Single Consul Datacenter in Multiple Kubernetes Clusters
Requires consul-helm v0.32.1 or higher.
This page describes how to deploy a single Consul datacenter in multiple Kubernetes clusters, with both servers and clients running in one cluster, and only clients running in the rest of the clusters. In this example, we will use two Kubernetes clusters, but this approach could be extended to using more than two.
Note: This deployment topology requires that your Kubernetes clusters have a flat network for both pods and nodes, so that pods or nodes from one cluster can connect to pods or nodes in another.
Deploying Consul servers and clients in the first cluster
First, we will deploy the Consul servers with Consul clients in the first cluster. For that, we will use the following Helm configuration:
Note that we are deploying in a secure configuration, with gossip encryption, TLS for all components, and ACLs. We are enabling the Consul Service Mesh and the controller for CRDs so that we can use them to later verify that our services can connect with each other across clusters.
We're also setting UI's service type to be NodePort
.
This is needed so that we can connect to servers from another cluster without using the pod IPs of the servers,
which are likely going to change.
To deploy, first we need to generate the Gossip encryption key and save it as a Kubernetes secret.
Now we can install our Consul cluster with Helm:
Note: The Helm release name must be unique for each Kubernetes cluster. That is because the Helm chart will use the Helm release name as a prefix for the ACL resources that it creates, such as tokens and auth methods. If the names of the Helm releases are the same, the Helm installation in subsequent clusters will clobber existing ACL resources.
Once the installation finishes and all components are running and ready, we need to extract the gossip encryption key we've created, the CA certificate and the ACL bootstrap token generated during installation, so that we can apply them to our second Kubernetes cluster.
Deploying Consul clients in the second cluster
Note: If multiple Kubernetes clusters will be joined to the Consul Datacenter, then the following instructions will need to be repeated for each additional Kubernetes cluster.
Now we can switch to the second Kubernetes cluster where we will deploy only the Consul clients that will join the first Consul cluster.
First, we need to apply credentials we've extracted from the first cluster to the second cluster:
To deploy in the second cluster, we will use the following Helm configuration:
Note that we're referencing secrets from the first cluster in ACL, gossip, and TLS configuration.
Next, we need to set up the externalServers
configuration.
The externalServers.hosts
and externalServers.httpsPort
refer to the IP and port of the UI's NodePort service deployed in the first cluster.
Set the externalServers.hosts
to any Node IP of the first cluster,
which you can see by running kubectl get nodes -o wide
.
Set externalServers.httpsPort
to the nodePort
of the cluster1-consul-ui
service.
In our example, the port is 31557
.
We set the externalServer.tlsServerName
to server.dc1.consul
. This the DNS SAN
(Subject Alternative Name) that is present in the Consul server's certificate.
We need to set it because we're connecting to the Consul servers over the node IP,
but that IP isn't present in the server's certificate.
To make sure that the hostname verification succeeds during the TLS handshake, we need to set the TLS
server name to a DNS name that is present in the certificate.
Next, we need to set externalServers.k8sAuthMethodHost
to the address of the second Kubernetes API server.
This should be the address that is reachable from the first cluster, and so it cannot be the internal DNS
available in each Kubernetes cluster. Consul needs it so that consul login
with the Kubernetes auth method will work
from the second cluster.
More specifically, the Consul server will need to perform the verification of the Kubernetes service account
whenever consul login
is called, and to verify service accounts from the second cluster it needs to
reach the Kubernetes API in that cluster.
The easiest way to get it is to set it from your kubeconfig
by running kubectl config view
and grabbing
the value of cluster.server
for the second cluster.
Lastly, we need to set up the clients so that they can discover the servers in the first cluster.
For this, we will use Consul's cloud auto-join feature
for the Kubernetes provider.
To use it we need to provide a way for the Consul clients to reach the first Kubernetes cluster.
To do that, we need to save the kubeconfig
for the first cluster as a Kubernetes secret in the second cluster
and reference it in the clients.join
value. Note that we're making that secret available to the client pods
by setting it in client.extraVolumes
.
Note: The kubeconfig you're providing to the client should have minimal permissions. The cloud auto-join provider will only need permission to read pods. Please see Kubernetes Cloud auto-join for more details.
Now we're ready to install!
Verifying the Consul Service Mesh works
When Transparent proxy is enabled, services in one Kubernetes cluster that need to communicate with a service in another Kubernetes cluster must have a explicit upstream configured through the "consul.hashicorp.com/connect-service-upstreams" annotation.
Now that we have our Consul cluster in multiple k8s clusters up and running, we will deploy two services and verify that they can connect to each other.
First, we'll deploy static-server
service in the first cluster:
Note that we're defining a Service intention so that our services are allowed to talk to each other.
Then we'll deploy static-client
in the second cluster with the following configuration:
Once both services are up and running, we can connect to the static-server
from static-client
: