About availability for Red Hat OpenShift Service on AWS

By design, pods are meant to exist for a short time. Appropriately scaling services so that multiple instances of your application pods are running can protect against issues with any individual pod or container. The OpenShift node scheduler can also make sure these workloads are distributed across different worker nodes to further improve resiliency.

When accounting for possible pod failures, it is also important to understand how storage is attached to your applications. Single persistent volumes attached to single pods cannot leverage the full benefits of pod scaling, whereas replicated databases, database services, or shared storage can.

To avoid disruption to your applications during planned maintenance, such as upgrades, it is important to define a Pod Disruption Budget. These are part of the Kubernetes API and can be managed with oc commands such as other object types. They allow for the specification of safety constraints on pods during operations, such as draining a node for maintenance.

Worker nodes are the virtual machines that contain your application pods. By default, a ROSA cluster has a minimum of two worker nodes for a single availability-zone cluster. In the event of a worker node failure, pods are relocated to functioning worker nodes, as long as there is enough capacity, until any issue with an existing node is resolved or the node is replaced. More worker nodes means more protection against single-node outages, and ensures proper cluster capacity for rescheduled pods in the event of a node failure.

Single-AZ ROSA clusters have at least three control plane and two infrastructure nodes in the same availability zone (AZ) in the private subnet.

Multi-AZ ROSA clusters have at least three control plane nodes and three infrastructure nodes that are preconfigured for high availability, either in a single zone or across multiple zones, depending on the type of cluster you have selected. Control plane and infrastructure nodes have the same resiliency as worker nodes, with the added benefit of being managed completely by Red Hat.

In the event of a complete control plane outage, the OpenShift APIs will not function, and existing worker node pods are unaffected. However, if there is also a pod or node outage at the same time, the control planes must recover before new pods or nodes can be added or scheduled.

All services running on infrastructure nodes are configured by Red Hat to be highly available and distributed across infrastructure nodes. In case of a complete infrastructure outage, these services are unavailable until these nodes have been recovered.

A zone failure from AWS affects all virtual components, such as worker nodes, block or shared storage, and load balancers that are specific to a single availability zone. To protect against a zone failure, ROSA provides the option for clusters that are distributed across three availability zones, known as multiple availability zone clusters. Existing stateless workloads are redistributed to unaffected zones in the event of an outage, as long as there is enough capacity.

If you have deployed a stateful application, then storage is a critical component and must be accounted for when thinking about high availability. A single block storage PV is unable to withstand outages even at the pod level. The best ways to maintain availability of storage are to use replicated storage solutions, shared storage that is unaffected by outages, or a database service that is independent of the cluster.