Version Routing in a Multicluster Service Mesh

If you’ve spent any time looking at Istio, you’ve probably noticed that it includes a lot of features that can be demonstrated with simple tasks¹ and examples² running on a single Kubernetes cluster. Because most, if not all, real-world cloud and microservices-based applications are not that simple and will need to have the services distributed and running in more than one location, you may be wondering if all these things will be just as simple in your real production environment.

Fortunately, Istio provides several ways to configure a service mesh so that applications can, more-or-less transparently, be part of a mesh where the services are running in more than one cluster, i.e., in a multicluster deployment. The simplest way to set up a multicluster mesh, because it has no special networking requirements, is using a replicated control plane model. In this configuration, each Kubernetes cluster contributing to the mesh has its own control plane, but each control plane is synchronized and running under a single administrative control.

In this article we’ll look at how one of the features of Istio, traffic management³, works in a multicluster mesh with a dedicated control plane topology. We’ll show how to configure Istio route rules to call remote services in a multicluster service mesh by deploying the Bookinfo sample⁴ with version v1 of the reviews service running in one cluster, versions v2 and v3 running in a second cluster.

Set up clusters

To start, you’ll need two Kubernetes clusters, both running a slightly customized configuration of Istio.

• Set up a multicluster environment with two Istio clusters by following the replicated control planes⁵ instructions.

• The kubectl command is used to access both clusters with the --context flag. Use the following command to list your contexts:

  $ kubectl config get-contexts
  CURRENT   NAME       CLUSTER    AUTHINFO       NAMESPACE
  *         cluster1   cluster1   user@foo.com   default
            cluster2   cluster2   user@foo.com   default
  

• Export the following environment variables with the context names of your configuration:

  $ export CTX_CLUSTER1=<cluster1 context name>
  $ export CTX_CLUSTER2=<cluster2 context name>
  

Deploy version v1 of the bookinfo application in cluster1

Run the productpage and details services and version v1 of the reviews service in cluster1:

$ kubectl label --context=$CTX_CLUSTER1 namespace default istio-injection=enabled
$ kubectl apply --context=$CTX_CLUSTER1 -f - <<EOF
apiVersion: v1
kind: Service
metadata:
  name: productpage
  labels:
    app: productpage
spec:
  ports:
  - port: 9080
    name: http
  selector:
    app: productpage
---
apiVersion: extensions/v1beta1
kind: Deployment
metadata:
  name: productpage-v1
spec:
  replicas: 1
  template:
    metadata:
      labels:
        app: productpage
        version: v1
    spec:
      containers:
      - name: productpage
        image: istio/examples-bookinfo-productpage-v1:1.10.0
        imagePullPolicy: IfNotPresent
        ports:
        - containerPort: 9080
---
apiVersion: v1
kind: Service
metadata:
  name: details
  labels:
    app: details
spec:
  ports:
  - port: 9080
    name: http
  selector:
    app: details
---
apiVersion: extensions/v1beta1
kind: Deployment
metadata:
  name: details-v1
spec:
  replicas: 1
  template:
    metadata:
      labels:
        app: details
        version: v1
    spec:
      containers:
      - name: details
        image: istio/examples-bookinfo-details-v1:1.10.0
        imagePullPolicy: IfNotPresent
        ports:
        - containerPort: 9080
---
apiVersion: v1
kind: Service
metadata:
  name: reviews
  labels:
    app: reviews
spec:
  ports:
  - port: 9080
    name: http
  selector:
    app: reviews
---
apiVersion: extensions/v1beta1
kind: Deployment
metadata:
  name: reviews-v1
spec:
  replicas: 1
  template:
    metadata:
      labels:
        app: reviews
        version: v1
    spec:
      containers:
      - name: reviews
        image: istio/examples-bookinfo-reviews-v1:1.10.0
        imagePullPolicy: IfNotPresent
        ports:
        - containerPort: 9080
EOF

Deploy bookinfo v2 and v3 services in cluster2

Run the ratings service and version v2 and v3 of the reviews service in cluster2:

$ kubectl label --context=$CTX_CLUSTER2 namespace default istio-injection=enabled
$ kubectl apply --context=$CTX_CLUSTER2 -f - <<EOF
apiVersion: v1
kind: Service
metadata:
  name: ratings
  labels:
    app: ratings
spec:
  ports:
  - port: 9080
    name: http
  selector:
    app: ratings
---
apiVersion: extensions/v1beta1
kind: Deployment
metadata:
  name: ratings-v1
spec:
  replicas: 1
  template:
    metadata:
      labels:
        app: ratings
        version: v1
    spec:
      containers:
      - name: ratings
        image: istio/examples-bookinfo-ratings-v1:1.10.0
        imagePullPolicy: IfNotPresent
        ports:
        - containerPort: 9080
---
apiVersion: v1
kind: Service
metadata:
  name: reviews
  labels:
    app: reviews
spec:
  ports:
  - port: 9080
    name: http
  selector:
    app: reviews
---
apiVersion: extensions/v1beta1
kind: Deployment
metadata:
  name: reviews-v2
spec:
  replicas: 1
  template:
    metadata:
      labels:
        app: reviews
        version: v2
    spec:
      containers:
      - name: reviews
        image: istio/examples-bookinfo-reviews-v2:1.10.0
        imagePullPolicy: IfNotPresent
        ports:
        - containerPort: 9080
---
apiVersion: extensions/v1beta1
kind: Deployment
metadata:
  name: reviews-v3
spec:
  replicas: 1
  template:
    metadata:
      labels:
        app: reviews
        version: v3
    spec:
      containers:
      - name: reviews
        image: istio/examples-bookinfo-reviews-v3:1.10.0
        imagePullPolicy: IfNotPresent
        ports:
        - containerPort: 9080
EOF

Access the bookinfo application

Just like any application, we’ll use an Istio gateway to access the bookinfo application.

