View autoscaler logs

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When you enable autoscaling, the autoscaler makes scaling decisions based on the options that you specify. Each scaling decision is logged by Cloud Logging. Read these logs in the Logs Explorer to better understand the autoscaler's scaling decisions.

Using the Logs Explorer, you can see events related to:

  • Autoscaler recommendation for resizing a managed instance group (MIG). If the autoscaler calculates a new recommended size for a MIG, the change in the recommended size is described in the autoscaler logs. See Resize log entry.
  • Changes in autoscaler status. For example, if the autoscaler was working as expected but then encountered an issue, the change in status would be described in the autoscaler logs. See Status change log entry.

Before you begin

  • Read the Autoscaling documentation.
  • Have an active autoscaler for which you want to view logs.
  • Review how to use the Logs Explorer.

View autoscaler logs

To view autoscaler logs, use the Google Cloud console, the gcloud CLI, or the Cloud Logging API.

Console

If you've turned on autoscaling, then you can see autoscaling logs in the Google Cloud console.

  1. Go to the Logs Explorer page.

    Go to Logs Explorer

  2. Pull down the Resource drop-down menu and select Autoscaler.

You can also navigate to the Logs Explorer page from your MIG as follows:

  1. In the Google Cloud console, go to the Instance groups page.

    Go to Instance groups

  2. Click the name of the MIG for which you want to view the logs.

  3. Click Monitoring tab.

  4. In the Monitoring tab, the Logs panel is at the bottom of the page. Open the panel by clicking the icon at the end of this panel.

  5. To open the Logs explorer page, click the View in logs explorer icon in this panel. The log explorer opens with a default query to fetch all autoscaler logs of the MIG.

gcloud

To look up all logs related to autoscaling, use the logging read command. For example:

gcloud logging read "resource.type=autoscaler" --limit 10 \
    --format json

To view the autoscaler logs of a specific MIG, specify the instance_group_manager_name as follows:

gcloud logging read "resource.type=autoscaler AND \
    resource.labels.instance_group_manager_name=example-igm" \
    --limit 10 --format json

To view the log entries related to autoscaler resize actions, specify the compute.autoscalers.resize method name:

gcloud logging read "resource.type=autoscaler AND \
    resource.labels.instance_group_manager_name=example-igm AND \
    protoPayload.methodName=compute.autoscalers.resize" \
    --limit 10 --format json

To view the log entries related to autoscaler status changes, specify the compute.autoscalers.changeStatus method name:

gcloud logging read "resource.type=autoscaler AND \
    resource.labels.instance_group_manager_name=example-igm AND \
    protoPayload.methodName=compute.autoscalers.changeStatus" \
    --limit 10 --format json

API

Make a request to the Logging V2 API. The request body must contain the filter parameter to use when searching for logs and the project you want to view logs for. For example, to make a request to get a list of resize actions of a specific MIG:

POST https://logging.googleapis.com/v2/entries:list

{
  "filter": "resource.type=autoscaler AND
    resource.labels.instance_group_manager_name=example-igm AND
    protoPayload.methodName=compute.autoscalers.resize",
  "pageSize": 10,
  "resourceNames": [
    "projects/example-project"
  ]
}

To make a request to get a list of status changes of a specific MIG:

POST https://logging.googleapis.com/v2/entries:list

{
  "filter": "resource.type=autoscaler AND
    resource.labels.instance_group_manager_name=example-igm AND
    protoPayload.methodName=compute.autoscalers.changeStatus",
  "pageSize": 10,
  "resourceNames": [
    "projects/example-project"
  ]
}

Resize log entry

When the autoscaler calculates a recommended size for your MIG, Compute Engine creates a log entry capturing the new and old recommended sizes. For zonal MIGs, the log entry also captures the reason for the change in recommended size.

The following is an example of a log entry for a zonal MIG. In this example, based on predictive autoscaling the autoscaler calculated 3 VMs to achieve the CPU utilization target of 35%. But, the maximum number of instances configured in the autoscaling policy is only 2. So, the autoscaler limits the recommended size to 2.

    {
    insertId: "1l68z7sg4jw7kzo"
    logName: "projects/example-project/logs/cloudaudit.googleapis.com%2Fsystem_event"
    protoPayload: {
      @type: "type.googleapis.com/google.cloud.audit.AuditLog"
      metadata: {
        @type: "type.googleapis.com/autoscaler.AutoscalerSizeChangeExplanation"
        autoscalingMode: "ON"
        autoscalingReason: {
          scalingLimit: {
            limitPolicy: {
              maxNumReplicas: 2
            }
            limitSize: 2
            name: "MAX_INSTANCES"
          }
          scalingSignal: {
            calculatedSize: 3
            calculationDetails: {
              servingSize: 1
              signalTarget: 0.35
              signalValue: 1.03533
            }
            name: "PREDICTED_CPU_UTILIZATION"
            signalPolicy: {
              coolDownPeriodSec: 15
              cpuUtilization: {
                predictiveMethod: "OPTIMIZE_AVAILABILITY"
                utilizationTarget: 0.35
              }
            }
          }
          summary: "The autoscaler's recommended size changed from 1 to 2
          because the autoscaler predicted that in 15 seconds the average CPU
          utilization across 1 serving instance will be 103.533%, which is above
          the utilization target of 35%. The calculated size was 3 to achieve
          the target CPU utilization but the size was limited by the maximum
          number of instances set in the autoscaling policy."
        }
        newSize: 2
        oldSize: 1
      }
      methodName: "compute.autoscalers.resize"
      resourceName: "projects/example-project/zones/us-east1-d/autoscalers/example-autoscaler"
      serviceName: "compute.googleapis.com"
    }
    receiveTimestamp: "2022-06-13T22:34:43.045973046Z"
    resource: {
      labels: {
        autoscaler_id: "1234567890123456789"
        autoscaler_name: "example-autoscaler"
        instance_group_manager_id: "1357908642148074125"
        instance_group_manager_name: "example-igm"
        location: "us-east1-d"
        project_id: "example-project"
      }
      type: "autoscaler"
    }
    severity: "INFO"
    timestamp: "2022-06-13T22:34:42.810216614Z"
    }

