Job retries and error handling in Ruby on Rails - Mini Project: Build & Apply

Job retries and error handling

📖 Scenario: You are building a background job in a Rails application that processes user data. Sometimes the job might fail due to temporary issues like network errors. You want to add retry logic and error handling to make the job more reliable.

🎯 Goal: Create a Rails Active Job class that retries up to 3 times on failure and logs errors when retries are exhausted.

📋 What You'll Learn

Create a job class named UserDataJob

Add a retry configuration to retry the job 3 times

Implement error handling to log a message when retries are exhausted

Use the perform method to simulate job work

💡 Why This Matters

🌍 Real World

Background jobs often fail due to temporary issues like network problems. Adding retries and error handling makes jobs more reliable and improves user experience.

💼 Career

Understanding job retries and error handling is essential for backend developers working with Rails to build robust and fault-tolerant applications.

1

Create the job class

Create a Rails job class called UserDataJob that inherits from ApplicationJob. Define an empty perform method that takes a parameter called user_id.

Ruby on Rails

class UserDataJob < ApplicationJob
  # Define perform method with user_id parameter
  def perform(user_id)
    # Your code here
  end
end

Hint

Use class UserDataJob < ApplicationJob and define def perform(user_id).

2

Add retry configuration

Add a retry configuration to UserDataJob to retry the job 3 times on failure using retry_on with StandardError.

Ruby on Rails

class UserDataJob < ApplicationJob
  retry_on StandardError, attempts: 3

  def perform(user_id)
    # Your code here
  end
end

Hint

Use retry_on StandardError, attempts: 3 inside the job class.

3

Implement job logic with error simulation

Inside the perform method, simulate job work by raising a StandardError with the message 'Temporary failure' to test retries.

Ruby on Rails

class UserDataJob < ApplicationJob
  retry_on StandardError, attempts: 3

  def perform(user_id)
    # Simulate a temporary failure
    # Your code here
  end
end

Hint

Use raise StandardError, 'Temporary failure' inside perform.

4

Add error handling after retries exhausted

Add an discard_on handler for StandardError that logs the message 'Job failed after retries' using Rails.logger.error.

Ruby on Rails

class UserDataJob < ApplicationJob
  retry_on StandardError, attempts: 3
  # Add discard_on handler below

  def perform(user_id)
    raise StandardError, 'Temporary failure'
  end
end

Hint

Use discard_on StandardError do |job, error| and inside log with Rails.logger.error.

Practice

1. What does the retry_on method do in a Rails Active Job?

easy

A. It retries the job automatically if a specified error occurs.

B. It discards the job immediately without retrying.

C. It schedules the job to run only once without retries.

D. It logs the error but does not retry the job.

Solution

1. Step 1: Understand retry_on purpose

   The retry_on method tells Rails to retry a job if a certain error happens during execution.

2. Step 2: Compare with other options

   Unlike discard_on, which stops retries, retry_on allows automatic retrying.

3. Final Answer:

   It retries the job automatically if a specified error occurs. -> Option A

4. Quick Check:

   retry_on = automatic retry [OK]

Hint: Remember: retry_on means try again on error [OK]

Common Mistakes:

• Confusing retry_on with discard_on
• Thinking retry_on discards the job
• Assuming retry_on logs errors only

2. Which of the following is the correct syntax to retry a job on ActiveRecord::Deadlocked error with 3 attempts in Rails?

easy

A. retry_on(ActiveRecord::Deadlocked, attempts: 3)

B. retry_on ActiveRecord::Deadlocked attempts=3

C. retry_on ActiveRecord::Deadlocked attempts: 3

D. retry_on(ActiveRecord::Deadlocked attempts: 3)

Solution

1. Step 1: Recall method call syntax in Ruby

   Methods with arguments and options require parentheses for clarity, especially with keyword arguments.

2. Step 2: Check each option's syntax

   retry_on(ActiveRecord::Deadlocked, attempts: 3) uses parentheses and correct keyword argument syntax: retry_on(ActiveRecord::Deadlocked, attempts: 3).