• Create the bookinfo gateway in cluster1:

  Zip

  $ kubectl apply --context=$CTX_CLUSTER1 -f samples/bookinfo/networking/bookinfo-gateway.yaml6
  

• Follow the Bookinfo sample instructions to determine the ingress IP and port and then point your browser to http://$GATEWAY_URL/productpage.

You should see the productpage with reviews, but without ratings, because only v1 of the reviews service is running on cluster1 and we have not yet configured access to cluster2.

Create a service entry and destination rule on cluster1 for the remote reviews service

As described in the setup instructions, remote services are accessed with a .global DNS name. In our case, it’s reviews.default.global, so we need to create a service entry and destination rule for that host. The service entry will use the cluster2 gateway as the endpoint address to access the service. You can use the gateway’s DNS name, if it has one, or its public IP, like this:

$ export CLUSTER2_GW_ADDR=$(kubectl get --context=$CTX_CLUSTER2 svc --selector=app=istio-ingressgateway \
    -n istio-system -o jsonpath="{.items[0].status.loadBalancer.ingress[0].ip}")

Now create the service entry and destination rule using the following command:

$ kubectl apply --context=$CTX_CLUSTER1 -f - <<EOF
apiVersion: networking.istio.io/v1alpha3
kind: ServiceEntry
metadata:
  name: reviews-default
spec:
  hosts:
  - reviews.default.global
  location: MESH_INTERNAL
  ports:
  - name: http1
    number: 9080
    protocol: http
  resolution: DNS
  addresses:
  - 240.0.0.3
  endpoints:
  - address: ${CLUSTER2_GW_ADDR}
    labels:
      cluster: cluster2
    ports:
      http1: 15443 # Do not change this port value
---
apiVersion: networking.istio.io/v1alpha3
kind: DestinationRule
metadata:
  name: reviews-global
spec:
  host: reviews.default.global
  trafficPolicy:
    tls:
      mode: ISTIO_MUTUAL
  subsets:
  - name: v2
    labels:
      cluster: cluster2
  - name: v3
    labels:
      cluster: cluster2
EOF

The address 240.0.0.3 of the service entry can be any arbitrary unallocated IP. Using an IP from the class E addresses range 240.0.0.0/4 is a good choice. Check out the gateway-connected multicluster example⁵ for more details.

Note that the labels of the subsets in the destination rule map to the service entry endpoint label (cluster: cluster2) corresponding to the cluster2 gateway. Once the request reaches the destination cluster, a local destination rule will be used to identify the actual pod labels (version: v1 or version: v2) corresponding to the requested subset.

Create a destination rule on both clusters for the local reviews service

Technically, we only need to define the subsets of the local service that are being used in each cluster (i.e., v1 in cluster1, v2 and v3 in cluster2), but for simplicity we’ll just define all three subsets in both clusters, since there’s nothing wrong with defining subsets for versions that are not actually deployed.

$ kubectl apply --context=$CTX_CLUSTER1 -f - <<EOF
apiVersion: networking.istio.io/v1alpha3
kind: DestinationRule
metadata:
  name: reviews
spec:
  host: reviews.default.svc.cluster.local
  trafficPolicy:
    tls:
      mode: ISTIO_MUTUAL
  subsets:
  - name: v1
    labels:
      version: v1
  - name: v2
    labels:
      version: v2
  - name: v3
    labels:
      version: v3
EOF

$ kubectl apply --context=$CTX_CLUSTER2 -f - <<EOF
apiVersion: networking.istio.io/v1alpha3
kind: DestinationRule
metadata:
  name: reviews
spec:
  host: reviews.default.svc.cluster.local
  trafficPolicy:
    tls:
      mode: ISTIO_MUTUAL
  subsets:
  - name: v1
    labels:
      version: v1
  - name: v2
    labels:
      version: v2
  - name: v3
    labels:
      version: v3
EOF

Create a virtual service to route reviews service traffic

At this point, all calls to the reviews service will go to the local reviews pods (v1) because if you look at the source code you will see that the productpage implementation is simply making requests to http://reviews:9080 (which expands to host reviews.default.svc.cluster.local), the local version of the service. The corresponding remote service is named reviews.default.global, so route rules are needed to redirect requests to the global host.

Apply the following virtual service to direct traffic for user jason to reviews versions v2 and v3 (⁵⁰⁄₅₀) which are running on cluster2. Traffic for any other user will go to reviews version v1.

$ kubectl apply --context=$CTX_CLUSTER1 -f - <<EOF
apiVersion: networking.istio.io/v1alpha3
kind: VirtualService
metadata:
  name: reviews
spec:
  hosts:
    - reviews.default.svc.cluster.local
  http:
  - match:
    - headers:
        end-user:
          exact: jason
    route:
    - destination:
        host: reviews.default.global
        subset: v2
      weight: 50
    - destination:
        host: reviews.default.global
        subset: v3
      weight: 50
  - route:
    - destination:
        host: reviews.default.svc.cluster.local
        subset: v1
EOF

Return to your browser and login as user jason. If you refresh the page several times, you should see the display alternating between black and red ratings stars (v2 and v3). If you logout, you will only see reviews without ratings (v1).

Summary

In this article, we’ve seen how to use Istio route rules to distribute the versions of a service across clusters in a multicluster service mesh with a replicated control plane model. In this example, we manually configured the .global service entry and destination rules needed to provide connectivity to one remote service, reviews. In general, however, if we wanted to enable any service to run either locally or remotely, we would need to create .global resources for every service. Fortunately, this process could be automated and likely will be in a future Istio release.