How to make a job idempotent for 1 run every month (Rails 5)?

Question

I need to generate invoices once per month, according to the EST/EDT timezone (the clients are throughout the country but in this industry billing happens at the same timezone).

I'm creating a GenerateInvoicesJob but I'm having trouble reasoning about a 100% perfect way to generate invoices so that there isn't any possible duplicate/confusion with regards to:

• Generate invoices only once per month
• Let the job run every day
• Make the job idempotent

Then the final point which is the hard one for me, how do I ensure that there are no bugs with EST/DST and 1 hour slipping through.

Here is my clock.rb:

every(1.day, 'GenerateInvoicesJob', tz: 'America/New_York', at: '04:00') do
  Delayed::Job.enqueue GenerateInvoicesJob.new, queue: 'high'
end

And here's the top of my job:

Unit.where(enabled: true)
  .joins(:user)
  .where('last_invoice_generated_at <= ?', Time.now.utc.end_of_month)
  .each do |unit|

  ActiveRecord::Base.transaction do
    unit.update_attributes(
      last_invoice_generated_at: Time.now.utc
    )
    invoice = Invoice.create!(
      ...
    )
    line_item = LineItem.create!(
      ...
    )
  end

I realize the direct conditional logic might be wrong, so that's not entirely my question... my main addition to that question is whats the best way overall to do this so I can make sure that all times in EST are 100% accounted for, including weird off-by-1-hour bugs, etc. This job is super important so I'm hesitant on a way to make it perfect.

On top of that I"m not sure whether I should store UTC in the database.... normally I know you always are supposed to store UTC, but I know UTC doesnt have DST so I'm afraid if I store it like that, the job could run one time and invoices would be not run properly

Answer 1

I would do something like this in the worker:

# `beginning_of_month` because we want to load units that haven't 
# been billed this month
units_to_bill = Unit.where(enabled: true)
  .where('last_invoice_generated_at < ?', Time.current.beginning_of_month)

# `find_each` because it needs less memory
units_to_bill.find_each do |unit|

  # Beginn a transaction to ensure all or nothing is updated
  Unit.transaction do

    # reload the unit, because it might have been updated by another 
    # task in the meantime
    unit.reload

    # lock the current unit for updates
    unit.lock!

    # check if the condition is still true
    if unit.last_invoice_generated_at < 1.month.ago

      # generate invoices
      # invoice = Invoice.create!(
      # line_item = LineItem.create!(

      # last step update unit
      unit.update_attributes(
        last_invoice_generated_at: Time.current
      )
    end
  end
end