Getting started with the Kubernetes Gateway API

Using the Gateway API to configure ingress traffic for your Kubernetes cluster.

Dec 14, 2022 | By Frank Budinsky - IBM

Whether you’re running your Kubernetes application services using Istio, or any service mesh for that matter, or simply using ordinary services in a Kubernetes cluster, you need to provide access to your application services for clients outside of the cluster. If you’re using plain Kubernetes clusters, you’re probably using Kubernetes Ingress resources to configure the incoming traffic. If you’re using Istio, you are more likely to be using Istio’s recommended configuration resources, Gateway and VirtualService, to do the job.

The Kubernetes Ingress resource has for some time been known to have significant shortcomings, especially when using it to configure ingress traffic for large applications and when working with protocols other than HTTP. One problem is that it configures both the client-side L4-L6 properties (e.g., ports, TLS, etc.) and service-side L7 routing in a single resource, configurations that for large applications should be managed by different teams and in different namespaces. Also, by trying to draw a common denominator across different HTTP proxies, Ingress is only able to support the most basic HTTP routing and ends up pushing every other feature of modern proxies into non-portable annotations.

To overcome Ingress’ shortcomings, Istio introduced its own configuration API for ingress traffic management. With Istio’s API, the client-side representation is defined using an Istio Gateway resource, with L7 traffic moved to a VirtualService, not coincidentally the same configuration resource used for routing traffic between services inside the mesh. Although the Istio API provides a good solution for ingress traffic management for large-scale applications, it is unfortunately an Istio-only API. If you are using a different service mesh implementation, or no service mesh at all, you’re out of luck.

Enter Gateway API

There’s a lot of excitement surrounding a new Kubernetes traffic management API, dubbed Gateway API, which has recently been promoted to Beta. Gateway API provides a set of Kubernetes configuration resources for ingress traffic control that, like Istio’s API, overcomes the shortcoming of Ingress, but unlike Istio’s, is a standard Kubernetes API with broad industry agreement. There are several implementations of the API in the works, including a Beta implementation in Istio, so now may be a good time to start thinking about how you can start moving your ingress traffic configuration from Kubernetes Ingress or Istio Gateway/VirtualService to the new Gateway API.

Whether or not you use, or plan to use, Istio to manage your service mesh, the Istio implementation of the Gateway API can easily be used to get started with your cluster ingress control. Even though it’s still a Beta feature in Istio, mostly driven by the fact that the Gateway API is itself still a Beta level API, Istio’s implementation is quite robust because under the covers it uses Istio’s same tried-and-proven internal resources to implement the configuration.

Gateway API quick-start

To get started using the Gateway API, you need to first download the CRDs, which don’t come installed by default on most Kubernetes clusters, at least not yet:

$ kubectl get crd gateways.gateway.networking.k8s.io &> /dev/null || \
  { kubectl kustomize "github.com/kubernetes-sigs/gateway-api/config/crd?ref=v1.1.0" | kubectl apply -f -; }

Once the CRDs are installed, you can use them to create Gateway API resources to configure ingress traffic, but in order for the resources to work, the cluster needs to have a gateway controller running. You can enable Istio’s gateway controller implementation by simply installing Istio with the minimal profile:

$ curl -L https://istio.io/downloadIstio | sh -
$ cd istio-1.22.3
$ ./bin/istioctl install --set profile=minimal -y

Your cluster will now have a fully-functional implementation of the Gateway API, via Istio’s gateway controller named istio.io/gateway-controller, ready to use.

Deploy a Kubernetes target service

To try out the Gateway API, we’ll use the Istio helloworld sample as an ingress target, but only running as a simple Kubernetes service without sidecar injection enabled. Because we’re only going to use the Gateway API to control ingress traffic into the “Kubernetes cluster”, it makes no difference if the target service is running inside or outside of a mesh.

