Do we need to implement concurrency with scheduled cron jobs?

[adeeb1]

Hi, and thank you for your amazing work on this gem! Under the Cron section of the README, it states:

Cron-style jobs are run on every GoodJob process (e.g. CLI or async execution mode) when config.good_job.enable_cron = true, but GoodJob's cron uses unique indexes to ensure that only a single job is enqeued at the given time interval.

Let's say I have two VMs each running GoodJob workers, and I'd like a daily cron job to run only one time each day. Would the default cron unique index work for this use case, or do I also need to implement concurrency to ensure the job runs only once?

[bensheldon]

@adeeb1 great question!

GoodJob cron will handle concurrency on its own. You do not need to implement concurrency separately.

You can double-check that. Your database should have a unique compound index on cron_key, cron_at:

good_job/spec/test_app/db/schema.rb

Line 52 in 8990bc0

t.index ["cron_key", "cron_at"], name: "index_good_jobs_on_cron_key_and_cron_at", unique: true

The cron_at column should be populated with the cron job's scheduled time, and thus prevent duplicate jobs from being enqueued at the same time.

[adeeb1]

Thanks for the quick response @bensheldon! At what point does it create this record? On server initialization? I had some difficulties finding the logic, so any pointers would be helpful.

[bensheldon]

GoodJob's cron implementation enqueues the ActiveJob job at the scheduled time. That timing is managed with Concurrent::ScheduledTasks:

good_job/lib/good_job/cron_manager.rb

Lines 84 to 99 in 8990bc0

	def create_task(cron_entry)
	cron_at = cron_entry.next_at
	delay = [(cron_at - Time.current).to_f, 0].max
	future = Concurrent::ScheduledTask.new(delay, args: [self, cron_entry, cron_at]) do |thr_scheduler, thr_cron_entry, thr_cron_at|
	# Re-schedule the next cron task before executing the current task
	thr_scheduler.create_task(thr_cron_entry)
	Rails.application.executor.wrap do
	cron_entry.enqueue(thr_cron_at) if thr_cron_entry.enabled?
	end
	end
	@tasks[cron_entry.key] = future
	future.add_observer(self.class, :task_observer)
	future.execute
	end

Here's where the CronEntry model rescues the duplicate entry:

good_job/app/models/good_job/cron_entry.rb

Lines 103 to 113 in 8990bc0

	def enqueue(cron_at = nil)
	GoodJob::CurrentThread.within do |current_thread|
	current_thread.cron_key = key
	current_thread.cron_at = cron_at
	configured_job = job_class.constantize.set(set_value)
	kwargs_value.present? ? configured_job.perform_later(*args_value, **kwargs_value) : configured_job.perform_later(*args_value)
	end
	rescue ActiveRecord::RecordNotUnique
	false
	end

[adeeb1]

Wonderful, thank you @bensheldon! I greatly appreciate the quick response.