Hi all,
MJS6
Actually yes, the single-partition mode will behave the same as
multi-partition mode with one single partition for all topics
So yes, the TopologyTestDriver constructors could be deprecated / removed
in the future.
MJS7:
Good catch indeed.
We can use -1 as default the default value to clearly indicate that no
explicit partition was set, making the routing strategy unambiguous
Concretely:
- Constructors without explicit partition should default to -1.
- In pipeInput:
- if partition == -1: delegate to TopologyTestDriverPartitioner
- if partition >= 0: use it directly (with range validation)
equals() / hashCode() methods include the partition field:
- existing code will now also produce -1 when no partition is set
explicitly
- backward compatibility for equality checks is preserved
Finally, -1 never appears in output records (readRecordsToList()), since
those always carry the resolved partition, it only exists as an input
sentinel value indicating “no explicit partition”.
I'll update the KIP accordingly.
thanks
Le jeu. 4 juin 2026 à 21:08, Matthias J. Sax <[email protected]> a écrit :
> Thanks for the update.
>
>
> Few more questions:
>
> MJS6: Would it be simpler to (in the long run) drop single-partition
> mode all together? We would only keep it now for backward compatibility
> if `new TopologyTestDriver()` is used, but would always use
> multi-partition mode when then new builder is used?
>
> It seems to add complexity to keep single-partition mode for the builder
> case?
>
> If we say, the new builder always does multi-partition mode, and we
> deprecate/remove `new TopologyTestDriver()` later, single-partition mode
> would get removed, too.
>
> Or is there something specific about single-partition mode that makes it
> worth keeping for the new builder case? I would believe, with the only
> exception that multi-partition mode does create more tasks, if all
> topics are defined with a single partition, the record piping would be
> the same as in single-partition mode? Or is there a difference?
>
>
>
> MJS7:
> > Update equals() and hashCode() to include the partition field. For
> records created via existing constructors, partition defaults to 0,
> preserving backward compatibility for existing assertEquals calls.
>
> Should it default to `-1` (or `Optional.empty()`) instead of `0`? Making
> the default `0` seems to not work with "multi-partition mode" but break
> it -- as it would skip the hashing/partitioning, as `0` would be set
> explicitly?
>
> And for `-1` as default and topics with a single partition, `-1` will
> get hashed/mapped to partition-0 correctly.
>
>
>
>
> -Matthias
>
>
> On 6/4/26 5:47 AM, Sébastien Viale wrote:
> > Hi,
> >
> > Thanks for the feedback and suggestions.
> >
> >
> > MJS1:
> >
> > I fixed the typo `woraround` => workaround`
> >
> >
> > MJS2:
> >
> > I fixed the inconsistency:
> >
> > - removed duplicate entry for getKeyValueStore
> > - added missing *WithHeaders variants for state store APIs
> >
> >
> > MJS3:
> >
> > The existing usages of TopologyTestDriver remain fully unchanged. In
> > particular, current code relying on the default behavior (i.e., no
> explicit
> > topic declaration) will continue to work as before.
> >
> > Topic declaration is only required when a test explicitly opts into
> > multi-partition behavior. Otherwise, topics continue to be created
> > implicitly with the current semantics.
> >
> > We will introduce a TopologyTestDriverBuilder class:
> >
> > public class TopologyTestDriverBuilder {
> > public TopologyTestDriverBuilder(Topology topology);
> > public TopologyTestDriverBuilder withConfig(Properties config);
> > public TopologyTestDriverBuilder withInitialWallClockTime(Instant
> > initialWallClockTime);
> > public TopologyTestDriverBuilder declareTopic(String topicName,
> > int partitions);
> > public TopologyTestDriver build();
> > }
> >
> > In this model, multi-partition mode is implicitly enabled when at least
> one
> > topic is declared with a partition count greater than 1.
> >
> > As a result, the explicit withMultiPartitionMode() is no longer needed.
> >
> > The existing TopologyTestDriver constructors remain unchanged for
> backward
> > compatibility.
> >
> > If it is ok for you all, deprecation of those constructors is
> intentionally
> > deferred to a follow-up KIP to keep the scope of this change focused.
> >
> >
> > MJS4:
> >
> > For this KIP, we wanted initially to make the partitioning behavior
> > deterministic and test-controlled.
> >
> > Supporting partitioner.class would indeed be a natural extension, and
> > aligns well with production behavior. We propose deferring this to a
> > follow-up KIP to keep this one focused
> >
> > However, we will implement the internal routing logic with an internal
> > TopologyTestDriverPartitioner that implements Partitioner. Which would
> > provide a natural extension point for future support of custom
> partitioners.
> >
> >
> > MJS5:
> > -
> >
> > Your proposal seems like the best compromise:
> > - - TestRecord.equals() and TestRecord.hashCode() will be updated to
> > include the partition field.
> > - For existing code using the current constructors (without a partition
> > argument), the partition defaults to 0, preserving backward compatibility
> > for existing assertEquals() assertions.
> >
> > - - A new utility method TestRecord.equalsIgnorePartition(TestRecord
> > other) will
> > be added for cases where the test author does not want to assert on
> > partition placement.
> > - This allows use with
> assertTrue(expected.equalsIgnorePartition(actual)) when
> > partition is irrelevant to the test.
> > -
> > - The KIP has been updated.
> > -
> > - Cheers
> >
> >
> > Le mer. 3 juin 2026 à 22:56, Matthias J. Sax <[email protected]> a écrit
> :
> >
> >> Thanks for the great discussion and KIP refinements.