The following table describes the fields from the log entry:

Property Value
autoscalingMode The autoscaling mode at the time when the entry was logged. Regardless of the mode, the autoscaler calculates the recommended size, which is represented by newSize in the log. Depending on the autoscaling mode, the autoscaler resizes the MIG as follows:
  • ON: The autoscaler resizes the MIG to newSize.
  • OFF: The autoscaler does not resize the MIG.
  • ONLY_SCALE_OUT: The autoscaler resizes the MIG only if newSize is greater than the target size of the MIG.
autoscalingReason Details about the reason for autoscaling. This field appears only for zonal MIGs.
autoscalingReason.scalingLimit Details about the autoscaling feature that limited the calculatedSize, which is the size that autoscaler calculated based on the autoscaling signal. The autoscaling features that can limit the calculatedSize are maximum or minimum number of instances, the scale-in controls, or the stabilization period. If no limit applies to calculatedSize, then the scalingLimit field does not appear in the logs.
autoscalingReason.scalingLimit.limitPolicy The configuration of the autoscaling feature that limited the calculatedSize. The limitPolicy field is not set when stabilization limits the calculatedSize because stabilization is a default feature of autoscaling.
autoscalingReason.scalingLimit.limitSize The number of VMs based on the autoscaling feature that limited the calculatedSize.
autoscalingReason.scalingLimit.name The name of the autoscaling feature that limited the calculatedSize.
autoscalingReason.scalingSignal Details about the autoscaling signal that is used to calculate the number of VMs the MIG should have. If the autoscaling policy has multiple signals, then the autoscaler considers the signal that requires the largest number of VMs.
autoscalingReason.scalingSignal.calculatedSize The number of VMs that the MIG should have based on the autoscaling signal. If scalingLimit applies, then the autoscaler limits calculatedSize to scalingLimit.limitSize.
autoscalingReason.scalingSignal.calculationDetails Details used to determine the calculatedSize.
  • servingSize: The number of VMs in the MIG excluding the VMs that are still in their cool down period.
  • signalTarget: The target value that the autoscaler maintains for CPU utilization (real-time), predicted CPU utilization, load balancing utilization, or a metric configured with utilization target.
  • singleInstanceAssignment: If scaling is based on a metric value that represents the total amount of work to be done, then singleInstanceAssignment is the quantity of work that each VM can handle.
  • signalValue: The value of the autoscaling signal. If signalTarget is set, then the autoscaler compares the signalValue with the signalTarget to calculate how many VMs are needed, which is represented by calculatedSize.
autoscalingReason.scalingSignal.name The name of the autoscaling signal based on which the autoscaler calculated the number of VMs, represented by calculatedSize in the log.
autoscalingReason.scalingSignal.signalPolicy The configuration of the autoscaling signal based on which the autoscaler determined the calculatedSize.
autoscalingReason.summary A description of the reason for autoscaling.
newSize The current recommended size of the MIG. Depending on the autoscalingMode, the autoscaler resizes the MIG to the newSize as follows:
  • ON: The autoscaler resizes the MIG to newSize.
  • OFF: The autoscaler does not resize the MIG.
  • ONLY_SCALE_OUT: The autoscaler resizes the MIG only if newSize is greater than the target size of the MIG.
oldSize The previous recommended size of the MIG.

For information about the other fields in the log entry, see Format of audit log entries.

Status change log entry

When the status of the autoscaler changes, Compute Engine creates a log entry capturing the old and new status messages. You can use these status messages to find out why your autoscaler is not behaving as you expect or to debug past issues you noticed. For some of the common status messages, see Commonly returned status messages.

The following fields in the protoPayload describe the old and new statuses:

  • metadata.newStatus.details: The new status of the autoscaler.
  • metadata.oldStatus.details: The previous status of the autoscaler.

The following is an example of a log entry that describes a status change. In this example, you might notice the new status message and check that your load balancing configuration is set up correctly.

    {
      insertId: "ivho6kg4icqfio"
      logName: "projects/example-project/logs/cloudaudit.googleapis.com%2Fsystem_event"
      protoPayload: {
        metadata: {
          @type: "type.googleapis.com/autoscaler.AutoscalerStatusChange"
          newStatus: {
            details: "The autoscaler is configured to scale based on a load
            balancing signal but the instance group has not received any
            utilization data from the load balancer or the utilization is
            constantly 0. Check that the load balancing configuration is
            working"
          }
          oldStatus: {
            details: "OK"
          },
          methodName: "compute.autoscalers.changeStatus"
          resourceName: "projects/example-project/zones/us-east1-b/autoscalers/example-autoscaler"
          serviceName: "compute.googleapis.com"
        }
        receiveTimestamp: "2022-06-08T03:54:12.332329320Z"
        resource: {
          labels: {
            autoscaler_id: "1234567890123456789"
            autoscaler_name: "example-autoscaler"
            instance_group_manager_id: "1357908642148074125"
            instance_group_manager_name: "example-igm"
            location: "us-east1-b"
            project_id: "example-project"
          }
          type: "autoscaler"
        }
        severity: "INFO"
        timestamp: "2022-06-08T03:54:10.675416812Z"
      }
    }

For information about the other fields in the log entry, see Format of audit log entries.

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