3. Final Answer:

   retry_on(ActiveRecord::Deadlocked, attempts: 3) -> Option A

4. Quick Check:

   Correct method call syntax = retry_on(ActiveRecord::Deadlocked, attempts: 3) [OK]

Hint: Use parentheses with keyword args for retry_on [OK]

Common Mistakes:

• Omitting parentheses with keyword arguments
• Using equal sign instead of colon for options
• Missing commas between arguments

3. Given this job code:

class MyJob < ApplicationJob
  retry_on StandardError, attempts: 2

  def perform
    raise StandardError, "fail"
  end
end

MyJob.perform_now

What happens when MyJob.perform_now is called?

medium

A. The job silently discards the error and finishes.

B. The job raises StandardError once and stops.

C. The job retries twice after failures, then raises error.

D. The job retries once after the first failure, then raises error.

Solution

1. Step 1: Understand retry_on with attempts: 2

   It means the job will try up to 2 times total (1 initial + 1 retry) before giving up.

2. Step 2: Analyze job behavior

   The job raises StandardError every time. It runs once, fails, retries once, then fails again and raises error.

3. Final Answer:

   The job retries once after the first failure, then raises error. -> Option D

4. Quick Check:

   attempts: 2 means 1 retry [OK]

Hint: attempts count includes first try, so retries = attempts - 1 [OK]

Common Mistakes:

• Thinking attempts means retries count
• Assuming job discards error automatically
• Believing job retries infinitely

4. Identify the error in this job code snippet:

class ExampleJob < ApplicationJob
  retry_on(StandardError, attempts: 3)

  def perform
    # job logic
  end
end

medium

A. The retry_on method should be called inside perform method.

B. The retry_on method should not have parentheses.

C. The retry_on method is correctly used; no error.

D. The retry_on method should be declared with a block.

Solution

1. Step 1: Check retry_on usage location

   retry_on is a class-level method and should be declared outside perform method, as shown.

2. Step 2: Verify syntax correctness

   Using parentheses with arguments is valid Ruby syntax here.

3. Final Answer:

   The retry_on method is correctly used; no error. -> Option C

4. Quick Check:

   retry_on class method syntax is correct [OK]

Hint: retry_on is a class method, not inside perform [OK]

Common Mistakes:

• Placing retry_on inside perform method
• Thinking parentheses are disallowed
• Expecting retry_on to require a block

5. You want a job to retry up to 5 times on Net::OpenTimeout errors but discard immediately on ArgumentError. Which code correctly implements this behavior?

hard

A. class NetworkJob < ApplicationJob
discard_on Net::OpenTimeout
retry_on ArgumentError, attempts: 5

def perform
# network call
end
end

B. class NetworkJob < ApplicationJob
retry_on Net::OpenTimeout, attempts: 5
discard_on ArgumentError

def perform
# network call
end
end

C. class NetworkJob < ApplicationJob
retry_on Net::OpenTimeout
retry_on ArgumentError, attempts: 5

def perform
# network call
end
end

D. class NetworkJob < ApplicationJob
discard_on Net::OpenTimeout, attempts: 5
discard_on ArgumentError

def perform
# network call
end
end

Solution

1. Step 1: Understand retry_on and discard_on roles

   retry_on retries job on specified error; discard_on stops retries and discards job immediately.

2. Step 2: Match errors to correct methods

   Net::OpenTimeout should retry up to 5 times, so use retry_on with attempts: 5. ArgumentError should discard immediately, so use discard_on.

3. Final Answer:

   Code with retry_on Net::OpenTimeout, attempts: 5 and discard_on ArgumentError -> Option B

4. Quick Check:

   retry_on for retry, discard_on for discard [OK]

Hint: retry_on = retry; discard_on = stop and discard [OK]

Common Mistakes:

• Swapping retry_on and discard_on for errors
• Missing attempts option for retry_on
• Using discard_on with attempts option