Re: [DISCUSS] KIP-820: Extend KStream process with new Processor API

[Matthias J. Sax]

For this KIP, I also see the value. I was just trying to make a step 
back and ask if it's a good short term solution. If we believe it is, I 
am fine with it.

(I am more worried about the header's KIP...)
Btw: I am still wondering if we can change existing `process()` as 
proposed in the KIP? It the propose change source compatible? (It's for 
sure not binary compatible, but this seems fine -- I don't think we 
guarantee binary compatibility).

Btw: would be good to clarify what is changes for process() -- should be 
return type change from `void` to `KStream<KOut, VOut>` as well as 
change of `ProcessorSupplier` generic types (output types change from 
`Void` to `KOut` and `VOut`?



-Matthias

On 2/23/22 11:32 AM, Guozhang Wang wrote:
Hi folks,

I agree with John that this KIP by itself could be a good improvement, and
I feel it aligns well with the eventual DSL 2.0 proposal so we do not need
to hold it until later.

Regarding the last point (i.e. whether we should do enforcement with a new
interface), here's my 2c: in the past we introduced public
`ValueTransfomer/etc` for two purposes, 1) to enforce the key is not
modifiable, 2) to indicate inside the library's topology builder itself
that since the key is not modified, the direct downstream does not need to
inject a repartition stage. I think we are more or less on the same page
that for purpose 1), doing runtime check could be sufficient; as for the
purpose of 2), as for this KIP itself I think it is similar to what we have
(i.e. just base on the function name "processValue" itself) and hence are
not sacrificed either. I do not know if
`KStream#processValue(ProcessorSupplier<K, V, Void, VOut>
processorSupplier)` will work, or work better, maybe Jorge could do some
digging and get back to us.


On Fri, Feb 18, 2022 at 8:24 AM John Roesler <vvcep...@apache.org> wrote:

Hello all,

While I sympathize with Matthias’s desire to wipe the slate clean and
redesign the dsl with full knowledge of everything we’ve learned in the
past few years, that would also be a pretty intense project on its own. It
seems better to leave that project for someone who is motivated to take it
on.

Reading between the lines, it seems like Jorge’s motivation is more along
the lines of removing a few specific pain points. I appreciate Matthias
extending the offer, but if Jorge doesn’t want to redesign the dsl right
now, we’re better off just accepting the work he’s willing to do.

Specifically, this KIP is quite a nice improvement. Looking at the KStream
interface, roughly half of it is devoted to various flavors of “transform”,
which makes it really hard on users to figure out which they are supposed
to use for what purpose. This kip let us drop all that complexity in favor
of just two methods, thanks to the fact that we now have the ability for
processors to specify their forwarding type.

By the way, I really like Matthias’s suggestion to set the KOut generic
bound to Void for processValues. Then, instead of doing an equality check
on the key during forward, you’d just set the key back to the one saved
before processing (with setRecordKey). This is both more efficient (because
we don’t have the equality check) and more foolproof for users (because
it’s enforced by the compiler instead of the runtime).

Thanks, all!
-John

On Fri, Feb 18, 2022, at 00:43, Jorge Esteban Quilcate Otoya wrote:
On Fri, 18 Feb 2022 at 02:16, Matthias J. Sax <mj...@apache.org> wrote:

It probably deserves its own thread to start discussing ideas.

Yes. My question was: if we think it's time to do a DSL 2.0, should we
drop this KIP and just fix via DSL 2.0 instead?


Good question. Would love to hear what others think about this.

I've stated my position about this here:

For this KIP specifically, I think about it as a continuation from
KIP-478. Therefore, it could make sense to have it as part of the current
version of the DSL.

I'd even add that if this KIP is adopted, I would not be that
disappointed
if KIP-634 is dropped in favor of a DSL v2.0 as the access to headers
provided by KIP-478's via Record API is much better than previous
`.context().headers()`.

But happy to reconsider if there is an agreement to focus efforts
towards a
DSL 2.0.


You're right. I'm not proposing the method signature.

What signature do you propose? I don't see an update on the KIP.