> >>
> >> Couple of comments:
> >>
> >>
> >> MJS1: (nit) there is a typo `woraround`
> >>
> >>
> >> MJS2: the KIP lists `getKeyValueStore` twice, and it's missing the newly
> >> added `*WithHeader` variants.
> >>
> >>
> >> MJS3: About the builder pattern. In general I like it, but it seems the
> >> applied patter is a little bit odd, and also not backward compatible (or
> >> maybe I misunderstand the proposal)?
> >>
> >> Existing code, would only call `new TopologyTestDriver()`, and it seems
> >> this code would break, and would need to get updated to add the
> >> dedicated setup phase? W/o declaring the topics explicitly, it seems
> >> later calls to `createInputTopic` and `createOutputTopic` would fail (at
> >> least on what the KIP says I believe; could also be a phrasing problem
> >> only)?
> >>
> >> I also think, that we would need to have some "builder helper class".
> >> Adding the methods to `TopologyTestDriver` can leave the object in an
> >> weird state. A classic builder pattern would suggest, that we add
> >>
> >> public class TTDBuilder { // or similar/better name
> >>
> >> public TTDBuilder(Topology topology);
> >>
> >> TTDBuilder withConfig(Properties config);
> >> TTDBuilder withInitialWallClockTime(Instant initialWallClockTime);
> >>
> >> TTDBuilder declareTopic(String topicName, int partitions);
> >>
> >> TopologyTestDriver build();
> >> }
> >>
> >> For this case, I don't think that we even need
> `withMultiPartitionMode()`?
> >>
> >> We would also deprecate the existing constructors `new
> >> TopologyTestDriver`, which we still need to support thought. For `new
> >> TopologyTestDriver()` we would just accept all topic names for
> >> `createInputTopic` and `createOutputTopic` and create these topic with a
> >> single partition. For this case, we would also still allow to use the
> >> existing `getStateStore()` methods, which we might also want to
> deprecate?
> >>
> >> (Maybe this is already proposed, but the KIP itself is not 100% clear
> >> about it -- at least to me.)
> >>
> >>
> >>
> >> MJS4: about partitioning; if the passed `Properties` configure
> >> `partitioner.class`, I think we could just use it allowing to plugin a
> >> global customer partitioner? Could also be a follow up KIP (maybe
> >> including the ability to define a customer partitioner per input topic).
> >>
> >> In general, I am wondering if we should implement the discussed
> >> partition-routing strategy just as an internal `TDDPartitioner extends
> >> Partitioner` (of course, this is an implementation detail). This would
> >> setup the code nicely to support `partitioner.class` config.
> >>
> >>
> >>
> >> MJS5: About testing and the new `partition` field on `TestRecord`. I
> >> think that both `equals()` and `hashCode()` should be considered. For
> >> existing code, all `TestRecord`s get set partition=0 and thus it's
> >> backward compatible. For new code, checking the expected partition sound
> >> desirable?
> >>
> >> I still believe that for some tests, the partition is not necessary. For
> >> this case, we could add `TestRecord.equalsIgnorePartition(TestRecord)`,
> >> and users can use `assertTrue()` for this case?
> >>
> >>
> >>
> >>
> >> -Matthias
> >>
> >>
> >>
> >>
> >> On 6/1/26 10:46 AM, Sébastien Viale wrote:
> >>> Thanks for your response.
> >>>
> >>> BB1:
> >>> I have removed the contradictory statement.
> >>>
> >>> LB5:
> >>> OK, I will keep the KIP as it is.
> >>>
> >>> I will call for the vote
> >>>
> >>> Cheers
> >>>
> >>> Le lun. 1 juin 2026 à 16:33, Bill Bejeck <[email protected]> a écrit
> :
> >>>
> >>>> Hi Sebastian,
> >>>>
> >>>> Thanks for the KIP! This will be a great, much needed addition.
> >>>> I realize I'm a bit late to the discussion. Overall things look good
> >> to me
> >>>> and I have one nit comment. I also think the KIP is ready for a vote.
> >>>>
> >>>> Regarding the treatment of null keys I agree with Lucas's comment for
> >> the
> >>>> treatment of null keys (round-robin) distribution.
> >>>> In "Partition Routing" section point 2 seems to reflection this
> change
> >> but
> >>>> then then it concludes with "a null key goes to a default partition (0
> >> in
> >>>> the test driver)" . I find this a little contradictory, can we either
> >>>> clarify or remove the statement?
> >>>>
> >>>> Thanks,
> >>>> Bill
> >>>>
> >>>> On Mon, Jun 1, 2026 at 8:33 AM Lucas Brutschy via dev <
> >>>> [email protected]>
> >>>> wrote:
> >>>>
> >>>>> Hi Sebastian,
> >>>>>
> >>>>> Thanks for the response!
> >>>>>
> >>>>> LB5: I was thinking of this solution but I think it introduces more
> >>>>> problems than it solves. Long-term, I feel it would be a footgun if
> >>>>> people try to compare test records _with_ partitions. And the current
> >>>>> implementation with a reasonable default partition should already be
> >>>>> backward compatible. There are other possible solutions, like a
> custom
> >>>>> assertion or an operation to "wipe" the partition, but they are
> >>>>> probably not worth the effort. As I mentioned before, I don't see a
> >>>>> better solution to this -- I would just keep the KIP as it is. I
> >>>>> mostly included it in the discussion to make people aware and maybe
> >>>>> spark an idea in someone.
> >>>>>
> >>>>> Cheers,
> >>>>> Lucas
> >>>>>
> >>>>
> >>>
> >>
> >>
> >
>
>
