Background email delivery in Ruby on Rails - Step-by-Step Execution

Concept Flow - Background email delivery

User triggers email send

↓

Create email job

↓

Add job to queue

↓

Background worker picks job

↓

Send email asynchronously

↓

Job completes, user continues

This flow shows how an email send request is turned into a background job, queued, and processed asynchronously so the user doesn't wait.

Execution Sample

Ruby on Rails

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

UserMailer.with(user: @user).welcome_email.deliver_later

This code defines an email and sends it in the background using deliver_later.

Execution Table

Step	Action	Evaluation	Result
1	Call deliver_later on welcome_email	Creates a job to send email	Job added to queue
2	Background worker picks job	Starts executing email send	Email sending begins
3	Mail method builds email content	Email content ready	Email ready to send
4	Email sent via SMTP or service	Email delivered	User receives email
5	Job marked complete	Worker ready for next job	Background process idle
6	User continues without waiting	No delay in user flow	Smooth user experience

💡 Job completes after email is sent, user flow is not blocked by email delivery

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	Final
job_status	none	queued	processing	building email	sent	completed
user_flow	waiting	continued	continued	continued	continued	continued

Key Moments - 3 Insights

Why doesn't the user wait for the email to send?

Because deliver_later creates a background job (see execution_table step 1) that runs separately, so the user flow continues immediately (step 6).

What happens if the background worker is busy?

The job stays queued until a worker is free (job_status stays 'queued' in variable_tracker), so email sending is delayed but user flow is unaffected.

How does Rails know what email to send in the background?

The mail method builds the email content inside the job (step 3), so the job has all info needed to send the email asynchronously.

Visual Quiz - 3 Questions

Look at the execution_table, what is the job_status after step 2?

A queued

B sent

C processing

D completed

💡 Hint

Check variable_tracker row for job_status after Step 2

At which step does the user flow continue without waiting?

A Step 3

B Step 6

C Step 1

D Step 4

💡 Hint

Look at execution_table step describing user continuing

If deliver_now was used instead of deliver_later, how would the execution_table change?

A User would wait until email is sent

B Job would be queued but not processed

C Background worker picks job immediately

D Email would never be sent

💡 Hint

deliver_now sends email immediately blocking user flow, unlike deliver_later

Concept Snapshot

Background email delivery in Rails:
- Use deliver_later to send emails asynchronously
- Creates a job added to a queue
- Background worker processes job and sends email
- User flow continues without waiting
- Improves app responsiveness and user experience

Full Transcript

Background email delivery in Rails works by turning an email send request into a background job using deliver_later. This job is added to a queue and processed by a background worker separately from the user's request. The worker builds the email content and sends it asynchronously. This means the user does not wait for the email to send and experiences a smooth flow. The job status changes from queued to processing to completed as the email is sent. If deliver_now is used instead, the user waits for the email to send, blocking the flow. Using background jobs improves app performance and user satisfaction.

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