Managed Instance Groups & Autoscaling

Managed Instance Groups (MIGs) are the foundation for building scalable, self-healing applications on Compute Engine. Combined with autoscaling, MIGs automatically adjust the number of VM instances in response to load, ensuring your application can handle traffic spikes while minimising costs during quiet periods.

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Autoscaling Overview

Autoscaling automatically adds or removes VM instances in a MIG based on configurable signals. When demand increases, the autoscaler creates new instances from the instance template. When demand decreases, it removes instances to reduce costs.

Autoscaling Signals

Signal	Description	Use Case
CPU utilisation	Average CPU usage across all instances	General workloads
HTTP load balancing utilisation	Requests per second per instance	Web applications behind a load balancer
Cloud Monitoring metrics	Custom or built-in Stackdriver metrics	Queue depth, custom business metrics
Schedule	Time-based scaling on a recurring schedule	Predictable traffic patterns

Configuring Autoscaling

# Set autoscaling based on CPU utilisation
gcloud compute instance-groups managed set-autoscaling web-mig \
  --zone=europe-west2-a \
  --min-num-replicas=2 \
  --max-num-replicas=20 \
  --target-cpu-utilization=0.60 \
  --cool-down-period=90