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https://issues.apache.org/jira/browse/CASSANDRA-6704?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13905500#comment-13905500
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Edward Capriolo commented on CASSANDRA-6704:
--------------------------------------------
Recent changes:
{code}
struct ScannerCreateDesc {
1:required string cfname,
2:required string filter_name,
3:required binary key,
4:optional binary start_column,
5:optional binary end_column,
6:required i32 slice_size,
7:optional ConsistencyLevel consistency_level=ConsistencyLevel.ONE,
8:optional map<string,binary> params
}
{code}
{code}
public interface ScanFilter {
public FilterReturn filter(ColumnOrSuperColumn col, ScannerState state);
}
{code}
Adding the params allows us to create more generic scanners. Before the scanner
Limit3 was implemented. However now we can do things like LIMIT X
{code}
@Override
public FilterReturn filter(ColumnOrSuperColumn col, ScannerState state) {
state.getFiltered().add(col);
return state.getFiltered().size()<
ByteBufferUtil.toInt(state.getParams().get("limit"))
? FilterReturn.FILTER_MORE : FilterReturn.FILTER_DONE;
}
{code}
Also thinking more broadly scanners could work from CQL, Hive has a feature
called UDTF (https://issues.apache.org/jira/browse/HIVE-1614) that takes in
zero to many columns and produces zero to many rows with one to many columns.
this roughly equates to the Scanner interface I am working on. Session level
tracking with need to record the position in the row so that a second
disconnected query can pick up where the first left off. I will draft this up
later.
> Create wide row scanners
> ------------------------
>
> Key: CASSANDRA-6704
> URL: https://issues.apache.org/jira/browse/CASSANDRA-6704
> Project: Cassandra
> Issue Type: New Feature
> Reporter: Edward Capriolo
> Assignee: Edward Capriolo
>
> The BigTable white paper demonstrates the use of scanners to iterate over
> rows and columns.
> http://static.googleusercontent.com/media/research.google.com/en/us/archive/bigtable-osdi06.pdf
> Because Cassandra does not have a primary sorting on row keys scanning over
> ranges of row keys is less useful.
> However we can use the scanner concept to operate on wide rows. For example
> many times a user wishes to do some custom processing inside a row and does
> not wish to carry the data across the network to do this processing.
> I have already implemented thrift methods to compile dynamic groovy code into
> Filters as well as some code that uses a Filter to page through and process
> data on the server side.
> https://github.com/edwardcapriolo/cassandra/compare/apache:trunk...trunk
> The following is a working code snippet.
> {code}
> @Test
> public void test_scanner() throws Exception
> {
> ColumnParent cp = new ColumnParent();
> cp.setColumn_family("Standard1");
> ByteBuffer key = ByteBuffer.wrap("rscannerkey".getBytes());
> for (char a='a'; a < 'g'; a++){
> Column c1 = new Column();
> c1.setName((a+"").getBytes());
> c1.setValue(new byte [0]);
> c1.setTimestamp(System.nanoTime());
> server.insert(key, cp, c1, ConsistencyLevel.ONE);
> }
>
> FilterDesc d = new FilterDesc();
> d.setSpec("GROOVY_CLASS_LOADER");
> d.setName("limit3");
> d.setCode("import org.apache.cassandra.dht.* \n" +
> "import org.apache.cassandra.thrift.* \n" +
> "public class Limit3 implements SFilter { \n " +
> "public FilterReturn filter(ColumnOrSuperColumn col,
> List<ColumnOrSuperColumn> filtered) {\n"+
> " filtered.add(col);\n"+
> " return filtered.size()< 3 ? FilterReturn.FILTER_MORE :
> FilterReturn.FILTER_DONE;\n"+
> "} \n" +
> "}\n");
> server.create_filter(d);
>
>
> ScannerResult res = server.create_scanner("Standard1", "limit3", key,
> ByteBuffer.wrap("a".getBytes()));
> Assert.assertEquals(3, res.results.size());
> }
> {code}
> I am going to be working on this code over the next few weeks but I wanted to
> get the concept our early so the design can see some criticism.
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