We’ll use the following command to deploy the helloworld service:

Zip
$ kubectl create ns sample
$ kubectl apply -f @samples/helloworld/helloworld.yaml@ -n sample

The helloworld service includes two backing deployments, corresponding to different versions (v1 and v2). We can confirm they are both running using the following command:

$ kubectl get pod -n sample
NAME                             READY   STATUS    RESTARTS   AGE
helloworld-v1-776f57d5f6-s7zfc   1/1     Running   0          10s
helloworld-v2-54df5f84b-9hxgww   1/1     Running   0          10s

Configure the helloworld ingress traffic

With the helloworld service up and running, we can now use the Gateway API to configure ingress traffic for it.

The ingress entry point is defined using a Gateway resource:

$ kubectl create namespace sample-ingress
$ kubectl apply -f - <<EOF
apiVersion: gateway.networking.k8s.io/v1beta1
kind: Gateway
metadata:
  name: sample-gateway
  namespace: sample-ingress
spec:
  gatewayClassName: istio
  listeners:
  - name: http
    hostname: "*.sample.com"
    port: 80
    protocol: HTTP
    allowedRoutes:
      namespaces:
        from: All
EOF

The controller that will implement a Gateway is selected by referencing a GatewayClass. There must be at least one GatewayClass defined in the cluster to have functional Gateways. In our case, we’re selecting Istio’s gateway controller, istio.io/gateway-controller, by referencing its associated GatewayClass (named istio) with the gatewayClassName: istio setting in the Gateway.

Notice that unlike Ingress, a Kubernetes Gateway doesn’t include any references to the target service, helloworld. With the Gateway API, routes to services are defined in separate configuration resources that get attached to the Gateway to direct subsets of traffic to specific services, like helloworld in our example. This separation allows us to define the Gateway and routes in different namespaces, presumably managed by different teams. Here, while acting in the role of cluster operator, we’re applying the Gateway in the sample-ingress namespace. We’ll add the route, below, in the sample namespace, next to the helloworld service itself, on behalf of the application developer.

Because the Gateway resource is owned by a cluster operator, it can very well be used to provide ingress for more than one team’s services, in our case more than just the helloworld service. To emphasize this point, we’ve set hostname to *.sample.com in the Gateway, allowing routes for multiple subdomains to be attached.

After applying the Gateway resource, we need to wait for it to be ready before retrieving its external address:

$ kubectl wait -n sample-ingress --for=condition=programmed gateway sample-gateway
$ export INGRESS_HOST=$(kubectl get -n sample-ingress gateway sample-gateway -o jsonpath='{.status.addresses[0].value}')

Next, we attach an HTTPRoute to the sample-gateway (i.e., using the parentRefs field) to expose and route traffic to the helloworld service:

$ kubectl apply -n sample -f - <<EOF
apiVersion: gateway.networking.k8s.io/v1beta1
kind: HTTPRoute
metadata:
  name: helloworld
spec:
  parentRefs:
  - name: sample-gateway
    namespace: sample-ingress
  hostnames: ["helloworld.sample.com"]
  rules:
  - matches:
    - path:
        type: Exact
        value: /hello
    backendRefs:
    - name: helloworld
      port: 5000
EOF

Here we’ve exposed the /hello path of the helloworld service to clients outside of the cluster, specifically via host helloworld.sample.com. You can confirm the helloworld sample is accessible using curl:

$ for run in {1..10}; do curl -HHost:helloworld.sample.com http://$INGRESS_HOST/hello; done
Hello version: v1, instance: helloworld-v1-78b9f5c87f-2sskj
Hello version: v2, instance: helloworld-v2-54dddc5567-2lm7b
Hello version: v1, instance: helloworld-v1-78b9f5c87f-2sskj
Hello version: v2, instance: helloworld-v2-54dddc5567-2lm7b
Hello version: v2, instance: helloworld-v2-54dddc5567-2lm7b
Hello version: v1, instance: helloworld-v1-78b9f5c87f-2sskj
Hello version: v1, instance: helloworld-v1-78b9f5c87f-2sskj
Hello version: v2, instance: helloworld-v2-54dddc5567-2lm7b
Hello version: v1, instance: helloworld-v1-78b9f5c87f-2sskj
Hello version: v2, instance: helloworld-v2-54dddc5567-2lm7b

Since no version routing has been configured in the route rule, you should see an equal split of traffic, about half handled by helloworld-v1 and the other half handled by helloworld-v2.

