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Multi-cluster communication

Linkerd’s multi-cluster functionality allows pods to connect to Kubernetes services across cluster boundaries in a way that is secure and fully transparent to the application. As of Linkerd 2.14, this feature supports two modes: hierarchical (using an gateway) and flat (without a gateway):

  • Flat mode requires that all pods on the source cluster be able to directly connect to pods on the destination cluster.
  • Hierarchical mode only requires that the gateway IP of the destination cluster be reachable by pods on the source cluster.

These modes can be mixed and matched.

Architectural diagram comparing hierarchical and flat network modes

Hierarchical mode places a bare minimum of requirements on the underlying network, as it only requires that the gateway IP be reachable. However, flat mode has a few advantages over the gateway approach used in hierarchical mode, including reducing latency and preserving client identity.

Service mirroring

Linkerd’s multi-cluster functionality uses a service mirror component that watches a target cluster for updates to services and mirrors those service updates locally to a source cluster.

Multi-cluster support is underpinned by a concept known as service mirroring. Mirroring refers to importing a service definition from another cluster, and it allows applications to address and consume multi-cluster services. The service mirror component runs on the source cluster; it watches a target cluster for updates to services and mirrors those updates locally in the source cluster. Only Kubernetes service objects that match a label selector are exported.

The label selector also controls the mode a service is exported in. For example, by default, services labeled with mirror.linkerd.io/exported=true will be exported in hierarchical (gateway) mode, whereas services labeled with mirror.linkerd.io/exported=remote-discovery will be exported in flat (pod-to-pod) mode. Since the configuration is service-centric, switching from gateway to pod-to-pod mode is trivial and does not require the extension to be re-installed.

The term “remote-discovery” refers to how the imported services should be interpreted by Linkerd’s control plane. Service discovery is performed by the destination service. Whenever traffic is sent to a target imported in “remote-discovery” mode, the destination service knows to look for all relevant information in the cluster the service has been exported from, not locally. In contrast, service discovery for a hierarchical (gateway mode) import will be performed locally; instead of routing directly to a pod, traffic will be sent to the gateway address on the target cluster.

Linkerd’s destination service performs remote discovery by connecting directly to multiple Kubernetes API servers. Whenever two clusters are connected together, a Kubernetes Secret is created in the control plane’s namespace with a kubeconfig file that allows an API client to be configured. The kubeconfig file uses RBAC to provide the “principle of least privilege”, ensuring the destination service may only access only the resources it needs.