Scheduled jobs in Ruby on Rails - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a scheduled job in Rails?

A scheduled job in Rails is a task set to run automatically at specific times or intervals without manual intervention. It helps automate repetitive work like sending emails or cleaning up data.

beginner

Which gem is commonly used in Rails to manage scheduled jobs?

The 'sidekiq-scheduler' gem is commonly used to schedule jobs in Rails. It works with Sidekiq to run background jobs at set times using cron-like syntax.

intermediate

How do you define a scheduled job using Sidekiq Scheduler?

You define a scheduled job by adding a YAML file (e.g., schedule.yml) with job names, cron timing, and the worker class to run. Sidekiq Scheduler reads this file and runs jobs automatically.

intermediate

What is the role of cron syntax in scheduled jobs?

Cron syntax specifies when a scheduled job should run, using fields for minute, hour, day, month, and weekday. It lets you set precise recurring times like 'every day at 3 AM'.

advanced

Why should scheduled jobs be idempotent in Rails?

Scheduled jobs should be idempotent so running them multiple times does not cause errors or duplicate effects. This ensures reliability if a job runs again due to retries or failures.

Which gem pairs with Sidekiq to schedule recurring jobs in Rails?

A sidekiq-scheduler

B delayed_job

C whenever

D resque

sidekiq-scheduler extends Sidekiq to run jobs on a schedule using cron syntax.

What does the cron expression '0 0 * * *' mean for a scheduled job?

A Run every hour

B Run every day at midnight

C Run every minute

D Run every Sunday

The expression means run at minute 0, hour 0, every day, which is midnight daily.

Where do you typically define scheduled jobs for Sidekiq Scheduler?

A In a YAML file like schedule.yml

B In the Rails routes file

C In the database schema

D In the application controller

Scheduled jobs are defined in a YAML file that Sidekiq Scheduler reads to know when to run jobs.

Why is it important for scheduled jobs to be idempotent?

A To improve UI responsiveness

B To reduce memory usage

C To avoid duplicate effects if run multiple times

D To speed up job execution

Idempotency prevents errors or duplicates if a job runs more than once.

Which Rails component is NOT directly involved in scheduled jobs?

A Active Job

B Sidekiq

C sidekiq-scheduler

D Action Mailbox

Action Mailbox handles inbound emails, not scheduled jobs.

Explain how you would set up a scheduled job in Rails using Sidekiq Scheduler.

Think about the gems, worker, schedule file, and Sidekiq process.

Why is cron syntax useful for scheduled jobs and how does it work?

Consider how you tell a job when to run repeatedly.

Practice

1. What is the main purpose of scheduled jobs in a Rails application?

easy

A. To manage database migrations automatically

B. To handle user authentication and sessions

C. To automate tasks to run at specific times without manual effort

D. To style the user interface dynamically

Solution

1. Step 1: Understand the role of scheduled jobs

   Scheduled jobs are designed to run tasks automatically at set times without needing a user to start them manually.

2. Step 2: Compare with other options

   User authentication, database migrations, and UI styling are handled by other parts of Rails, not scheduled jobs.

3. Final Answer:

   To automate tasks to run at specific times without manual effort -> Option C

4. Quick Check:

   Scheduled jobs automate tasks = C [OK]

Hint: Scheduled jobs run tasks automatically on a schedule [OK]

Common Mistakes:

• Confusing scheduled jobs with user authentication
• Thinking scheduled jobs handle database migrations
• Assuming scheduled jobs style the UI

2. Which of the following is the correct way to define a job class in Rails?

easy

A. def MyJob
# task code
end

B. class MyJob
def perform
# task code
end
end

C. module MyJob
def perform
# task code
end
end

D. class MyJob < ApplicationJob
def perform
# task code
end
end

Solution

1. Step 1: Check the inheritance pattern for jobs

   In Rails, jobs must inherit from ApplicationJob to integrate with Active Job framework.

