@Override
  public CNSSubscription getSubscription(String arn) throws Exception {

    // read form index to get composite col-name

    CmbColumnSlice<String, String> slice =
        cassandraHandler.readColumnSlice(
            AbstractDurablePersistence.CNS_KEYSPACE,
            columnFamilySubscriptionsIndex,
            arn,
            null,
            null,
            1,
            CMB_SERIALIZER.STRING_SERIALIZER,
            CMB_SERIALIZER.STRING_SERIALIZER,
            CMB_SERIALIZER.STRING_SERIALIZER);

    if (slice != null) {

      // get Column from main table

      String colName = slice.getColumns().get(0).getName();
      CnsSubscriptionProtocol protocol = getEndpointAndProtoIndexValProtocol(colName);
      String endpoint = getEndpointAndProtoIndexValEndpoint(colName);
      CmbComposite columnName = cassandraHandler.getCmbComposite(endpoint, protocol.name());
      CmbColumn<CmbComposite, String> column =
          cassandraHandler.readColumn(
              AbstractDurablePersistence.CNS_KEYSPACE,
              columnFamilySubscriptions,
              Util.getCnsTopicArn(arn),
              columnName,
              CMB_SERIALIZER.STRING_SERIALIZER,
              CMB_SERIALIZER.COMPOSITE_SERIALIZER,
              CMB_SERIALIZER.STRING_SERIALIZER);

      if (column != null) {
        CNSSubscription s = extractSubscriptionFromColumn(column, Util.getCnsTopicArn(arn));
        s.checkIsValid();
        return s;
      }
    }

    return null;
  }
  /**
   * Enumerate all subs in a topic
   *
   * @param nextToken The ARN of the last sub-returned or null is first time call.
   * @param topicArn
   * @param protocol
   * @param pageSize
   * @param hidePendingArn
   * @return The list of subscriptions given a topic. Note: if nextToken is provided, the returned
   *     list will not contain it for convenience
   * @throws Exception
   */
  public List<CNSSubscription> listSubscriptionsByTopic(
      String nextToken,
      String topicArn,
      CnsSubscriptionProtocol protocol,
      int pageSize,
      boolean hidePendingArn)
      throws Exception {

    if (nextToken != null) {
      if (getSubscription(nextToken) == null) {
        throw new SubscriberNotFoundException("Subscriber not found for arn " + nextToken);
      }
    }

    // read from index to get composite-col-name corresponding to nextToken
    CmbComposite nextTokenComposite = null;

    if (nextToken != null) {
      CmbColumnSlice<String, String> slice =
          cassandraHandler.readColumnSlice(
              AbstractDurablePersistence.CNS_KEYSPACE,
              columnFamilySubscriptionsIndex,
              nextToken,
              null,
              null,
              1,
              CMB_SERIALIZER.STRING_SERIALIZER,
              CMB_SERIALIZER.STRING_SERIALIZER,
              CMB_SERIALIZER.STRING_SERIALIZER);
      if (slice == null) {
        throw new IllegalArgumentException("Could not find any subscription with arn " + nextToken);
      }
      // get Column from main table
      String colName = slice.getColumns().get(0).getName();
      CnsSubscriptionProtocol tokProtocol = getEndpointAndProtoIndexValProtocol(colName);
      String endpoint = getEndpointAndProtoIndexValEndpoint(colName);
      nextTokenComposite = cassandraHandler.getCmbComposite(endpoint, tokProtocol.name());
    }

    List<CNSSubscription> l = new ArrayList<CNSSubscription>();

    CNSTopic t = PersistenceFactory.getTopicPersistence().getTopic(topicArn);

    if (t == null) {
      throw new TopicNotFoundException("Resource not found.");
    }

    // read pageSize at a time

    CmbColumnSlice<CmbComposite, String> cols =
        cassandraHandler.readColumnSlice(
            AbstractDurablePersistence.CNS_KEYSPACE,
            columnFamilySubscriptions,
            topicArn,
            nextTokenComposite,
            null,
            pageSize,
            CMB_SERIALIZER.STRING_SERIALIZER,
            CMB_SERIALIZER.COMPOSITE_SERIALIZER,
            CMB_SERIALIZER.STRING_SERIALIZER);

    if (nextToken != null && cols.size() > 0) {
      cols.getColumns().remove(0);
    }

    while (l.size() < pageSize) {

      if (cols == null || cols.size() == 0) {
        return l;
      }

      for (CmbColumn<CmbComposite, String> col : cols.getColumns()) {

        CNSSubscription sub = extractSubscriptionFromColumn(col, topicArn);

        // ignore invalid subscriptions coming from Cassandra

        try {
          sub.checkIsValid();
        } catch (CMBException ex) {
          logger.error("event=invalid_subscription " + sub.toString(), ex);
          continue;
        }

        if (protocol != null && protocol != sub.getProtocol()) {
          continue;
        }

        if (hidePendingArn) {
          if (sub.isConfirmed()) {
            l.add(sub);
          } else {
            sub.setArn("PendingConfirmation");
            l.add(sub);
          }
        } else {
          l.add(sub);
        }

        if (l.size() == pageSize) {
          return l;
        }
      }

      nextTokenComposite = cols.getColumns().get(cols.size() - 1).getName();
      cols =
          cassandraHandler.readColumnSlice(
              AbstractDurablePersistence.CNS_KEYSPACE,
              columnFamilySubscriptions,
              topicArn,
              nextTokenComposite,
              null,
              pageSize,
              CMB_SERIALIZER.STRING_SERIALIZER,
              CMB_SERIALIZER.COMPOSITE_SERIALIZER,
              CMB_SERIALIZER.STRING_SERIALIZER);

      if (cols.size() > 0) {
        cols.getColumns().remove(0);
      }
    }

    return l;
  }
 private static String getEndpointAndProtoIndexVal(
     String endpoint, CnsSubscriptionProtocol protocol) {
   return protocol.name() + ":" + endpoint;
 }