如何确保相同的延迟作业不会同时两次（互斥）

Question

For a project I want to make sure that there a certain job is not running twice at the same time. This job is an importer and just does not make sense to run again if it is still running. If we detect that the job is already running, I want to raise an exception so I get alerted about long running jobs.

Answer 1

I found a way to solve this. We introduce a new custom job . When the job is performed, we check if there is already another job running:

# checks if the job is already running when it is supposed to be performed.
# if it is already running, then we raise an error
# jobs with different arguments but same object and method raise an error.
class ExclusiveJob < Struct.new(:object, :meth, :args)
  class ExclusiveJobError < RuntimeError; end

  def initialize(object, meth, *args)
    super(object, meth, args)
  end

  def perform
    same_and_current_jobs = Delayed::Job
      .where(last_error: nil)
      .where.not(locked_at: nil)
      .collect { |job| YAML.load(job.handler) }
      .select { |handler| handler.is_a?(self.class) && handler.object == object && handler.meth == meth }
    
    raise ExclusiveJobError, "Tried to perform \"#{identifier}\", but it is already running" if same_and_current_jobs.count > 1 # we have more than this job in the pipeline

    object.send(meth, *args)
  end

  protected

  # just for display purposes
  def identifier
    "#{object}##{meth}"
  end
end

Please mind that this code is not ideal because we are relying on the internal data model of delayed job (however it is officially documented ). Also, please mind that this class does not take method arguments into account, ie we skip the job if another job with the same receiver & method is found. And, we are not using ActiveJob, there are probably ways to solve this using callbacks .

To enqueue such an exclusive job:

Delayed::Job.enqueue ExclusiveJob.new(MySuperService, :run, 'arg1', :arg2)