Without commenting on if this is a good idea, let me show how you can use Accumulo to store the seven values needed to perform a rolling average. Log into the shell. Create the table. Then configure iterators to retain seven values instead of just the default single value. Finally insert some values.

bin/accumulo shell -u root -p password
> createtable rolling
rolling> config -t rolling -s table.iterator.scan.vers.opt.maxVersions=7
rolling> config -t rolling -s table.iterator.minc.vers.opt.maxVersions=7
rolling> config -t rolling -s table.iterator.majc.vers.opt.maxVersions=7

rolling> insert 2012.02.20 "" "" 21
rolling> insert 2012.02.20 "" "" 22
rolling> insert 2012.02.20 "" "" 23
rolling> insert 2012.02.20 "" "" 24
rolling> insert 2012.02.20 "" "" 25
rolling> insert 2012.02.20 "" "" 26
rolling> insert 2012.02.20 "" "" 27
rolling> insert 2012.02.20 "" "" 28
rolling> insert 2012.02.20 "" "" 29
rolling> insert 2012.02.20 "" "" 30

rolling> insert 2012.02.21 "" "" 51
rolling> insert 2012.02.21 "" "" 52
rolling> insert 2012.02.21 "" "" 53
rolling> insert 2012.02.21 "" "" 54
rolling> insert 2012.02.21 "" "" 55
rolling> insert 2012.02.21 "" "" 56
rolling> insert 2012.02.21 "" "" 57
rolling> insert 2012.02.21 "" "" 58
rolling> insert 2012.02.21 "" "" 59
rolling> insert 2012.02.21 "" "" 60

You can use the 'scan' command to see all information in the table. Or you can use 'scan -b 2012.02.21 -e 2012.02.21' to see information about a single row id. You can use Java program to calculate the rolling average:

public class RollingAverageDriver {

 public static void main(String[] args) throws AccumuloException, AccumuloSecurityException, TableNotFoundException, TableExistsException {
  String instanceName = "development";
  String zooKeepers = "localhost";
  String user = "root";
  byte[] pass = "password".getBytes();
  String tableName = "rolling";

  ZooKeeperInstance instance = new ZooKeeperInstance(instanceName, zooKeepers);
  Connector connector = instance.getConnector(user, pass);

  Scanner scanner = connector.createScanner(tableName, new Authorizations());

  RollingAverageCalculator raCalculator = new RollingAverageCalculator(scanner, 7);
  int rollingAverage = raCalculator.calculate("2012.02.21");

  System.out.println("7 Day Rolling Average: " + rollingAverage);


  System.out.println("END");
 }
}

Of course, you'll also need the RollingAverageCalculator class:

public class RollingAverageCalculator {

 Scanner scanner = null;
 int minNumberOfValues = 0;

 public RollingAverageCalculator(Scanner scanner, int minNumberOfValues) {
  super();
  this.scanner = scanner;
  this.minNumberOfValues = minNumberOfValues;
 }

 public int calculate(final String rowId) {
  scanner.setRange(new Range(rowId, rowId));

  int sum = 0;
  int count = 0;
  Iterator<Map.Entry<Key,Value>> iterator = scanner.iterator();
  while (iterator.hasNext()) {
   Map.Entry<Key,Value> entry = iterator.next();
   Value value = entry.getValue();
   String sValue = new String(value.get());
   sum += Integer.parseInt(sValue);
   count++;
  }
  return count < minNumberOfValues ? 0 : (sum / count);
 }
}

It should be fairly straightforward to change the code to perform any kind of rolling average.