private List<CNSSubscription> listSubscriptions(
      String nextToken, CnsSubscriptionProtocol protocol, String userId, boolean hidePendingArn)
      throws Exception {

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

    // Algorithm is to keep reading in chunks of 100 till we've seen 500 from the nextToken-ARN
    List<CNSSubscription> l = new ArrayList<CNSSubscription>();
    // read form index to get sub-arn

    while (l.size() < 100) {

      CmbColumnSlice<String, String> slice =
          cassandraHandler.readColumnSlice(
              AbstractDurablePersistence.CNS_KEYSPACE,
              columnFamilySubscriptionsUserIndex,
              userId,
              nextToken,
              null,
              500,
              CMB_SERIALIZER.STRING_SERIALIZER,
              CMB_SERIALIZER.STRING_SERIALIZER,
              CMB_SERIALIZER.STRING_SERIALIZER);

      if (slice == null) {
        return l;
      }

      // get Column from main table
      List<CmbColumn<String, String>> cols = slice.getColumns();

      if (nextToken != null) {
        cols.remove(0);
      }

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

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

        String subArn = col.getName();
        CNSSubscription subscription = getSubscription(subArn);

        if (subscription == null) {
          throw new IllegalStateException(
              "Subscriptions-user-index contains subscription-arn which doesn't exist in subscriptions-index. subArn:"
                  + subArn);
        }

        // ignore invalid subscriptions coming from Cassandra

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

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

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

        if (l.size() == 100) {
          return l;
        }
      }

      nextToken = cols.get(cols.size() - 1).getName();
    }

    return l;
  }
  /**
   * 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;
  }
  @Override
  public CNSSubscription subscribe(
      String endpoint, CnsSubscriptionProtocol protocol, String topicArn, String userId)
      throws Exception {

    // subscription is unique by protocol + endpoint + topic

    final CNSSubscription subscription = new CNSSubscription(endpoint, protocol, topicArn, userId);

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

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

    // check if queue exists for cqs endpoints

    if (protocol.equals(CnsSubscriptionProtocol.cqs)) {

      CQSQueue queue =
          PersistenceFactory.getQueuePersistence()
              .getQueue(com.comcast.cqs.util.Util.getRelativeQueueUrlForArn(endpoint));

      if (queue == null) {
        throw new CMBException(
            CMBErrorCodes.NotFound, "Queue with arn " + endpoint + " does not exist.");
      }
    }

    subscription.setArn(Util.generateCnsTopicSubscriptionArn(topicArn, protocol, endpoint));

    // attempt to delete existing subscription

    /*Composite superColumnName = new Composite(subscription.getEndpoint(), subscription.getProtocol().name());

    HSuperColumn<Composite, String, String> superCol = readColumnFromSuperColumnFamily(columnFamilySubscriptions, subscription.getTopicArn(), superColumnName, new StringSerializer(), new CompositeSerializer(), StringSerializer.get(), StringSerializer.get(), CMBProperties.getInstance().getReadConsistencyLevel());

    if (superCol != null) {
    	CNSSubscription exisitingSub = extractSubscriptionFromSuperColumn(superCol, topicArn);
              deleteIndexes(exisitingSub.getArn(), exisitingSub.getUserId(), exisitingSub.getToken());
    	deleteSuperColumn(subscriptionsTemplate, exisitingSub.getTopicArn(), superColumnName);
    }*/

    // then set confirmation stuff and update cassandra

    CNSSubscription retrievedSubscription = getSubscription(subscription.getArn());

    if (!CMBProperties.getInstance().getCNSRequireSubscriptionConfirmation()) {

      subscription.setConfirmed(true);
      subscription.setConfirmDate(new Date());

      insertOrUpdateSubsAndIndexes(subscription, null);

      if (retrievedSubscription == null) {
        cassandraHandler.incrementCounter(
            AbstractDurablePersistence.CNS_KEYSPACE,
            columnFamilyTopicStats,
            subscription.getTopicArn(),
            "subscriptionConfirmed",
            1,
            CMB_SERIALIZER.STRING_SERIALIZER,
            CMB_SERIALIZER.STRING_SERIALIZER);
      }

    } else {

      // protocols that cannot confirm subscriptions (e.g. redisPubSub)
      // get an automatic confirmation here
      if (!protocol.canConfirmSubscription()) {
        subscription.setConfirmed(true);
        subscription.setConfirmDate(new Date());
        insertOrUpdateSubsAndIndexes(subscription, null);

        // auto confirm subscription to cqs queue by owner
      } else if (protocol.equals(CnsSubscriptionProtocol.cqs)) {

        String queueOwner = com.comcast.cqs.util.Util.getQueueOwnerFromArn(endpoint);

        if (queueOwner != null && queueOwner.equals(userId)) {

          subscription.setConfirmed(true);
          subscription.setConfirmDate(new Date());

          insertOrUpdateSubsAndIndexes(subscription, null);
          if (retrievedSubscription == null) {
            cassandraHandler.incrementCounter(
                AbstractDurablePersistence.CNS_KEYSPACE,
                columnFamilyTopicStats,
                subscription.getTopicArn(),
                "subscriptionConfirmed",
                1,
                CMB_SERIALIZER.STRING_SERIALIZER,
                CMB_SERIALIZER.STRING_SERIALIZER);
          }
        } else {

          // use cassandra ttl to implement expiration after 3 days
          insertOrUpdateSubsAndIndexes(subscription, 3 * 24 * 60 * 60);
          if (retrievedSubscription == null) {
            cassandraHandler.incrementCounter(
                AbstractDurablePersistence.CNS_KEYSPACE,
                columnFamilyTopicStats,
                subscription.getTopicArn(),
                "subscriptionPending",
                1,
                CMB_SERIALIZER.STRING_SERIALIZER,
                CMB_SERIALIZER.STRING_SERIALIZER);
          }
        }

      } else {

        // use cassandra ttl to implement expiration after 3 days
        insertOrUpdateSubsAndIndexes(subscription, 3 * 24 * 60 * 60);
        if (retrievedSubscription == null) {
          cassandraHandler.incrementCounter(
              AbstractDurablePersistence.CNS_KEYSPACE,
              columnFamilyTopicStats,
              subscription.getTopicArn(),
              "subscriptionPending",
              1,
              CMB_SERIALIZER.STRING_SERIALIZER,
              CMB_SERIALIZER.STRING_SERIALIZER);
        }
      }
    }

    CNSSubscriptionAttributes attributes =
        new CNSSubscriptionAttributes(topicArn, subscription.getArn(), userId);
    PersistenceFactory.getCNSAttributePersistence()
        .setSubscriptionAttributes(attributes, subscription.getArn());

    return subscription;
  }