migrate_concurrently.rb

# concurrent migration strategy
# when migrating through this script
# you can launch it on multiple processes
# and they will deal with the concurrency by
# themselves without interrupting or crashing
class MigrateConcurrently
  class Error < StandardError; end

  DELAY_BETWEEN_CYCLES_IN_SECONDS = 3
  MAX_CYCLES = 100

  def initialize; end

  def perform
    return true unless concurrency_error? { run_migrate }

    wait_and_try_again
  end

  private

  def wait_and_try_again
    log_cycle
    wait
    next_cycle
    perform
  end

  def run_migrate
    ActiveRecord::Base.connection.migration_context.migrate
  end

  def concurrency_error?(&block)
    yield
    false
  rescue ActiveRecord::ConcurrentMigrationError
    true
  end

  def log_cycle
    Logger.info(
      message: "Migration is already running, waiting to try again",
      type: "infrastructure_run_migrate",
      delay_between_cycles_in_seconds: DELAY_BETWEEN_CYCLES_IN_SECONDS,
      cycle: cycles.current,
      max_cycles: cycles.max,
    )
  end

  def wait
    sleep DELAY_BETWEEN_CYCLES_IN_SECONDS
  end

  def next_cycle
    raise_if_max_cycles_reached
    cycles.next
  end

  def raise_if_max_cycles_reached
    return unless cycles.max?

    raise Error, """
    Max cycles reached after trying #{cycles.current} times.
    Something might be wrong with the migration process.
    """
  end

  def cycles
    @cycles ||= Cycles.new(max: MAX_CYCLES)
  end
end