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From "Benedict (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-6704) Create wide row scanners
Date Sat, 15 Feb 2014 13:51:20 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-6704?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13902405#comment-13902405
] 

Benedict commented on CASSANDRA-6704:
-------------------------------------

bq.  Essentially the same thing, user code running inside cassandra.

Sort of, but there are some important differences: 1) as Brandon says, the code is clearly
vetted by the database dev team deploying triggers, which can't be said here; and 2) we're
all Java experts here, and the execution context is the normal execution context of Cassandra,
which again we're all familiar with. Helping users with issues from dynamic class compilation
/ loading of languages we don't understand is quite a different matter IMO, especially once
sandboxing is introduced (which really would be essential as C*'s internal APIs are *not*
safe to be accessed, nor protected, and could be used dangerously). It's not clear to me this
will be pain free from our side to ensure it always works, either.

Also, with triggers we can more easily justify API breakages across minor/major versions that
require some work when upgrading, as they're well contained within their Cassandra deployment,
however if we expose internal APIs to client code we will necessarily see more pushback on
rapid development of these APIs, as the difficulty for users to migrate will be increased.

bq. The language that you chose to implement the filter with is your call.

This only seems to make my issue (1) worse, to my eyes

bq. think about all the cql iterations like cql2 , execute_cql, execute_cql_3. Set keyspace
set _consistency level.

Well, these things are all still present. We may retire CQL2 soon, but that has the advantage
of having very quickly been superseded by CQL3, which to my knowledge does not have dramatically
different syntax anyway - and yet it still has stuck around. It's not yet clear what this
would be superseded by, or if the functionality would map easily, and maintaining a deprecated
access method doesn't reduce the support burden.

I think it's a pretty nice underlying goal, but it's a really heavyweight feature that needs
to be approached cautiously, and as Sylvain says, preferably coherently.

I do wonder if it mightn't be possible to offer this as an easy to apply patch in the meantime,
outside of the main Apache repository. There are definitely some users that would be happy
with the security risks and would love this to play with, but those people are power users
who would be comfortable applying a simple patch to their C* instance, and would not contribute
excessively to the support burden as they'd be competent enough to figure out any issues they
have.

Just my 2c, anyway.

> 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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