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Accumulo >> mail # user >> WholeRowIterator, BatchScanner, and fetchColumnFamily don't play well together?


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Re: WholeRowIterator, BatchScanner, and fetchColumnFamily don't play well together?
John,

This was a fun one, but we figured it out. Thanks for providing code --
that helped a lot. The quick workaround is to set the priority of the
WholeRowIterator to 21, above the VersioningIterator. Turns out the two
iterators are not commutative, so order matters.

Solution: when you set up your WholeRowIterator, use priority 21 or greater:
  batchScanner.setScanIterators(21 /* voila */,
WholeRowIterator.class.getName(), UUID.randomUUID().toString());

Here's what's happening:

First, a little background on notation. When you scan for a range including
the row "bar", that range can be notated (bear with me):
  [bar : [] 9223372036854775807 false,bar%00; : [] 9223372036854775807
false)
This shows two complete keys, separated by a comma, with empty column
family, column qualifier, column visibility, a MAX_LONG timestamp, and
delete flag set to false. The second key has a row that is the same as the
first, but with a 0 byte value added on (%00;). In this notation, the [
means the left key is inclusive, and the ) means that the right key is
exclusive.

In your query, you added a column family filter, so we (the Accumulo client
library) got tricky and narrowed your range in addition to doing filtering.
Here's what the narrowed range looks like:
  [bar foo: [] 9223372036854775807 false,bar foo%00;: []
9223372036854775807 false)
You can see the column family foo specified on the left, and foo%00;
specified on the right side. This will select only everything in the foo
column family within the row bar.

When the VersioningIterator seeks to a range it does a couple of
interesting things. First, it widens the range to include all of the
possible versions of a key by setting the left-hand side timestamp to
MAX_LONG. This is done to get an accurate count of the versions so that it
knows which versions to skip. Second, it scans through the versions,
skipping anything after the start of the range it was given. This way, you
can seek directly to the nth version of a key and maintain a consistent
last version. Skipping keys that don't fit in the range works great until
we throw in an iterator that transforms keys, modifying their columns.

Enter the WholeRowIterator. The WholeRowIterator groups and encodes all
key/value pairs in a row into a single key/value pair to guarantee
isolation. This new key looks like:
  bar : [] 9223372036854775807 false
Effectively, we're taking the key in column family "foo" and moving it to
column family "". This breaks the second interesting behavior of the
VersioningIterator, which will skip over everything that is not in the
narrowed range (including this key).

So, the conflict is actually the confluence of the WholeRowIterator, the
VersioningIterator, and setting to a single row range with a column filter
(resulting in a range that is narrower than one row). This is also not
specific to the BatchScanner. If you set the range of your Scanner to (new
Range(new Text("bar"))), just like the BatchScanner, the Scanner will
display the same behavior.

Cheers,
Adam
On Mon, Jul 9, 2012 at 12:42 PM, John Armstrong <[EMAIL PROTECTED]> wrote:

> Hi everybody.
>
> I've run across an unexpected behavior when using WholeRowIterator on a
> BatchScanner.  In case it matters, we're using cloudbase-1.3.4.
>
> When I tell it to fetchColumnFamily(new Text("foo")) I get no results
> back, though there are definitely records in that column family and in the
> row ranges I'm scanning.  This doesn't happen when I use a scanner on that
> column family, though in that case I'm scanning over the entire table.
>
> To be more explicit, some constants:
>
> List<Range> ranges = new ArrayList<Range>();
> ranges.add(new Range(new Text("bar")));
> Text CF = new Text("foo");
>
> getNewScanner() and getNewBatchScanner() create scanners for the
> appropriate table name, authorization, and number of threads.
>
> BatchScanner batchScanner = getNewBatchScanner();
> batchScanner.**fetchColumnFamily(CF);
> batchScanner.setRanges(ranges)**;
>
> returns all the entries in row "bar" and column family "foo".