Service Sync for Consul on Kubernetes

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 K8s 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 to be accessed using kube-dns and Kubernetes-specific environment variables. This integration makes it very easy to automate external service discovery, including hosted services like databases.

Installation and configuration

The service sync feature deploys a long-running process which can run either inside or outside of a Kubernetes cluster. However, running this process 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 a Consul agent as well as any additional access information such as ACL tokens.

To install the sync process, 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 value enables service syncing in both direction. You can also disable a direction so that only Kubernetes services sync to Consul, or only Consul services sync to Kubernetes.

To only enable syncing Consul services to Kubernetes use the config:

syncCatalog:
  enabled: true
  toConsul: false
  toK8S: true

To only enable syncing Kubernetes services to Consul use:

syncCatalog:
  enabled: true
  toConsul: true
  toK8S: false

Refer to the Helm configuration for more information.

Authentication

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

If running consul-k8s using the Helm chart, then this authentication is handled for you.

If running consul-k8s outside of Kubernetes, a valid kubeconfig file must be provided with cluster and authentication information. The sync process looks into the default locations for both in-cluster and out-of-cluster authentication. If kubectl works, then the sync program should work.

If ACLs are configured on the Consul cluster, you need to provide a Consul ACL token. Review the ACL rules when creating this token so that it only allows the necessary privileges. The catalog sync process accepts this token by using the CONSUL_HTTP_TOKEN environment variable. This token should be set as a Kubernetes secret and referenced in the Helm chart.

Kubernetes to Consul

This sync registers Kubernetes services to the Consul catalog automatically.

This sync enables discovery and connection to Kubernetes services using native Consul service discovery protocols 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.

Each synced service is registered onto a Consul node called k8s-sync. This node is not a real node. Instead, the catalog sync process is monitoring Kubernetes and syncing the services to Consul.

Kubernetes service types

Not all Kubernetes services are externally accessible. The sync program by default only syncs services of the following types or configurations. If a service type is not listed below, then the sync program ignores 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 is 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. By default it uses the external IP of the node but this can be configured through the nodePortSyncType helm option.

The service instance's port is set to the first defined node port of the service unless set specifically in the consul.hashicorp.com/service-port annotation. Refer to Service Ports for more information.

LoadBalancer

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

The service instance's port is set to the first defined port of the service unless set specifically in the consul.hashicorp.com/service-port annotation. Refer to Service Ports for more information.

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 resolves to this set of IP addresses rather than a virtual IP.

If an external IP list is present, a service instance in Consul is 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.

The service instance's port is set to the first defined port of the service unless set specifically with the consul.hashicorp.com/service-port annotation. Refer to Service Ports for more information.

ClusterIP

ClusterIP services are synced by default as of consul-k8s version 0.3.0. Each pod that is an endpoint for the service is synced as a Consul service instance with its IP set to the pod IP and its port set to the targetPort.

The service instance's port can be overridden with the consul.hashicorp.com/service-port annotation. Refer to Service Ports for more information.

Enable and disable sync

By default, all valid service types are synced from every Kubernetes namespace except for kube-system and kube-public.

To only sync specific services, first modify the annotation to set the default to false:

syncCatalog:
  enabled: true
  default: false

Then, explicitly enable syncing specific services with the consul.hashicorp.com/service-sync annotation:

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

You can allow or deny syncing from specific Kubernetes namespaces by setting the k8sAllowNamespaces and k8sDenyNamespaces keys:

syncCatalog:
  enabled: true
  default: true
  k8sAllowNamespaces: ['*']
  k8sDenyNamespaces: ['kube-system', 'kube-public']

In the default configuration, services from all namespaces except for kube-system and kube-public are synced.

To only sync from specific namespaces, you can list only those namespaces in the k8sAllowNamespaces key:

syncCatalog:
  enabled: true
  default: true
  k8sAllowNamespaces: ['my-ns-1', 'my-ns-2']
  k8sDenyNamespaces: []

To sync from every namespace except specific namespaces, use * in the allow list and then specify the non-syncing namespaces in the deny list:

syncCatalog:
  enabled: true
  default: true
  k8sAllowNamespaces: ['*']
  k8sDenyNamespaces: ['no-sync-ns-1', 'no-sync-ns-2']

Service name

When a Kubernetes service is synced to Consul, the name of the service in Consul by default is the value of the name metadata on that Kubernetes service. This makes it so that service sync works with zero configuration changes. This setting 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 registers 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 between deployments inside and outside of Kubernetes.

Service ports

When syncing the Kubernetes service to Consul, the Consul service port is the first defined port in the service. Additionally, all ports become 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 be a name of a port (recommended) or an exact port value.

Service tags

A service registered in Consul from Kubernetes always has the tag "k8s" added to it. Additional tags can be specified with a comma-separated annotation value. These custom tags automatically include the "k8s" tag, which can't be disabled. When specifying values, use commas without 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 sets the external-source key to kubernetes. This can be used from the APLI, CLI and UI to filter service instances that are set in k8s. The Consul UI displays 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 Enterprise Namespaces

Consul Enterprise supports Consul namespaces. These can be used when syncing from Kubernetes to Consul. However, namespaces are not supported when syncing from Consul to Kubernetes.

There are three options available:

1. Single Destination Namespace – Sync all Kubernetes services, regardless of namespace, into the same Consul namespace.

   global:
     enableConsulNamespaces: true
   
   syncCatalog:
     enabled: true
     consulNamespaces:
       consulDestinationNamespace: 'my-consul-ns'
   

2. Mirror Namespaces - Each Kubernetes service is synced to a Consul namespace with the same name as its Kubernetes namespace.

   For example, service foo in Kubernetes namespace ns-1 is synced to the Consul namespace ns-1. If a mirrored namespace does not exist in Consul, it is created automatically.

   global:
     enableConsulNamespaces: true
   
   syncCatalog:
     enabled: true
     consulNamespaces:
       mirroringK8S: true
   
   addK8SNamespaceSuffix: false
   

3. Mirror Namespaces With Prefix - Each Kubernetes service is synced to a Consul namespace with the same name as its Kubernetes namespace with a prefix. For example, given a prefix k8s-, service foo in Kubernetes namespace ns-1 syncs to the Consul namespace k8s-ns-1.

   global:
     enableConsulNamespaces: true
   
   syncCatalog:
     enabled: true
     consulNamespaces:
       mirroringK8S: true
       mirroringK8SPrefix: 'k8s-'
   
   addK8SNamespaceSuffix: false
   

Consul to Kubernetes

This syncs Consul services into first-class Kubernetes services. The sync creates an ExternalName for each Consul service. The "external name" is the Consul DNS name.

For example, given a Consul service foo:

apiVersion: v1
kind: Service
metadata:
  name: foo
  ...
spec:
  externalName: foo.service.consul
  type: ExternalName

With Consul To Kubernetes syncing enabled, DNS requests of the form <consul-service-name> are serviced by Consul DNS. From a different Kubernetes namespace than where Consul is deployed, the DNS request would need to be <consul-service-name>.<consul-namespace>.

Enable and disable sync

All Consul services visible to the sync process based on its given ACL token are 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 matches the name of the Consul service exactly.

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 is not synced. This does not match the Kubernetes to Consul behavior, but given the current implementation we must do this because Kubernetes cannott mix both CNAME and Endpoint-based services.

Kubernetes service labels and annotations

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

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