My bad. I have clarified this in the KIP's public interfaces now:

```

New methods:

    - KStream<K,VOut> KStream#processValues(ProcessorSupplier<K, V, K,
VOut>
    processorSupplier, String... stateStoreNames)
    - KStream<K,VOut> KStream#processValues(ProcessorSupplier<K, V, K,
VOut>
    processorSupplier, Named named, String... stateStoreNames)

Modified methods:

    - KStream<KOut,VOut> KStream#process(ProcessorSupplier<K, V, KOut,
VOut>
    processorSupplier, String... stateStoreNames)
    - KStream<KOut,VOut> KStream#process(ProcessorSupplier<K, V, KOut,
VOut>
    processorSupplier, Named named, String... stateStoreNames)

```



Not sure if I understand how this would look like. Do you mean
checking
it
on the Record itself or somewhere else?

@Guozhang: I am not worried about the runtime overhead. I am worries
about user experience. It's not clear from the method signature, that
you are not allowed to change the key, what seems to be bad API desig.
Even if I understand the desire to keep the API surface ares small -- I
would rather have a compile time enforcement than a runtime check.

For example, we have `map()` and `mapValues()` and `mapValues()` returns
a `Value V` (enforces that that key is not changes) instead of a
`KeyValue<KIn,VOut>` and we use a runtime check to check that the key is
not changed.

Naively, could we enforce something similar by setting the output key
type as `Void`.

    KStream#processValue(ProcessorSupplier<K, V, Void, VOut>
processorSupplier)

Not sure if this would work or not?

Or it might be worth to add a new interface, `ValueProcessorSupplier`
that ensures that the key is not modified?


This is an important discussion, even more so with a DSL v2.0.

At the moment, the DSL just flags whether partitioning is required based
on
the DSL operation. As mentioned, `mapValues()` enforces only the value
has
changed through the DSL, though the only _guarantee_ we have is that
Kafka
Streams "owns" the implementation, and we can flag this properly.

With a hypothetical v2.0 based on Record API, this will be harder to
enforce with the current APIs. e.g. with `mapValues(Record<K, V>
record)`,
nothing would stop users from using
`record.withKey("needs_partitioning")`.

The approach defined on this KIP is similar to what we have at the moment
on `ValueTransformer*` where it validates at runtime that the users are
not
calling `forward` with `ForwardingDisabledProcessorContext`.
`ValueProcessorSupplier` is not meant to be a public API. Only to be used
internally on `processValues` implementation.

At first, `KStream#processValue(ProcessorSupplier<K, V, Void, VOut>
processorSupplier)` won't work as it will require the `Processor`
implementation to actually change the key. Will take a deeper look to
validate if this could solve this issue.




-Matthias


On 2/17/22 10:56 AM, Guozhang Wang wrote:
Regarding the last question Matthias had, I wonder if it's similar to
my
first email's point 2) above? I think the rationale is that, since
reference checks are relatively very cheap, it is worthwhile to pay
this
extra runtime checks and in return to have a single consolidated
ProcessorSupplier programming interface (i.e. we would eventually
deprecate ValueTransformerWithKeySupplier).

On Wed, Feb 16, 2022 at 10:57 AM Jorge Esteban Quilcate Otoya <
quilcate.jo...@gmail.com> wrote:

Thank you Matthias, this is great feedback.

Adding my comments below.

On Wed, 16 Feb 2022 at 00:42, Matthias J. Sax <mj...@apache.org>
wrote:

Thanks for the KIP.

In alignment to my reply to KIP-634, I am wondering if we are
heading
into the right direction, or if we should consider to re-design the
DSL
from scratch?


I'm very excited about the idea of a DLS v2.0. It probably deserves
its
own
thread to start discussing ideas.

For this KIP specifically, I think about it as a continuation from
KIP-478.
Therefore, it could make sense to have it as part of the current
version of
the DSL.



Even if we don't do a DSL 2.0 right now, I have some concerns about
this
KIP:

(1) I am not sure if the propose changed is backward compatible? We
currently have:

     void KStream#process(ProcessorSupplier, String...)

The newly proposed method:

     KStream KStream#process(ProcessorSupplier)

seems to be an incompatible change?

The KIP states:

Modified method KStream#process should be compatible with previous
version, that at the moment is fixed to a Void return type.

Why is it backward compatible? Having both old and new #process()
seems
not to be compatible to me? Or are you proposing to _change_ the
method
signature (if yes, the `String...` parameter to add a state store
seems
to be missing)? For this case, it seems that existing programs
would at
least need to be recompiled -- it would only be a source compatible
change, but not a binary compatible change?


You're right. I'm not proposing the method signature.
Totally agree about compatibility issue. I was only considering
source
compatibility and was ignorant that changing from void to a specific
type
would break binary compatibility.
I will update the KIP to reflect this:

Modifications to method KStream#process are source compatible with
previous version, though not binary compatible. Therefore will
require
users to recompile their applications with the latest version.


