Update ClusterClient documentation to include the new discovery feature

[Arkatufus]

Changes

When the Cluster Cannot Be Reached at All

Watching ClusterClient Actor Termination

It is possible to make the cluster client stop entirely if it cannot find a receptionist it can talk to within a configurable interval. This is configured with the reconnect-timeout, which defaults to off. This can be useful when initial contacts are provided from some kind of service registry, cluster node addresses are entirely dynamic and the entire cluster might shut down or crash, be restarted on new addresses. Since the client will be stopped in that case a monitoring actor can watch it and upon Terminate a new set of initial contacts can be fetched and a new cluster client started.

Contact Auto-Discovery Using Akka.Discovery

Note

This feature is currently considered as an advanced feature and is not currently compatible with Akka.Discovery Akka.Hosting extensions.

This feature should not be used until Akka.Management 1.5.26 is released.

This feature is added in Akka.NET 1.5.26. Instead of watching for actor termination manually, you can leverage Akka.Discovery to discover cluster client contact points inside a dynamic environment such as Kubernetes, AWS, or anywhere else with Azure Table

The HOCON configuration to set these are:

akka.cluster.client
{
  use-initial-contacts-discovery = false

  discovery
  {
    method = <method>
    actor-system-name = null
    receptionist-name = receptionist
    service-name = null
    port-name = null
    discovery-retry-interval = 1s
    discovery-timeout = 60s
  }
}

To enable contact auto-discovery, you will need to:

• Set akka.cluster.client.use-initial-contacts-discovery to true.
• Set akka.cluster.client.discovery.service-name that matches the service name of the Akka.Discovery extension that you used:
  • For Akka.Discovery.KubernetesApi, this is the pod-label-selector HOCON setting or the KubernetesDiscoveryOptions.PodLabelSelector options property.
  • For Akka.Discovery.AwsApi, this is
    • EC2: the akka.discovery.aws-api-ec2-tag-based.tag-key HOCON setting or the Akka.Hosting Ec2ServiceDiscoveryOptions.TagKey options property.
    • ECS: the akka.discovery.aws-api-ecs.tags HOCON setting or the Akka.Hosting EcsServiceDiscoveryOptions.Tags options property.
  • For Akka.Discovery.Azure, this is the service-name HOCON setting or the AkkaDiscoveryOptions.ServiceName options property.
• Set akka.cluster.client.discovery.method to a valid discovery method name listed under akka.discovery.
• Set akka.cluster.client.discovery.actor-system-name to the target cluster ActorSystem name.
• OPTIONAL. Set akka.cluster.client.discovery,port-name if the discovery extension that you're using depends on port names.
• OPTIONAL. Set akka.cluster.client.discovery.receptionist-name if you're using a non-default receptionist name.

Using Akka.Discovery For Both Akka.Cluster.Tools.Client And Akka.Management.Cluster.Bootstrap

If you need to use Akka.Discovery with both ClusterClient AND ClusterBootstrap, you will have to make sure that you have TWO different Akka.Discovery settings living side-by-side under the akka.discovery HOCON setting object.

Akka.Discovery.KubernetesApi Example

In your YAML file:

• Make sure that you tag the instances that will run the cluster client receptionists with an extra tag. If your ClusterBootstrap is tagged with the YAML value metadata.labels.app: cluster, then you will need to add another tag to the instances that runs the Receptionists, e.g. metadata.labels.contact: cluster-client like so:

  metadata:
    labels:
      app: cluster
      contact: cluster-client

• Make sure you name the Akka remoting port

  spec:
    template:
      spec:
        containers:
          ports:
          - containerPort: 2552 # This is the remoting port, change this to match yours
            protocol: TCP
            name: akka-remote # this is important

In your cluster client Akka.NET node HOCON settings:

• Copy the akka.discovery.kubernetes-api HOCON section and paste it above or under the original settings. You can also copy the value from here
• Rename the HOCON section to akka.discovery.kubernetes-api-cluster-client. The key name does not matter, what matters is that the name does not collide with any other setting section name under akka.discovery.
• Make sure you change akka.discovery.kubernetes-api-cluster-client.pod-label-selector to "contact={0}" to match what we have in the YAML file.
• Make sure you change akka.cluster.client.discovery.service-name to "cluster-client" to match what we have in the YAML file.
• Make sure you change akka.cluster.client.discovery.port-name value to "akka-remote" to match what we have in the YAML file.
• Keep the akka.discovery.method HOCON value to "kubernetes-api", this is the discovery extension that will be used by ClusterBootstrap.
• Change the akka.cluster.client.discovery.method value from "<method>" to "kubernetes-api-cluster-client", this is the discovery extension that will be used by ClusterClient. If not set, this will default to the value set in akka.discovery.method, which is NOT what we want.

Akka.Discovery.Azure Example

In your cluster receptionist Akka.NET node HOCON settings:

• Copy the akka.discovery.azure HOCON section and paste it above or under the original settings. You can also copy the value from here
• Rename the HOCON section to akka.discovery.azure-cluster-client. The key name does not matter, what matters is that the name does not collide with any other setting section name under akka.discovery.
• Change akka.discovery.azure-cluster-client.public-port to the remoting port of the Akka.NET node.
• Change akka.discovery.azure-cluster-client.service-name to "cluster-client". The name does not matter, what matters is that this name HAS to match the service name we'll be using in akka.cluster.client.discovery.service-name.
• [OPTIONAL] change akka.discovery.azure-cluster-client.table-name to akkaclusterreceptionists to separate the discovery table from ClusterBootstrap entries.
• Make sure that you start the discovery extension in the receptionist side. This needs to be done because the extension is responsible for updating the Azure table.

  Discovery.Get(myActorSystem).LoadServiceDiscovery("azure-cluster-client");

In your cluster client Akka.NET node HOCON settings:

• If you're USING ClusterBootstrap in the cluster client side:
  • You WILL NEED to perform the same HOCON configuration cloning process above.
  • Make sure you change akka.cluster.client.discovery.service-name to "cluster-client" to match what we have in the receptionist node HOCON file.
  • You WILL NEED to keep the akka.discovery.method HOCON value to "azure", this is the discovery extension that will be used by ClusterBootstrap.
  • Change the akka.cluster.client.discovery.method value from "<method>" to "azure-cluster-client", this is the discovery extension that will be used by ClusterClient. If not set, this will default to the value set in akka.discovery.method, which is NOT what we want.
  • [OPTIONAL] IF you opt to change the table name in the receptionist side, you WILL NEED to change akka.discovery.azure-cluster-client.table-name to match the table name with the receptionist side, it was akkaclusterreceptionists in our example above
• If you're NOT USING ClusterBootstrap in the cluster client side:
  • You DO NOT NEED to perform the HOCON configuration cloning process as you're not using ClusterBootstrap.
  • Make sure you change akka.cluster.client.discovery.service-name to "cluster-client" to match what we have in the receptionist node HOCON file.
  • You can keep the akka.discovery.method HOCON value to "azure".
  • You can change the akka.cluster.client.discovery.method value from "<method>" to "azure", this is the discovery extension that will be used by ClusterClient. If not set, this will default to the value set in akka.discovery.method, which will be the same.
  • [OPTIONAL] IF you opt to change the table name in the receptionist side, you WILL NEED to change akka.discovery.azure-cluster-client.table-name to match the table name with the receptionist side, it was akkaclusterreceptionists in our example above.