Kubernetes clusters can scale. Open-source Kubernetes supports up to 5,000 Nodes, and GKE supports up to 15,000 Nodes. But scaling out a single cluster can only get you so far: if your cluster’s control plane goes down, your entire platform goes down; if the Cloud region running your cluster has a service interruption, so does your app.
Many organizations choose, instead, to operate multiple Kubernetes clusters. Besides availability, there are lots of reasons to consider multi-cluster, such as allocating a cluster to each development team, splitting workloads between cloud and on-prem, or providing burst capability for traffic spikes.
But operating a multi-cluster platform comes with its own challenges. How to consistently administer many clusters at once? How to keep the clusters secure? How to deploy and monitor applications running across multiple clusters? How to seamlessly fail over from one region to another?
This post introduces a few tools that can help platform teams more easily administer a multi-cluster Kubernetes environment.
The platform base layer, with Config Sync
In the last post, we explored how thoughtful platform abstractions can help reduce toil for app developers- including for a multi-cluster environment, where automation such as CI/CD handles all interactions with the staging and production clusters. But equally important is the platform base layer, the Kubernetes resources and configuration that are shared across services. Your platform base layer might consist of Namespaces, role-based access control, and shared workloads like Prometheus.
Platform abstractions depend on the existence of these base-layer resources. And so does the security and stability of your platform as a whole. It’s important that these resources not only get deployed, but also stay put. CI/CD is great for deploying resources, but what about making sure resources stay deployed? What if a Kubernetes Namespace gets deleted? Or a Prometheus StatefulSet is modified?
Kubernetes’ job is to ensure that the cluster’s actual state matches the desired state. But sometimes, the “desired” state isn’t desired at all – it’s a developer who mistakenly modified a resource, or a bad actor that’s gained access into the system. For this reason, a platform base layer needs more than a one-and-done CI/CD pipeline. A tool called Config Sync can help with this.
Config Sync is a Google Cloud product that can sync Kubernetes resources from a Git repository to one or more GKE or Anthos clusters. Unlike CI/CD tools like Cloud Build, Config Sync watches your clusters constantly, making sure that the intended resource state in the cluster always matches what’s in Git. Config Sync is designed primarily for base-layer resources like namespaces and RBAC. In this way, Config Sync is complementary to, not a replacement for, CI/CD.