Ouch. That can be a painful discovery after a client upgrade. It can
break a lot of app code.
I can see the reason for custom hash algorithm (lots of db products do
this, usually for stability, but sometimes for other hash properties
(Oracle has some cool guarantees around modifying number of partitions
and data movement)).
I'm wondering if, in the interest of painless upgrades, we should add
a configuration flag for topics - old.hash.algorithm that will keep
existing behavior. Sounds like a rather ugly hack (and things can
still break in new versions of Java), but I can't see a better
alternative at the moment.
Gwen
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
>
