» Syncing Kubernetes and Consul Services

The services in Kubernetes and Consul can be automatically synced so that Kubernetes services are available to Consul agents and services in Consul can be available as first-class Kubernetes services. This functionality is provided by the consul-k8s project and can be automatically installed and configured using the Consul Helm chart.

Why sync Kubernetes services to Consul? Kubernetes services synced to the Consul catalog enable Kubernetes services to be accessed by any node that is part of the Consul cluster, including other distinct Kubernetes clusters. For non-Kubernetes nodes, they can access services using the standard Consul DNS or HTTP API.

Why sync Consul services to Kubernetes? Syncing Consul services to Kubernetes services enables non-Kubernetes services (such as external to the cluster) to be accessed in a native Kubernetes way: using kube-dns, environment variables, etc. This makes it very easy to automate external service discovery, including hosted services like databases.

» Installation and Configuration

The service sync is done using an external long-running process in the consul-k8s project. This process can run either in or out of a Kubernetes cluster. However, running this within the Kubernetes cluster is generally easier since it is automated using the Helm chart.

The Consul server cluster can run either in or out of a Kubernetes cluster. The Consul server cluster does not need to be running on the same machine or same platform as the sync process. The sync process needs to be configured with the address to the Consul cluster as well as any additional access information such as ACL tokens.

To install the sync, enable the catalog sync feature using Helm values and upgrade the installation using helm upgrade for existing installs or helm install for a fresh install.

syncCatalog:
  enabled: true

This will enable services to sync in both directions. You can also choose to only sync Kubernetes services to Consul or vice versa by disabling a direction. See the Helm configuration for more information.

» Authentication

The sync process must authenticate to both Kubernetes and Consul to read and write services.

For Consul, the process accepts both the standard CLI flag -token and the environment variable CONSUL_HTTP_TOKEN. This should be set to an Consul ACL token if ACLs are enabled. This can also be configured using the Helm chart to read from a Kubernetes secret.

For Kubernetes, a valid kubeconfig file must be provided with cluster and auth information. The sync process will look into the default locations for both in-cluster and out-of-cluster authentication. If kubectl works, then the sync program should work.

» Kubernetes to Consul

This sync registers Kubernetes services to the Consul catalog automatically.

This enables discovery and connection to Kubernetes services using native Consul service discovery such as DNS or HTTP. This is particularly useful for non-Kubernetes nodes. This also causes all discoverable services to be part of a central service catalog in Consul for further syncing into alternate Kubernetes clusters or other platforms.

» Kubernetes Service Types

Not all Kubernetes services are externally accessible. The sync program by default will only sync services of the following types or configurations. If a service type is not listed below, then the sync program will ignore that service type.

» NodePort

NodePort services register a static port that every node in the K8S cluster listens on.

For NodePort services, a Consul service instance will be created for each node that has the representative pod running. While Kubernetes configures a static port on all nodes in the cluster, this limits the number of service instances to be equal to the nodes running the target pods.

The service instances will be registered to the Kubernetes node name that each instance lives on. This is guaranteed unique by Kubernetes. An existing node entry will be used if it is already part of the Consul cluster (for example if you're running a client agent on all Kubernetes nodes). This allows the normal agent health checks for that node to continue working.

» LoadBalancer

For LoadBalancer services, a single service instance will be registered with the external IP of the created load balancer. Because this is already a load balancer, only one service instance will be registered with Consul rather than registering each individual pod endpoint.

» External IPs

Any service type may specify an "external IP" configuration. The external IP must be configured by some other system, but any service discovery will resolve to this set of IP addresses rather than a virtual IP.

If an external IP list is present, a service instance in Consul will be created for each external IP. It is assumed that if an external IP is present that it is routable and configured by some other system.

» Sync Enable/Disable

By default, all valid services (as explained above) are synced. This default can be changed as configuration to the sync process. Syncing can also be explicitly enabled or disabled using an annotation:

kind: Service
apiVersion: v1
metadata:
  name: my-service
  annotations:
    "consul.hashicorp.com/service-sync": false

» Service Name

When a Kubernetes service is synced to Consul, the name of the service in Consul by default will be the value of the "name" metadata on that Kubernetes service. This makes it so that service sync works with zero configuration changes. This can be overridden using an annotation to specify the Consul service name:

kind: Service
apiVersion: v1
metadata:
  name: my-service
  annotations:
    "consul.hashicorp.com/service-name": my-consul-service

If a conflicting service name exists in Consul, the sync program will register additional instances to that same service. Therefore, services inside and outside of Kubernetes should have different names unless you want either side to potentially connect. This default behavior also enables gracefully transitioning a service from outside of K8S to inside, and vice versa.

» Service Ports

When syncing the Kubernetes service to Consul, the Consul service port will be the first defined port in the service. Additionally, all ports will be registered in the service instance metadata with the key "port-X" where X is the name of the port and the value is the externally accessible port.

The default service port can be overridden using an annotation:

kind: Service
apiVersion: v1
metadata:
  name: my-service
  annotations:
    "consul.hashicorp.com/service-port": "http"

The annotation value may a name of a port (recommended) or an exact port value.

» Service Tags

A service registered in Consul from Kubernetes will always have the tag "k8s" added to it. Additional tags can be specified with a comma-separated annotation value as shown below. This will also automatically include the "k8s" tag which can't be disabled. The values should be specified comma-separated without any additional whitespace.

kind: Service
apiVersion: v1
metadata:
  name: my-service
  annotations:
    "consul.hashicorp.com/service-tags": "primary,foo"

» Service Meta

A service registered in Consul from Kubernetes will set the external-source key to "kubernetes". This can be used by API consumers, the UI, CLI, etc. to filter service instances that are set in k8s. The Consul UI (in Consul 1.2.3 and later) will read this value to show a Kubernetes icon next to all externally registered services from Kubernetes.

Additional metadata can be specified using annotations. The "KEY" below can be set to any key. This allows setting multiple meta values:

kind: Service
apiVersion: v1
metadata:
  name: my-service
  annotations:
    "consul.hashicorp.com/service-meta-KEY": "value"

» Consul to Kubernetes

This syncs Consul services into first-class Kubernetes services. Each Consul service is synced to an ExternalName service in Kubernetes. The external name is configured to be the Consul DNS entry.

This enables external services to be discovered using native Kubernetes tooling. This can be used to ease software migration into or out of Kubernetes, across platforms, to and from hosted services, and more.

» Sync Enable/Disable

All Consul services visible to the sync process based on its given ACL token will be synced to Kubernetes.

There is no way to change this behavior per service. For the opposite sync direction (Kubernetes to Consul), you can use Kubernetes annotations to disable a sync per service. This is not currently possible for Consul to Kubernetes sync and the ACL token must be used to limit what services are synced.

In the future, we hope to support per-service configuration.

» Service Name

When a Consul service is synced to Kubernetes, the name of the Kubernetes service will exactly match the name of the Consul service.

To change this default exact match behavior, it is possible to specify a prefix to be added to service names within Kubernetes by using the -k8s-service-prefix flag. This can also be specified in the Helm configuration.

If a conflicting service is found, the service will not be synced. This does not match the Kubernetes to Consul behavior, but given the current implementation we must do this because Kubernetes can't mix both CNAME and Endpoint-based services.

» Kubernetes Service Labels and Annotations

Any Consul services synced to Kubernetes will be labeled and annotated. An annotation consul.hashicorp.com/synced will be set to "true" to note that this is a synced service from Consul.

Additionally, a label consul=true will be specified so that label selectors can be used with kubectl and other tooling to easily filter all Consul-synced services.