2. Step 2: Verify the method definition

   The perform method defines the task to run. class MyJob < ApplicationJob def perform # task code end end correctly defines a class inheriting ApplicationJob with perform method.

3. Final Answer:

   class MyJob < ApplicationJob\n def perform\n # task code\n end\nend -> Option D

4. Quick Check:

   Job class inherits ApplicationJob = A [OK]

Hint: Jobs inherit ApplicationJob and define perform method [OK]

Common Mistakes:

• Defining job without inheriting ApplicationJob
• Using module instead of class for jobs
• Defining job as a method instead of a class

3. Given the following job class, what will be the output when MyJob.perform_now is called?

class MyJob < ApplicationJob
  def perform
    puts 'Job executed'
  end
end

medium

A. Job executed

B. No output, job runs silently

C. Error: perform method missing

D. Job executed asynchronously

Solution

1. Step 1: Understand perform_now behavior

   Calling perform_now runs the job immediately and synchronously, executing the perform method.

2. Step 2: Check the perform method output

   The perform method prints 'Job executed' to the console, so this string will appear as output.

3. Final Answer:

   Job executed -> Option A

4. Quick Check:

   perform_now runs perform immediately = B [OK]

Hint: perform_now runs job immediately and prints output [OK]

Common Mistakes:

• Thinking perform_now runs job silently
• Confusing perform_now with asynchronous perform_later
• Expecting an error due to missing method

4. Identify the error in this job scheduling code snippet:

class CleanupJob < ApplicationJob
  def perform
    User.delete_all
  end
end

# Schedule in config/schedule.rb
every 1.day do
  runner 'CleanupJob.perform_later'
end

medium

A. No error, code is correct

B. perform_later should be perform_now in schedule

C. Using runner instead of calling rake task

D. runner expects a method call, not a class method

Solution

1. Step 1: Examine the job class definition

   The class correctly inherits from ApplicationJob and defines the perform method.

2. Step 2: Verify scheduling syntax

   The config/schedule.rb uses standard Whenever syntax: every 1.day with runner 'CleanupJob.perform_later', which executes Ruby code to enqueue the job asynchronously. No errors present.

3. Final Answer:

   No error, code is correct -> Option A

4. Quick Check:

   runner 'Job.perform_later' in schedule.rb = D [OK]

Hint: runner 'Job.perform_later' correctly enqueues job [OK]

Common Mistakes:

• Thinking runner expects only instance methods
• Confusing runner with rake tasks
• Believing perform_later is invalid in cron schedules

5. You want to schedule a job that runs every day at midnight to archive old records. Which of these is the best way to set this up using the whenever gem?

class ArchiveJob < ApplicationJob
  def perform
    Record.archive_old
  end
end

# config/schedule.rb
?

Which snippet correctly schedules the job?

hard

A. every 1.day, at: '12:00 pm' do
runner 'ArchiveJob.perform_later'
end

B. every :day, at: '00:00' do
runner 'ArchiveJob.perform_later'
end

C. every 24.hours do
rake 'archive:old_records'
end

D. every :midnight do
runner 'ArchiveJob.perform_now'
end

Solution

1. Step 1: Check whenever syntax for daily jobs at midnight

   The whenever gem uses symbols like :day and time strings like '00:00' to schedule jobs at midnight.

2. Step 2: Verify the job call method

   Using perform_later runs the job asynchronously, which is preferred for scheduled jobs to avoid blocking.

3. Step 3: Compare options

   every :day, at: '00:00' do runner 'ArchiveJob.perform_later' end uses correct symbol :day, time '00:00', and perform_later with runner, matching best practice.

4. Final Answer:

   every :day, at: '00:00' do\n runner 'ArchiveJob.perform_later'\nend -> Option B

5. Quick Check:

   whenever daily at midnight with perform_later = D [OK]

Hint: Use :day and '00:00' with perform_later in whenever [OK]

Common Mistakes:

• Using perform_now instead of perform_later for scheduled jobs
• Incorrect time format like '12:00 am' instead of '00:00'
• Using rake task instead of runner for Active Job