Service Topology enables a service to route traffic based upon the Node topology of the cluster. For example, a service can specify that traffic be preferentially routed to endpoints that are on the same Node as the client, or in the same availability zone.
By default, traffic sent to a
NodePort Service may be routed to
any backend address for the Service. Since Kubernetes 1.7 it has been possible
to route “external” traffic to the Pods running on the Node that received the
traffic, but this is not supported for
ClusterIP Services, and more complex
topologies — such as routing zonally — have not been possible. The
Service Topology feature resolves this by allowing the Service creator to
define a policy for routing traffic based upon the Node labels for the
originating and destination Nodes.
By using Node label matching between the source and destination, the operator may designate groups of Nodes that are “closer” and “farther” from one another, using whatever metric makes sense for that operator’s requirements. For many operators in public clouds, for example, there is a preference to keep service traffic within the same zone, because interzonal traffic has a cost associated with it, while intrazonal traffic does not. Other common needs include being able to route traffic to a local Pod managed by a DaemonSet, or keeping traffic to Nodes connected to the same top-of-rack switch for the lowest latency.
If your cluster has Service Topology enabled, you can control Service traffic
routing by specifying the
topologyKeys field on the Service spec. This field
is a preference-order list of Node labels which will be used to sort endpoints
when accessing this Service. Traffic will be directed to a Node whose value for
the first label matches the originating Node’s value for that label. If there is
no backend for the Service on a matching Node, then the second label will be
considered, and so forth, until no labels remain.
If no match is found, the traffic will be rejected, just as if there were no
backends for the Service at all. That is, endpoints are chosen based on the first
topology key with available backends. If this field is specified and all entries
have no backends that match the topology of the client, the service has no
backends for that client and connections should fail. The special value
be used to mean “any topology”. This catch-all value, if used, only makes sense
as the last value in the list.
topologyKeys is not specified or empty, no topology constraints will be applied.
Consider a cluster with Nodes that are labeled with their hostname, zone name,
and region name. Then you can set the
topologyKeys values of a service to direct
traffic as follows.
["kubernetes.io/hostname", "topology.kubernetes.io/zone", "topology.kubernetes.io/region"]. This may be useful, for example, in cases where data locality is critical.
Service topology is not compatible with
therefore a Service cannot use both of these features. It is possible to use
both features in the same cluster on different Services, just not on the same
Valid topology keys are currently limited to
topology.kubernetes.io/region, but will
be generalized to other node labels in the future.
Topology keys must be valid label keys and at most 16 keys may be specified.
The catch-all value,
"*", must be the last value in the topology keys, if
it is used.
The following are common examples of using the Service Topology feature.
A Service that only routes to node local endpoints. If no endpoints exist on the node, traffic is dropped:
apiVersion: v1 kind: Service metadata: name: my-service spec: selector: app: my-app ports: - protocol: TCP port: 80 targetPort: 9376 topologyKeys: - "kubernetes.io/hostname"
A Service that prefers node local Endpoints but falls back to cluster wide endpoints if node local endpoints do not exist:
apiVersion: v1 kind: Service metadata: name: my-service spec: selector: app: my-app ports: - protocol: TCP port: 80 targetPort: 9376 topologyKeys: - "kubernetes.io/hostname" - "*"
A Service that prefers zonal then regional endpoints. If no endpoints exist in either, traffic is dropped.
apiVersion: v1 kind: Service metadata: name: my-service spec: selector: app: my-app ports: - protocol: TCP port: 80 targetPort: 9376 topologyKeys: - "topology.kubernetes.io/zone" - "topology.kubernetes.io/region"
A Service that prefers node local, zonal, then regional endpoints but falls back to cluster wide endpoints.
apiVersion: v1 kind: Service metadata: name: my-service spec: selector: app: my-app ports: - protocol: TCP port: 80 targetPort: 9376 topologyKeys: - "kubernetes.io/hostname" - "topology.kubernetes.io/zone" - "topology.kubernetes.io/region" - "*"
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