Configure weight-based version routing

Among other “traffic shaping” features, you can use Gateway API to send all of the traffic to one of the versions or split the traffic based on request percentages. For example, you can use the following rule to distribute the helloworld traffic 90% to v1, 10% to v2:

$ kubectl apply -n sample -f - <<EOF
apiVersion: gateway.networking.k8s.io/v1beta1
kind: HTTPRoute
metadata:
  name: helloworld
spec:
  parentRefs:
  - name: sample-gateway
    namespace: sample-ingress
  hostnames: ["helloworld.sample.com"]
  rules:
  - matches:
    - path:
        type: Exact
        value: /hello
    backendRefs:
    - name: helloworld-v1
      port: 5000
      weight: 90
    - name: helloworld-v2
      port: 5000
      weight: 10
EOF

Gateway API relies on version-specific backend service definitions for the route targets, helloworld-v1 and helloworld-v2 in this example. The helloworld sample already includes service definitions for the helloworld versions v1 and v2, we just need to run the following command to define them:

Zip
$ kubectl apply -n sample -f @samples/helloworld/gateway-api/helloworld-versions.yaml@

Now, we can run the previous curl commands again:

$ for run in {1..10}; do curl -HHost:helloworld.sample.com http://$INGRESS_HOST/hello; done
Hello version: v1, instance: helloworld-v1-78b9f5c87f-2sskj
Hello version: v1, instance: helloworld-v1-78b9f5c87f-2sskj
Hello version: v1, instance: helloworld-v1-78b9f5c87f-2sskj
Hello version: v1, instance: helloworld-v1-78b9f5c87f-2sskj
Hello version: v1, instance: helloworld-v1-78b9f5c87f-2sskj
Hello version: v1, instance: helloworld-v1-78b9f5c87f-2sskj
Hello version: v1, instance: helloworld-v1-78b9f5c87f-2sskj
Hello version: v1, instance: helloworld-v1-78b9f5c87f-2sskj
Hello version: v2, instance: helloworld-v2-54dddc5567-2lm7b
Hello version: v1, instance: helloworld-v1-78b9f5c87f-2sskj

This time we see that about 9 out of 10 requests are now handled by helloworld-v1 and only about 1 in 10 are handled by helloworld-v2.

Gateway API for internal mesh traffic

You may have noticed that we’ve been talking about the Gateway API only as an ingress configuration API, often referred to as north-south traffic management, and not an API for service-to-service (aka, east-west) traffic management within a cluster.

If you are using a service mesh, it would be highly desirable to use the same API resources to configure both ingress traffic routing and internal traffic, similar to the way Istio uses VirtualService to configure route rules for both. Fortunately, the Kubernetes Gateway API is working to add this support. Although not as mature as the Gateway API for ingress traffic, an effort known as the Gateway API for Mesh Management and Administration (GAMMA) initiative is underway to make this a reality and Istio intends to make Gateway API the default API for all of its traffic management in the future.

The first significant Gateway Enhancement Proposal (GEP) has recently been accepted and is, in-fact, already available to use in Istio. To try it out, you’ll need to use the experimental version of the Gateway API CRDs, instead of the standard Beta version we installed above, but otherwise, you’re ready to go. Check out the Istio request routing task to get started.

Summary

In this article, we’ve seen how a light-weight minimal install of Istio can be used to provide a Beta-quality implementation of the new Kubernetes Gateway API for cluster ingress traffic control. For Istio users, the Istio implementation also lets you start trying out the experimental Gateway API support for east-west traffic management within the mesh.

Much of Istio’s documentation, including all of the ingress tasks and several mesh-internal traffic management tasks, already includes parallel instructions for configuring traffic using either the Gateway API or the Istio configuration API. Check out the Gateway API task for more information about the Gateway API implementation in Istio.

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