/**
   * Create a single {@link com.rackspacecloud.blueflood.service.SingleRollupWriteContext} from the
   * given {@link com.rackspacecloud.blueflood.types.IMetric} and Granularity.
   *
   * @param destGran
   * @param metric
   * @return
   * @throws IOException
   */
  protected SingleRollupWriteContext createSingleRollupWriteContext(
      Granularity destGran, IMetric metric) throws IOException {

    Locator locator = metric.getLocator();

    Points<SimpleNumber> points = new Points<SimpleNumber>();
    points.add(
        new Points.Point<SimpleNumber>(
            metric.getCollectionTime(), new SimpleNumber(metric.getMetricValue())));

    BasicRollup rollup = BasicRollup.buildRollupFromRawSamples(points);

    return new SingleRollupWriteContext(
        rollup,
        locator,
        destGran,
        CassandraModel.getBasicColumnFamily(destGran),
        metric.getCollectionTime());
  }
  /**
   * For a given list of metrics, figure out the shard and slot they belong to and for those shard
   * and slot combinations, get all the locators from metrics_delayed_locator column family.
   *
   * @param metrics
   * @return
   * @throws IOException
   */
  protected Set<Locator> retrieveLocatorsByShardAndSlot(List<IMetric> metrics) throws IOException {
    Set<String> slotKeys = new HashSet<String>();

    for (IMetric metric : metrics) {
      int shard = Util.getShard(metric.getLocator().toString());
      int slot = DELAYED_METRICS_STORAGE_GRANULARITY.slot(metric.getCollectionTime());
      SlotKey slotKey = SlotKey.of(DELAYED_METRICS_STORAGE_GRANULARITY, slot, shard);

      slotKeys.add(slotKey.toString());
    }

    Set<Locator> locatorsFromDB = new HashSet<Locator>();
    for (String slotKeyStr : slotKeys) {
      locatorsFromDB.addAll(delayedLocatorIO.getLocators(SlotKey.parse(slotKeyStr)));
    }

    return locatorsFromDB;
  }