gistfile1.rb

Delayed::Backend::ActiveRecord::Job.class_eval do
  def self.build_ready_scope(worker, max_run_time)
    ready_scope = ready_to_run(worker.name, max_run_time)
    ready_scope = ready_scope.by_priority

    ready_scope = ready_scope.where(
      RandomUserIdQuery::SQL, db_time_now: db_time_now,
                              worker_name: worker.name,
                              locked_at_expired_at: db_time_now - max_run_time
    )

    ready_scope
  end

  def self.reserve(worker, max_run_time = Delayed::Worker.max_run_time)
    reserve_with_scope(build_ready_scope(worker, max_run_time), worker, db_time_now)
  end
end

class RandomUserIdQuery
  def generate_sql
    <<-EOSQL
      user_id =
        (
          WITH
            #{top_priority_cte},
            #{eligible_user_ids_cte}
          SELECT user_id
          FROM eligible_user_ids
          ORDER BY random()
          LIMIT 1
        )
    EOSQL
  end

  private

 # A job is runnable if it isn't locked or permanently failed, and the run_at time is not in the future
  def runnable_filter
    <<-EOSQL
      (
        run_at <= :db_time_now
        AND (locked_at IS NULL OR locked_at < :locked_at_expired_at)
        OR locked_by = :worker_name
      )
      AND failed_at IS NULL
      AND "delayed_jobs"."blocked" = 'f'
    EOSQL
  end

  # Calculate the highest priority among runnable jobs
  def top_priority_cte
    <<-EOSQL
      top_priority AS
      (
        SELECT MIN(priority) AS top_priority
        FROM delayed_jobs
        WHERE #{runnable_filter}
      )
    EOSQL
  end

  def priority_filter
    'delayed_jobs.priority = (SELECT top_priority FROM top_priority)'
  end

  # Only consider users that have a currently-runnable job and that has the highest priority among jobs
  def eligible_user_ids_cte
    <<-EOSQL
      eligible_user_ids AS
        (
          SELECT DISTINCT user_id
          FROM delayed_jobs
          WHERE #{runnable_filter} AND #{priority_filter}
        )
    EOSQL
  end

  # We only need to generate this SQL once, the first time this file is loaded
  SQL = new.generate_sql.freeze
end