚠️ BottleneckRequest Handling blocking on email delivery

Core Web Vital Affected

INP

This affects page load speed and user interaction responsiveness by offloading email sending from the main request cycle.

Optimization Tips

1Never send emails synchronously during web requests.

2Use deliver_later with a background job processor like Sidekiq or Delayed Job.

3Monitor background job queues to ensure timely email delivery.

Performance Quiz - 3 Questions

What is the main performance problem with sending emails synchronously during a web request?

A It causes layout shifts on the page

B It increases CSS paint time

C It blocks the request, delaying page load and interaction

D It reduces image loading speed

DevTools: Performance

How to check: Record a performance profile while triggering the email sending action. Look for long tasks blocking the main thread during the request.

What to look for: Long blocking tasks or delayed Time to Interactive indicate synchronous email sending.

Practice

1. What does deliver_later do in Rails email sending?

easy

A. Previews the email without sending it

B. Sends the email immediately and blocks the app until done

C. Schedules the email to be sent at a specific future time

D. Queues the email to be sent in the background without blocking the app

Solution

1. Step 1: Understand deliver_later purpose

   deliver_later queues the email to be sent asynchronously, so the app doesn't wait for the email to finish sending.

2. Step 2: Compare with other options

   deliver_now sends immediately and blocks, scheduling for future time requires extra setup, and previewing doesn't send email.

3. Final Answer:

   Queues the email to be sent in the background without blocking the app -> Option D

4. Quick Check:

   deliver_later = background email sending [OK]

Hint: Remember: deliver_later means send email later in background [OK]

Common Mistakes:

• Confusing deliver_later with deliver_now
• Thinking deliver_later schedules for a specific time
• Assuming deliver_later previews email

2. Which of the following is the correct syntax to send a Rails mailer email in the background?

easy

A. UserMailer.welcome_email.deliver_later(user)

B. UserMailer.welcome_email(user).send_later

C. UserMailer.welcome_email(user).deliver_later

D. UserMailer.deliver_later.welcome_email(user)

Solution

1. Step 1: Recall correct method chaining

   In Rails, you call the mailer method first with parameters, then call deliver_later on the result.

2. Step 2: Check each option's syntax

   UserMailer.welcome_email(user).deliver_later matches the correct pattern: UserMailer.welcome_email(user).deliver_later. Others have incorrect method order or non-existent methods.

3. Final Answer:

   UserMailer.welcome_email(user).deliver_later -> Option C

4. Quick Check:

   Mailer method then deliver_later [OK]

Hint: Call mailer method first, then .deliver_later [OK]

Common Mistakes:

• Swapping method order
• Using non-existent methods like send_later
• Passing parameters after deliver_later

3. Given this code snippet in a Rails app with Active Job configured:

class UserMailer < ApplicationMailer
  def welcome_email(user)
    mail(to: user.email, subject: 'Welcome!')
  end
end

UserMailer.welcome_email(@user).deliver_later

What happens when this code runs?

medium

A. The email is queued to be sent by a background job and the app continues immediately

B. The email is sent immediately and the app waits until it finishes

C. The email is not sent because deliver_later requires extra parameters

D. The code raises an error because deliver_later is not defined

Solution

1. Step 1: Understand deliver_later behavior with Active Job

   When Active Job is set up, calling deliver_later queues the email to be sent asynchronously by a background worker.

2. Step 2: Analyze the code snippet

   The mailer method is called correctly, and deliver_later queues the job. The app does not wait for sending to complete.

3. Final Answer:

   The email is queued to be sent by a background job and the app continues immediately -> Option A

4. Quick Check:

   deliver_later queues email, no blocking [OK]

Hint: deliver_later queues email, app keeps running [OK]

Common Mistakes:

• Thinking deliver_later sends email immediately
• Assuming deliver_later needs extra parameters
• Believing deliver_later is undefined without setup

4. You wrote this code to send an email in the background:

UserMailer.welcome_email(user).deliver_later

But emails are not being sent. What is the most likely cause?

medium

A. You forgot to configure a background job adapter like Sidekiq or Async

B. You should use deliver_now instead of deliver_later

C. The mailer method welcome_email is missing the mail call

D. deliver_later requires a block to work

Solution

1. Step 1: Check background job adapter setup

   deliver_later queues jobs but requires a background job adapter (like Sidekiq, Async, or Delayed Job) to process the queue.

2. Step 2: Evaluate other options

   Using deliver_now sends immediately but blocks. Missing mail call causes no email but not related to background delivery. deliver_later does not require a block.

3. Final Answer:

   You forgot to configure a background job adapter like Sidekiq or Async -> Option A

4. Quick Check:

   Background job adapter needed for deliver_later [OK]

Hint: Check if background job adapter is set up [OK]

Common Mistakes:

• Using deliver_later without job adapter
• Confusing deliver_now with deliver_later
• Thinking deliver_later needs a block

5. You want to send a welcome email only if the user has a confirmed email address, and send it in the background. Which code snippet correctly applies this logic?

hard

A. UserMailer.welcome_email(user).deliver_later unless user.email_confirmed?

B. UserMailer.welcome_email(user).deliver_later if user.email_confirmed?

C. if user.email_confirmed?
UserMailer.welcome_email(user).deliver_now
end

D. UserMailer.welcome_email(user).deliver_later(user.email_confirmed?)

Solution

1. Step 1: Apply conditional check before sending email

   The email should be sent only if user.email_confirmed? is true, so the condition must guard the delivery call.

2. Step 2: Use deliver_later for background sending

   UserMailer.welcome_email(user).deliver_later if user.email_confirmed? correctly uses the condition and calls deliver_later to send asynchronously. if user.email_confirmed? UserMailer.welcome_email(user).deliver_now end sends immediately, not in background. UserMailer.welcome_email(user).deliver_later unless user.email_confirmed? sends when email is not confirmed (wrong logic). UserMailer.welcome_email(user).deliver_later(user.email_confirmed?) passes a boolean as argument incorrectly.

3. Final Answer:

   UserMailer.welcome_email(user).deliver_later if user.email_confirmed? -> Option B

4. Quick Check:

   Condition then deliver_later for background send [OK]

Hint: Check condition before calling deliver_later [OK]

Common Mistakes:

• Sending email immediately instead of background
• Using wrong condition logic
• Passing condition as argument to mailer