I am also wondering if/how this change related to KIP-401:



https://cwiki.apache.org/confluence/pages/viewpage.action?pageId=97553756

   From a high level it might not conflict, but I wanted to double
check?


Wasn't aware of this KIP, thanks for sharing! I don't think there is
conflict between KIPs, as far as I understand.



For `KStream#processValues()`, my main concern is the added runtime
check if the key was modified or not -- it seems to provide bad user
experience -- enforcing that the key is not modified on an API
level,
would seem to be much better.

Last, what is the purpose of `setRecordKey()` and
`clearRecordKey()`? I
am not sure if I understand their purpose?


Both methods set/clear the context (current key) to be used when
checking
keys on forward(record) implementation.

enforcing that the key is not modified on an API level, would seem
to
be
much better.

Not sure if I understand how this would look like. Do you mean
checking
it
on the Record itself or somewhere else?



-Matthias


On 2/15/22 11:53 AM, John Roesler wrote:
My apologies, this feedback was intended for KIP-634.
-John

On Tue, Feb 15, 2022, at 13:15, John Roesler wrote:
Thanks for the update, Jorge,

I've just looked over the KIP again. Just one more small
concern:

5) We can't just change the type of Record#headers() to a
new fully qualified type. That would be a source-
incompatible breaking change for users.

Out options are:
* Deprecate the existing method and create a new one with
the new type
* If the existing Headers is "not great but ok", then maybe
we leave it alone.

Thanks,
-John

On Mon, 2022-02-14 at 13:58 -0600, Paul Whalen wrote:
No specific comments, but I just wanted to mention I like the
direction of
the KIP.  My team is a big user of "transform" methods because of
the
ability to chain them, and I have always found the terminology
challenging
to explain alongside "process".  It felt like one concept with
two
names.
So moving towards a single API that is powerful enough to handle
both
use
cases seems absolutely correct to me.

Paul

On Mon, Feb 14, 2022 at 1:12 PM Jorge Esteban Quilcate Otoya <
quilcate.jo...@gmail.com> wrote:

Got it. Thanks John, this make sense.

I've updated the KIP to include the deprecation of:

      - KStream#transform
      - KStream#transformValues
      - KStream#flatTransform
      - KStream#flatTransformValues



On Fri, 11 Feb 2022 at 15:16, John Roesler <vvcep...@apache.org

wrote:

Thanks, Jorge!

I think it’ll be better to keep this KIP focused on KStream
methods
only.
I suspect that the KTable methods may be more complicated than
just
that
proposed replacement, but it’ll also be easier to consider that
question
in
isolation.

The nice thing about just deprecating the KStream methods and
not
the
Transform* interfaces is that you can keep your proposal just
scoped
to
KStream and not have any consequences for the rest of the DSL.

Thanks again,
John

On Fri, Feb 11, 2022, at 06:43, Jorge Esteban Quilcate Otoya
wrote:
Thanks, John.

4) I agree that we shouldn't deprecate the Transformer*
classes, but do you think we should deprecate the
KStream#transform* methods? I'm curious if there's any
remaining reason to have those methods, or if your KIP
completely obviates them.

Good catch.
I considered that deprecating `Transformer*` and `transform*`
would
go
hand
in hand — maybe it happened similarly with old `Processor` and
`process`?
Though deprecating only `transform*` operations could be a
better
signal
for users than non deprecating anything at all and pave the
way
to
it's
deprecation.

Should this deprecation also consider including
`KTable#transformValues`?
The approach proposed on the KIP:
`ktable.toStream().processValues().toTable()` seems fair to
me,
though
I
will have to test it further.

I'm happy to update the KIP if there's some consensus around
this.
Will add the deprecation notes these days and wait for any
additional
feedback on this topic before wrapping up the KIP.


On Fri, 11 Feb 2022 at 04:03, John Roesler <
vvcep...@apache.org>
wrote:

Thanks for the update, Jorge!

I just read over the KIP again, and I'm in support. One more
question came up for me, though:

4) I agree that we shouldn't deprecate the Transformer*
classes, but do you think we should deprecate the
KStream#transform* methods? I'm curious if there's any
remaining reason to have those methods, or if your KIP
completely obviates them.

