def getMinInstancesInService()

in magenta-lib/src/main/scala/magenta/tasks/AWS.scala [493:544]
45 lines of code
6 McCabe index (conditional complexity)

  def getMinInstancesInService(
      tagRequirements: List[TagMatch],
      client: AutoScalingClient,
      reporter: DeployReporter
  ): Int = {
    groupWithTags(tagRequirements, client, reporter) match {
      case Some(asg) =>
        val max = asg.maxSize
        val desired = asg.desiredCapacity
        val seventyFivePercent: Int = (max * 0.75).toInt
        val minInstancesInService = Math.min(
          seventyFivePercent,
          desired
        )

        // Happy path. We have enough headroom to double the instances, then half once healthy.
        if (2 * desired <= max) {
          reporter.info(
            s"Max=$max. Desired=$desired. Setting MinInstancesInService=$minInstancesInService."
          )
        }

        // We have to take some running instances out of service, at the cost of possibly reducing service availability.
        else if (minInstancesInService < desired) {
          reporter.warning(
            s"""Max=$max. Desired=$desired. Setting MinInstancesInService=$minInstancesInService.
               |This deployment will temporarily reduce the number of in-service instances.
               |The number of instances may go as low as $minInstancesInService, which is less than the current desired capacity of $desired.
               |To ensure a quick deployment we cannot set "MinInstancesInService" to more than 75% of the maximum number of instances.
               |You should consider increasing your application's maximum capacity so that we always have at least 25% headroom for deployments.
               |""".stripMargin
          )
        }

        // There isn't enough room to double capacity, but we don't have to take any instances out of service.
        else {
          reporter.warning(
            s"""Max=$max. Desired=$desired. Setting MinInstancesInService=$minInstancesInService.
               |This deployment will happen more slowly, in multiple steps.
               |The current number of in-service instances will be preserved and the application will be updated in batches of at most ${max - minInstancesInService}.
               |You could consider increasing your application's maximum capacity to double of your expected maximum so that deployments can happen in a single step.
               |""".stripMargin
          )
        }

        minInstancesInService
      case _ =>
        reporter.fail(
          s"No autoscaling group found with tags $tagRequirements. Creating a new stack? Initially choose the ${Strategy.Dangerous} strategy."
        )
    }
  }