Error when using alarm.add_alarm_action() with StepScalingAction

[sathvik-k]

For context, I'm trying to create a cloudwatch alarm that scales down an ecs task.

This is the cdk snippet

alarm = cloudwatch.Alarm(
    self,
    id="alarm",
    metric=ecs_service.target_group.metric_healthy_host_count(),
    comparison_operator=cloudwatch.ComparisonOperator.LESS_THAN_OR_EQUAL_TO_THRESHOLD,
    threshold=0,  # Trigger the alarm when there are no healthy hosts
    evaluation_periods=1,
    treat_missing_data=cloudwatch.TreatMissingData.NOT_BREACHING,
)

# Scale down to 0 tasks when the alarm is triggered
scalable_target = applicationautoscaling.ScalableTarget(
    self,
    id="<>_scalable_target",
    service_namespace=applicationautoscaling.ServiceNamespace.ECS,
    resource_id=f"service/{cluster.cluster_name}/{ecs_service.service.service_name}",
    scalable_dimension="ecs:service:DesiredCount",
    min_capacity=0,
    max_capacity=1
)
step_scaling_action = applicationautoscaling.StepScalingAction(
    self,
    id="<>_scale_down_action",
    scaling_target=scalable_target,
    adjustment_type=applicationautoscaling.AdjustmentType.CHANGE_IN_CAPACITY,
)
step_scaling_action.add_adjustment(
    lower_bound=0,
    upper_bound=1,
    adjustment=-1,
)
alarm.add_alarm_action(step_scaling_action)

When doing cdk synth I get back this error trace that I'm unable to decipher:

jsii.errors.JavaScriptError:
  @jsii/kernel.RuntimeError: TypeError: a.bind is not a function
      at Kernel._ensureSync (/private/var/folders/zn/b3j3d5ys6cnbcmtyn345hs_w0000gn/T/tmpbgrq4lu1/lib/program.js:8872:27)
      at Kernel.invoke (/private/var/folders/zn/b3j3d5ys6cnbcmtyn345hs_w0000gn/T/tmpbgrq4lu1/lib/program.js:8272:34)
      at KernelHost.processRequest (/private/var/folders/zn/b3j3d5ys6cnbcmtyn345hs_w0000gn/T/tmpbgrq4lu1/lib/program.js:11482:36)
      at KernelHost.run (/private/var/folders/zn/b3j3d5ys6cnbcmtyn345hs_w0000gn/T/tmpbgrq4lu1/lib/program.js:11442:22)
      at Immediate._onImmediate (/private/var/folders/zn/b3j3d5ys6cnbcmtyn345hs_w0000gn/T/tmpbgrq4lu1/lib/program.js:11443:46)
      at process.processImmediate (node:internal/timers:478:21)

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/Users/sathvik/work/<>/.deploy/app.py", line 6, in <module>
    HypatiaConsumersStack(
  File "/Users/sathvik/work/hyp-env/lib/python3.10/site-packages/jsii/_runtime.py", line 111, in __call__
    inst = super().__call__(*args, **kwargs)
  File "/Users/sathvik/work/<>/.deploy/<>/<>stack.py", line 201, in __init__
    alarm.add_alarm_action(step_scaling_action)
  File "/Users/sathvik/work/hyp-env/lib/python3.10/site-packages/aws_cdk/aws_cloudwatch/__init__.py", line 12688, in add_alarm_action
    return typing.cast(None, jsii.invoke(self, "addAlarmAction", [*actions]))
  File "/Users/sathvik/work/hyp-env/lib/python3.10/site-packages/jsii/_kernel/__init__.py", line 149, in wrapped
    return _recursize_dereference(kernel, fn(kernel, *args, **kwargs))
  File "/Users/sathvik/work/hyp-env/lib/python3.10/site-packages/jsii/_kernel/__init__.py", line 401, in invoke
    response = self.provider.invoke(
  File "/Users/sathvik/work/hyp-env/lib/python3.10/site-packages/jsii/_kernel/providers/process.py", line 378, in invoke
    return self._process.send(request, InvokeResponse)
  File "/Users/sathvik/work/hyp-env/lib/python3.10/site-packages/jsii/_kernel/providers/process.py", line 340, in send
    raise RuntimeError(resp.error) from JavaScriptError(resp.stack)
RuntimeError: TypeError: a.bind is not a function

Does anyone have any insight into why this is failing?

[sathvik-k]

Resolved, the fix here was to first cast the step scaling action to an AutoScalingAction like auto_scaling_action = cloudwatch_actions.AutoScalingAction(step_scaling_action).

Because I was using ApplicationLoadBalancedFargateService it also seems like the more idiomatic way of creating an autoscaling action is to create a ScalableTarget from auto_scale_task_count and then use ScalableTarget.scale_on_metric.

[github-actions[bot]]

Hello! Reopening this discussion to make it searchable.