Thanks,
-John

On Thu, 2022-02-10 at 21:32 +0000, Jorge Esteban Quilcate
Otoya wrote:
Thank you both for your feedback!

I have added the following note on punctuation:

```
NOTE: The key validation can be defined when processing the
message.
Though, with punctuations it won't be possible to define the
key
for
validation before forwarding, therefore it won't be
possible to
forward
from punctuation.
This is similar behavior to how `ValueTransformer`s behave
at
the
moment.
```

Also make it explicit also that we are going to apply
referencial
equality
for key validation.

I hope this is covering all your feedback, let me know if
I'm
missing
anything.

Cheers,
Jorge.

On Wed, 9 Feb 2022 at 22:19, Guozhang Wang <
wangg...@gmail.com

wrote:

I'm +1 on John's point 3) for punctuations.

And I think if people are on the same page that a reference
equality
check
per record is not a huge overhead, I think doing that
enforcement
is
better
than documentations and hand-wavy undefined behaviors.


Guozhang

On Wed, Feb 9, 2022 at 11:27 AM John Roesler <
vvcep...@apache.org

wrote:

Thanks for the KIP Jorge,

I'm in support of your proposal.

1)
I do agree with Guozhang's point (1). I think the cleanest
approach. I think it's cleaner and better to keep the
enforcement internal to the framework than to introduce a
public API or context wrapper for processors to use
explicitly.

2) I tend to agree with you on this one; I think the
equality check ought to be fast enough in practice.

3) I think this is implicit, but should be explicit in the
KIP: For the `processValues` API, because the framework
sets
the key on the context before calling `process` and then
unsets it afterwards, there will always be no key set
during
task puctuation. Therefore, while processors may still
register punctuators, they will not be able to forward
anything from them.

This is functionally equivalent to the existing
transformers, by the way, that are also forbidden to
forward
anything during punctuation.

For what it's worth, I think this is the best tradeoff.

The only alternative I see is not to place any restriction
on forwarded keys at all and just document that if users
don't maintain proper partitioning, they'll get undefined
behavior. That might be more powerful, but it's also a
usability problem.

Thanks,
-John

On Wed, 2022-02-09 at 11:34 +0000, Jorge Esteban Quilcate
Otoya wrote:
Thanks Guozhang.

Does `ValueProcessorContext` have to be a public API? It
seems
to me
that this can be completely abstracted away from user
interfaces
as an
internal class

Totally agree. No intention to add these as public APIs.
Will
update
the
KIP to reflect this.

in the past the rationale for enforcing it at the
interface layer rather than do runtime checks is that it
is
more
efficient.
I'm not sure how much overhead it may incur to check if
the
key
did
not
change: if it is just a reference equality check maybe
it's
okay.
What's
your take on this?

Agree, reference equality should cover this validation
and
the
overhead
impact should not be meaningful.
Will update the KIP to reflect this as well.


On Tue, 8 Feb 2022 at 19:05, Guozhang Wang <
wangg...@gmail.com>
wrote:

Hello Jorge,

Thanks for bringing this KIP! I think this is a nice
idea
to
consider
using
a single overloaded function name for #process, just a
couple
quick
questions after reading the proposal:

1) Does `ValueProcessorContext` have to be a public
API? It
seems to
me
that this can be completely abstracted away from user
interfaces
as
an
internal class, and we call the `setKey` before calling
user-instantiated
`process` function, and then in its overridden
`forward` it
can
just
check
if the key changes or not.
2) Related to 1) above, in the past the rationale for
enforcing
it at
the
interface layer rather than do runtime checks is that
it is
more
efficient.
I'm not sure how much overhead it may incur to check if
the
key
did
not
change: if it is just a reference equality check maybe
it's
okay.
What's
your take on this?


Guozhang

On Tue, Feb 8, 2022 at 5:17 AM Jorge Esteban Quilcate
Otoya
<
quilcate.jo...@gmail.com> wrote:

Hi Dev team,

I'd like to start a new discussion thread on Kafka
Streams
KIP-820:











https://cwiki.apache.org/confluence/display/KAFKA/KIP-820%3A+Extend+KStream+process+with+new+Processor+API

This KIP is aimed to extend the current
`KStream#process`
API
to
return
output values that could be chained across the
topology,
as
well as
introducing a new `KStream#processValues` to use
processor
while
validating
keys haven't change and repartition is not required.

Looking forward to your feedback.

Regards,
Jorge.



--
-- Guozhang




--
-- Guozhang