Change default KafkaEx partitioner to correctly match the Java client

[mjparrott]

One way of fixing #398

[dantswain]

This makes sense to me. @barthez did the original murmur implementation - do you have any sense if the original may have just been a typo?

I am a little concerned about breaking implementation for anyone who is already using the existing algorithm. Maybe we can provide a legacy partitioner?

If nothing else @mjparrott you'll need to update the test cases, which should be relatively straightforward.

[kennethito]

I think providing a legacy partitioner and some breaking change documentation is a great idea.

[joshuawscott]

I thought these tests had been copied from the Java tests, but it doesn't look like that's actually the case. Perhaps we should remove these test cases, and use the same ones that the Java implementation uses:
https://github.com/apache/kafka/blob/8ab0994919752cd4870e771221ba934a6a539a67/clients/src/test/java/org/apache/kafka/common/utils/UtilsTest.java#L66-L78

    @Test
    public void testMurmur2() {
        Map<byte[], Integer> cases = new java.util.HashMap<>();
        cases.put("21".getBytes(), -973932308);
        cases.put("foobar".getBytes(), -790332482);
        cases.put("a-little-bit-long-string".getBytes(), -985981536);
        cases.put("a-little-bit-longer-string".getBytes(), -1486304829);
        cases.put("lkjh234lh9fiuh90y23oiuhsafujhadof229phr9h19h89h8".getBytes(), -58897971);
        cases.put(new byte[]{'a', 'b', 'c'}, 479470107);

        for (Map.Entry<byte[], Integer> c : cases.entrySet()) {
            assertEquals(c.getValue().intValue(), murmur2(c.getKey()));
        }
    }

[jbruggem]

I am a little concerned about breaking implementation for anyone who is already using the existing algorithm. Maybe we can provide a legacy partitioner?

Indeed, it's touchy. With kayrock we'll have a major release, so having a breaking change is OK, but we need:

• clear documentation on the matter, in the documentation itself and in the release notes
• a way to activate the old behaviour for those in need of it

[joshuawscott]

Breaking changes are allowed in semver before 1.0, so I have no issue with breaking this. This is a bug in any case, the intention and documentation is that the partitioning matches the java client, so if it isn't doing that now, it's broken, and changing it to match is fine, IMO

[mjparrott]

I added a LegacyPartitioner module which can be used in place of the DefaultPartitioner which implements the old behaviour. This modules are mostly just a copy-paste of each other. The only difference is the umurmur2 function which is called.

It looks like the one of the builds is failing due to the code copy-paste - any thoughts?

[joshuawscott]

I'm ok if you put in a comment to disable that check in credo for that line or function
http://rrrene.org/2017/06/01/credo-config-comments/