Hi David,
Absolutely. What you have here is a classic producer-consumer model.
Your BatchScanner is producing results, which you then consume by your
scanner, and ultimately return those results to the client.
The problem with your below implementation is that you're not going to
be polling your batchscanner as aggressively as you could be. You are
blocking while you can fetch each of those new Ranges from the Scanner
before fetching new ranges. Have you considered splitting up the
BatchScanner and Scanner code into two different threads?
You could easily use a ArrayBlockingQueue (or similar) to pass results
from the BatchScanner to the Scanner. I would imagine that this would
give you a fair improvement in performance.
Also, it doesn't appear that there's a reason you can't use a
BatchScanner for both lookups?
One final warning, your current implementation could also hog heap very
badly if your batchscanner returns too many records. The
producer/consumer I proposed should help here a little bit, but you
should still be asserting upper-bounds to avoid running out of heap
space in your client.
On 5/20/14, 1:10 PM, Slater, David M. wrote:
Hey everyone,
I'm trying to improve the query performance of batchscans on my data table. I
first scan over index tables, which returns a set of rowIDs that correspond to
the records I am interested in. This set of records is fairly randomly (and
uniformly) distributed across a large number of tablets, due to the randomness
of the UID and the query itself. Then I want to scan over my data table, which
is setup as follows:
row colFam colQual value
rowUID -- -- byte[] of data
These records are fairly small (100s of bytes), but numerous (I may return
50000 or more). The method I use to obtain this follows. Essentially, I turn
the rows returned from the first query into a set of ranges to input into the
batchscanner, and then return those rows, retrieving the value from them.
// returns the data associated with the given collection of rows
public Collection<byte[]> getRowData(Collection<Text> rows, Text dataType,
String tablename, int queryThreads) throws TableNotFoundException {
List<byte[]> values = new ArrayList<byte[]>(rows.size());
if (!rows.isEmpty()) {
BatchScanner scanner = conn.createBatchScanner(tablename, new
Authorizations(), queryThreads);
List<Range> ranges = new ArrayList<Range>();
for (Text row : rows) {
ranges.add(new Range(row));
}
scanner.setRanges(ranges);
for (Map.Entry<Key, Value> entry : scanner) {
values.add(entry.getValue().get());
}
scanner.close();
}
return values;
}
Is there a more efficient way to do this? I have index caches and bloom filters
enabled (data caches are not), but I still seem to have a long query lag. Any
thoughts on how I can improve this?
Thanks,
David
