This was actually intentional.
The problem with relying on hashCode is that
(1) it is often a very bad hash function,
(2) it is not guaranteed to be consistent from run to run (i.e. if you
restart the jvm the value of hashing the same key can change!),
(3) it is not available outside the jvm so non-java producers can't use the
same function.
In general at the moment different producers don't use the same hash code
so I think this is not quite as bad as it sounds. Though it would be good
to standardize things.
I think the most obvious thing we could do here would be to do a much
better job of advertising this in the docs, though, so people don't get
bitten by it.
-Jay
On Fri, Apr 24, 2015 at 5:48 PM, James Cheng <jch...@tivo.com> wrote:
> Hi,
>
> I was playing with the new producer in 0.8.2.1 using partition keys
> ("semantic partitioning" I believe is the phrase?). I noticed that the
> default partitioner in 0.8.2.1 does not partition items the same way as the
> old 0.8.1.1 default partitioner was doing. For a test item, the old
> producer was sending it to partition 0, whereas the new producer was
> sending it to partition 4.
>
> Digging in the code, it appears that the partitioning logic is different
> between the old and new producers. Both of them hash the key, but they use
> different hashing algorithms.
>
> Old partitioner:
> ./core/src/main/scala/kafka/producer/DefaultPartitioner.scala:
>
> def partition(key: Any, numPartitions: Int): Int = {
> Utils.abs(key.hashCode) % numPartitions
> }
>
> New partitioner:
>
> ./clients/src/main/java/org/apache/kafka/clients/producer/internals/Partitioner.java:
>
> } else {
> // hash the key to choose a partition
> return Utils.abs(Utils.murmur2(record.key())) % numPartitions;
> }
>
> Where murmur2 is a custom hashing algorithm. (I'm assuming that murmur2
> isn't the same logic as hashCode, especially since hashCode is
> overrideable).
>
> Was it intentional that the hashing algorithm would change between the old
> and new producer? If so, was this documented? I don't know if anyone was
> relying on the old default partitioner, as opposed to going round-robin or
> using their own custom partitioner. Do you expect it to change in the
> future? I'm guessing that one of the main reasons to have a custom hashing
> algorithm is so that you are full control of the partitioning and can keep
> it stable (as opposed to being reliant on hashCode()).
>
> Thanks,
> -James
>
>
