Class DNSMembershipProvider

java.lang.Object
org.apache.catalina.tribes.membership.MembershipProviderBase
org.apache.catalina.tribes.membership.cloud.CloudMembershipProvider
org.apache.catalina.tribes.membership.cloud.DNSMembershipProvider
All Implemented Interfaces:
ChannelListener, Heartbeat, MembershipProvider
public class DNSMembershipProvider extends CloudMembershipProvider
A MembershipProvider that uses DNS to retrieve the members of a cluster.

Relying solely on DNS to determine cluster membership requires that all DNS caching between the cluster nodes and the authoritative name server must honour the TTL set by the authoritative name server. Experience has shown that that is often not the case. Therefore, to ensure that new cluster members are not excluded from the cluster due to a stale DNS cache, this membership service accepts messages from any node regardless of whether it or not is listed as a cluster member in DNS and adds that node to the cluster. The DNS entry is, effectively, used by new nodes to identify the other nodes in the cluster to which cluster messages should be sent.

If more control is required over cluster membership, users are strongly encouraged to use KubernetesMembershipProvider instead. Alternatively, the EncryptInterceptor may be used to ensure that only nodes with knowledge of the shared key are able to participate in the cluster.

TODO: Make the "accept messages from any node and add that node to the cluster" behaviour optional.

Configuration example for Kubernetes

server.xml 

<Server ...

  <Service ...

    <Engine ...

      <Host ...

        <Cluster className="org.apache.catalina.ha.tcp.SimpleTcpCluster">
          <Channel className="org.apache.catalina.tribes.group.GroupChannel">
            <Membership className="org.apache.catalina.tribes.membership.cloud.CloudMembershipService"
                membershipProviderClassName="org.apache.catalina.tribes.membership.cloud.DNSMembershipProvider"/>
          </Channel>
        </Cluster>
        ...
minimal example for the Service my-tomcat-app-membership, note the selector
dns-membership-service.yml 

apiVersion: v1
kind: Service
metadata:
  annotations:
    service.alpha.kubernetes.io/tolerate-unready-endpoints: "true"
    description: "The service for tomcat cluster membership."
  name: my-tomcat-app-membership
spec:
  clusterIP: None
  selector:
    app: my-tomcat-app
First Tomcat pod minimal example, note the labels that must correspond to the selector in the service.
tomcat1.yml 

apiVersion: v1
kind: Pod
metadata:
  name: tomcat1
  labels:
    app: my-tomcat-app
spec:
  containers:
  - name: tomcat
    image: tomcat
    ports:
    - containerPort: 8080
Environment variable configuration
DNS_MEMBERSHIP_SERVICE_NAME=my-tomcat-app-membership

  • Constructor Details

    • DNSMembershipProvider

      public DNSMembershipProvider()
      Construct a new DNSMembershipProvider.

  • Method Details

    • start

      public void start(int level) throws Exception
      Description copied from interface: MembershipProvider
      Start the membership provider.
      Specified by:
      start in interface MembershipProvider
      Overrides:
      start in class CloudMembershipProvider
      Parameters:
      level - the readiness level
      • Channel.DEFAULT - will start all services
      • Channel.MBR_RX_SEQ - starts the membership receiver
      • Channel.MBR_TX_SEQ - starts the membership broadcaster
      Throws:
      Exception - if an error occurs

    • stop

      public boolean stop(int level) throws Exception
      Description copied from interface: MembershipProvider
      Stop the membership provider.
      Specified by:
      stop in interface MembershipProvider
      Overrides:
      stop in class CloudMembershipProvider
      Parameters:
      level - the readiness level
      • Channel.DEFAULT - will stop all services
      • Channel.MBR_RX_SEQ - stops the membership receiver
      • Channel.MBR_TX_SEQ - stops the membership broadcaster
      Returns:
      true if successful
      Throws:
      Exception - if an error occurs

    • fetchMembers

      protected Member[] fetchMembers()
      Description copied from class: CloudMembershipProvider
      Fetch current cluster members from the cloud orchestration.
      Specified by:
      fetchMembers in class CloudMembershipProvider
      Returns:
      the member array

    • accept

      public boolean accept(Serializable msg, Member sender)
      Description copied from interface: ChannelListener
      Invoked by the channel to determine if the listener will process this message or not.
      Specified by:
      accept in interface ChannelListener
      Overrides:
      accept in class CloudMembershipProvider
      Parameters:
      msg - Serializable
      sender - Member
      Returns:
      boolean