Package org.apache.catalina.tribes.membership.cloud

Class 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 Detail

      • DNSMembershipProvider

        public DNSMembershipProvider()
        Construct a new DNSMembershipProvider.

    • Method Detail

      • start

        public void start​(int level)
           throws java.lang.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:
        java.lang.Exception - if an error occurs

      • stop

        public boolean stop​(int level)
             throws java.lang.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:
        java.lang.Exception - if an error occurs

      • accept

        public boolean accept​(java.io.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