An enterprise runs a batch and transaction processing workload on a Compute Engine regional Managed Instance Group (MIG). The workload faces diverse bottlenecks depending on customer behavior: Sudden surges in incoming HTTP requests require rapid scaling based on CPU utilization. Periodic asynchronous processing requires scaling based on a Cloud Monitoring custom metric tracking message backlog depth. A recurring morning traffic surge requires pre-provisioning capacity via scaling schedules. You need to configure an autoscaling policy that combines these signals to ensure the application maintains high performance without dropping requests. How does the Compute Engine autoscaler evaluate these multiple signals to determine the target size of the MIG?

The autoscaler calculates the recommended number of VM instances independently for each signal and provisions the largest resulting instance count.: ### What Managed Instance Group Multi-Signal Autoscaling Is Compute Engine autoscaling allows a **Managed Instance Group (MIG)** to automatically adjust its number of virtual machine instances according to changing workload demands. When an autoscaling policy is configured with multiple signals—such as **CPU utilization**, **Cloud Monitoring custom metrics**, **load balancing serving capacity**, and **scaling schedules**—the autoscaler independently evaluates the required capacity for every single signal during each collection cycle. ### How It Meets the Workload Requirements In this scenario, the workload has multiple independent bottlenecks: - **CPU spikes:** The autoscaler calculates the VM count required to keep average CPU utilization at or below the target threshold. - **Queue backlog:** The autoscaler calculates the VM count necessary to process custom Cloud Monitoring queue depth metrics. - **Scheduled morning spikes:** The scaling schedule defines a minimum required replica count for specific time windows. By taking the **maximum (largest number of VMs)** among all individual recommendations, the autoscaler guarantees that whichever bottleneck or schedule demands the most capacity is fully satisfied, preventing performance degradation or request drops. ### Key Advantages and Capabilities - **Bottleneck resilience:** Protects workloads against multi-dimensional bottlenecks where memory, CPU, or message queue depth might independently spike at different times. - **Signal diversity:** Supports up to 1 CPU utilization signal, 1 load balancing serving capacity signal, up to 5 Cloud Monitoring custom metrics, and up to 128 scaling schedules per MIG. - **Bounded elasticity:** Respects configured `minNumReplicas` and `maxNumReplicas` bounds across all combined evaluations. - **Predictive & proactive scaling:** Combines reactive real-time signals with proactive scheduled capacity. ### Why This Is Optimal Picking the largest recommended instance count ensures high availability and eliminates under-provisioning. If one metric demands 20 VMs due to high CPU while a queue metric only requires 8 VMs, scaling to 20 VMs ensures the CPU-bound requests do not fail.

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Mig Autoscaling Multiple Signals Evaluation

Section: Scalability to meet growth requirements

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An enterprise runs a batch and transaction processing workload on a Compute Engine regional Managed Instance Group (MIG). The workload faces diverse bottlenecks depending on customer behavior:

Sudden surges in incoming HTTP requests require rapid scaling based on CPU utilization.
Periodic asynchronous processing requires scaling based on a Cloud Monitoring custom metric tracking message backlog depth.
A recurring morning traffic surge requires pre-provisioning capacity via scaling schedules.

You need to configure an autoscaling policy that combines these signals to ensure the application maintains high performance without dropping requests.

How does the Compute Engine autoscaler evaluate these multiple signals to determine the target size of the MIG?

The autoscaler evaluates signals by strict priority hierarchy, where scaling schedules override Cloud Monitoring metrics, which in turn override CPU utilization.

The autoscaler sums together the recommended instance counts calculated for each individual signal up to the configured maximum number of replicas.

The autoscaler calculates the recommended number of VM instances independently for each signal and provisions the largest resulting instance count.

The autoscaler calculates the arithmetic average across all recommended instance counts from the configured signals to smooth out sudden scaling fluctuations.

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Mig Autoscaling Multiple Signals Evaluation

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