Autoscaling is a powerful feature in Kubernetes that ensures your applications scale dynamically to handle increasing or decreasing traffic. However, one common question is: How fast can Kubernetes scale out, and how much traffic can it handle?

Two Levels of Horizontal Scaling

In Kubernetes, autoscaling operates on two levels: Pod-level autoscaling and Node-level autoscaling.

1. Pod-level Autoscaling (Horizontal Pod Autoscaler - HPA)

The Horizontal Pod Autoscaler (HPA) monitors the resource usage of your pods, such as CPU or memory, and automatically scales the number of replicas up or down based on demand. Here’s what you need to know:

  • Scaling Speed: Pod-level autoscaling is generally fast, typically scaling out in less than a minute depending on how the cluster is configured. However, certain configurations can make scaling even faster:
    • PriorityClass: Pods can have different priorities based on their importance. Critical pods with higher priority can be scheduled faster during scaling events. This ensures that important workloads are prioritized when resources are constrained.
    • Pinned and Pre-scaled HPA: You can configure the HPA to pre-scale pods if you anticipate spikes in traffic. This allows the system to respond quicker to traffic surges without waiting for resource thresholds to be breached.
  • Traffic Capacity: The amount of traffic your pods can handle depends on the resource allocation (e.g., CPU, memory) for each pod. If each pod can handle a fixed number of requests per second, scaling out additional pods ensures that the overall system can manage larger traffic loads. By carefully configuring pod resource limits and HPA thresholds, you can optimize the system to balance resource efficiency and traffic capacity.

2. Node-level Autoscaling (Cluster Autoscaler or Karpenter)

When scaling pods isn’t enough, Kubernetes can also scale nodes (virtual machines) in the cluster to accommodate more pods.

  • Scaling Speed: Scaling nodes can take longer than scaling pods because it involves provisioning new instances from your cloud provider (AWS, GCP, etc.). Typically, scaling out nodes can take a few minutes, depending on the cloud provider’s infrastructure and the size of the instance. To optimize node-level scaling:
    • Karpenter: A newer alternative to Cluster Autoscaler, Karpenter optimizes node scaling by efficiently provisioning nodes with the exact resources required. It is often faster than the traditional autoscaler and can bring up nodes in seconds.
    • Over-provisioning: To mitigate the time it takes to scale nodes, you can “over-provision” nodes. This means keeping a small buffer of idle nodes that are ready to handle a sudden surge in traffic. This approach ensures that your system can scale instantly without waiting for new nodes to spin up.
  • Traffic Capacity: At the node level, the capacity to handle traffic is related to how many pods can be scheduled on the available nodes. By scaling out nodes, you increase the cluster’s total resource pool, allowing for more pods and thus more traffic handling capability.

Conclusion

Kubernetes autoscaling is highly dynamic, with two distinct layers working together to ensure your application scales as needed.

  • Pod-level scaling is rapid, generally happening in less than a minute, especially when pre-scaled or with proper PriorityClass settings.
  • Node-level scaling may take a few minutes, but tools like Karpenter and over-provisioning can help speed up the process.

By effectively managing both pod and node autoscaling, you can ensure that your application can handle large traffic surges while maintaining efficiency.