Job retries and error handling in Ruby on Rails - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of job retries in Rails Active Job?

Job retries automatically try to run a failed job again after an error occurs. This helps handle temporary problems like network issues without losing the job.

beginner

How do you specify the number of retries for a job in Rails?

You use the <code>retry_on</code> method inside your job class and pass the error type and options like <code>wait</code> time and <code>attempts</code> count.

intermediate

What does the discard_on method do in a Rails job?

It tells Rails to stop retrying the job if a specific error happens. The job is discarded immediately without retrying.

intermediate

How can you customize the wait time between retries in Rails jobs?

You can pass a wait option to retry_on with a number of seconds or a block that returns the wait time. This controls how long Rails waits before retrying.

intermediate

What happens if a job keeps failing after all retries are exhausted?

The job is marked as failed and removed from the retry queue. You can handle this by defining rescue_from in the job to log or notify about the failure.

Which method in Rails Active Job is used to retry a job on specific errors?

A retry_on

B discard_on

C rescue_from

D retry_job

The retry_on method tells Rails to retry the job when certain errors happen.

What does discard_on do in a Rails job?

A Retries the job immediately

B Stops retrying and discards the job on specific errors

C Logs the error but retries anyway

D Schedules the job for later

discard_on stops retries and discards the job when the specified error occurs.

How can you set a custom wait time between retries in Rails jobs?

A Using <code>sleep</code> inside the job perform method

B Using <code>delay</code> option in <code>discard_on</code>

C Using <code>wait_for</code> method

D Using <code>wait</code> option in <code>retry_on</code>

The wait option in retry_on sets how long to wait before retrying.

If a job fails after all retries, what can you do to handle it?

A Use <code>retry_on</code> again

B Nothing, the job is lost

C Use <code>rescue_from</code> to catch and handle the failure

D Use <code>discard_on</code> to retry

rescue_from lets you handle errors after retries are exhausted.

Which of these is NOT a valid way to handle errors in Rails jobs?

A retry_job_forever

B discard_on

C rescue_from

D retry_on

retry_job_forever is not a valid Rails Active Job method.

Explain how job retries work in Rails Active Job and how you can control retry behavior.

Think about how Rails tries again after errors and how you tell it when to stop.

Describe how you would handle a job failure after all retry attempts have been used in Rails.

Consider what happens when retries run out and how to respond.

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