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SCALING.md

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Scaling

Autoscaling is one of the key features in Kubernetes. This means that Kubernetes will automatically increase the number of pods that run a service as the demand for the service increases, and reduce the number of pods when the demand drops. This improves service quality, simplifies management, and reduces operational costs by avoiding the need to over-provision services to the peak load. Most importantly, autoscaling saves you from having to guess the number of nodes or pods needed to run your workload: Kubernetes will automatically and dynamically resize your workload based on demand.

Further factors to autoscale your WebRTC workload are:

  • smaller load on each instance: this might result in better and more stable performance;
  • smaller blast radius: less calls will be affected if a pod fails for some reason.

Autoscaling a production service, especially one as sensitive to latency and performance as WebRTC, can be challenging. This guide will provide the basics on autoscaling; see the official docs for more detail.

Horizontal scaling

It is a good practice to scale your STUNner deployment horizontally (that is, by adding or removing stunnderd pods) instead of vertically (that is, by increasing the resource limits of your pods) when demand increases. We advice to set the resource limits and requests of the stunnerd pods to the bare minimum (this can be set in the Dataplane template used for provisioning stunnerd pods) and let Kubernetes to automatically scale out the STUNner dataplane by adding more stunnerd pods if needed. Note that HPA uses the requested amount of resources to determine when to scale-up or down the number of instances.

STUNner comes with a full support for horizontal scaling using the the Kubernetes built-in HorizontalPodAutoscaler (HPA). The triggering event can be based on arbitrary metric, say, the number of active client connections per STUNner dataplane pod. Below we use the CPU utilization for simplicity.

Scaling STUNner up occurs by Kubernetes adding more pods to the STUNner dataplane deployment and load-balancing client requests across the running pods. This should (theoretically) never interrupt existing calls, but new calls should be automatically routed by the cloud load balancer to the new endpoint(s). Automatic scale-up means that STUNner should never become the bottleneck in the system. Note that in certain cases scaling STUNner up would require adding new Kubernetes nodes to your cluster: most modern hosted Kubernetes services provide horizontal node autoscaling out of the box to support this.

Scaling STUNner down, however, is trickier. Intuitively, when a running STUNner dataplane pod is terminated on scale-down, all affected clients with active TURN allocations on the terminating pod would be disconnected. This would then require clients to go through an ICE restart to re-connect, which may cause prolonged connection interruption and may not even be supported by all browsers.

In order to avoid client disconnects on scale-down, STUNner supports a feature called graceful shutdown. This means that stunnerd pods would refuse to terminate as long as there are active TURN allocations on them, and automatically remove themselves only once all allocations are deleted or timed out. It is important that terminating pods will not be counted by the HorizontalPodAutoscaler towards the average CPU load, and hence would not affect autoscaling decisions. In addition, new TURN allocation requests would never be routed by Kubernetes to terminating stunnerd pods.

Graceful shutdown enables full support for scaling STUNner down without affecting active client connections. As usual, however, some caveats apply:

  1. The default is to provision stunnerd pods with at most 2 CPU cores and 16 listener threads, both can be customized in the Dataplane template used to provision stunnerd pods.
  2. Currently the max lifetime for stunnerd to remain alive is 1 hour after being deleted: this means that stunnerd will remain active only for 1 hour after it has been deleted/scaled-down even if active allocations would last longer. You can adjust the grace period in the terminationGracePeriod setting in the Dataplane template.
  3. STUNner pods may remain alive well after the last client connection is gone. This occurs when an allocation is left open by a client (e.g., spontaneous UDP client-side connection closure cannot be reliably detected by the server). As the default TURN refresh lifetime is 10 minutes it may take 10 minutes until all allocations time out, letting stunnerd to finally terminate. In such cases stunnerd may refuse to stop after a kubectl delete. Use kubectl delete pod --grace-period=0 --force stunner-XXX to force removal.

Example

Below is a simple HorizontalPodAutoscaler config for autoscaling stunnerd. The example assumes that the Kubernetes metric server is available in the cluster.

apiVersion: autoscaling/v2
kind: HorizontalPodAutoscaler
metadata:
  name: hpa-stunner
  namespace: stunner
spec:
  scaleTargetRef:
    apiVersion: apps/v1
    kind: Deployment
    name: stunner
  minReplicas: 1
  maxReplicas: 10
  metrics:
  - type: Resource
    resource:
      name: cpu
      target:
        type: Utilization
        averageUtilization: 300

Here, scaleTargetRef selects the STUNner Deployment named stunnerd as the scaling target and the deployment will always run at least 1 pod and at most 10 pods. Understanding how Kubernetes chooses the number of running pods is, however, a bit tricky.

Suppose that the configured resources in the STUNner deployment are the following.

resources:
  limits:
    cpu: 2
    memory: 512Mi
  requests:
    cpu: 500m
    memory: 128Mi

Initially, there is only a single stunnerd pod in the cluster. As new calls arrive, CPU utilization is increasing. Scale out will be triggered when CPU usage of the stunnerd pod reaches 1500 millicore CPU (three times the requested CPU). If more calls come and the total CPU usage of the stunnerd pods reaches 3000 millicore, which amounts to 1500 millicore on average, scale out would happen again. When users leave, load will drop and the total CPU utilization will fall under 3000 millicore. At this point Kubernetes will automatically scale-in and remove one of the stunnerd instances. Recall, this would never affect existing connections thanks to graceful shutdown.