increment_connection_stat_worker.rb

class IncrementConnectionStatWorker
  include Sidekiq::Worker

  def perform(activity_id)
    a = Activity.find(activity_id)

    # I had previously used .find_or_initialize_by
    # but that proved to not be thread-safe on the incrementing
    stat = ConnectionStat.find_by(
      day:           Time.zone.now.to_date,
      connection_id: a.connection_id
    )

    if stat.present?
      # Pessimistic locking here seemed
      # to do the trick removing race conditions
      # when incrementing
      stat.with_lock do
        stat.activities_analyzed += 1
        stat.save!
      end
    else
      # It's possible the presence check for ConnectionStat
      # could be susceptible to a race condition, so I created
      # a compount unique index on [:day, :connection_id] -- 
      # causing the insert to fail, the job to retry and properly
      # go down the path of incrementing the existing record
      ConnectionStat.create!(
        day:                 Time.zone.now.to_date,
        connection_id:       a.connection_id,
        activities_analyzed: 1
      )
    end
  end
end