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Re: [DISCUSS] Long-term goal of making flink-table Scala-free


Hi Hequn,

thanks for your feedback. Yes, migrating the test cases is another issue that is not represented in the document but should naturally go along with the migration.

I agree that we should migrate the main API classes quickly within this 1.8 release after the module split has been performed. Help here is highly appreciated!

I forgot that Java supports static methods in interfaces now, but actually I don't like the design of calling `TableEnvironment.get(env)`. Because people often use `TableEnvironment tEnd = TableEnvironment.get(env)` and then wonder why there is no `toAppendStream` or `toDataSet` because they are using the base class. However, things like that can be discussed in the corresponding issue when it comes to implementation.

@Vino: I think your work fits nicely to these efforts.

@everyone: I will wait for more feedback until end of this week. Then I will convert the design document into a FLIP and open subtasks in Jira, if there are no objections?

Regards,
Timo

Am 24.11.18 um 13:45 schrieb vino yang:
Hi hequn,

I am very glad to hear that you are interested in this work.
As we all know, this process involves a lot.
Currently, the migration work has begun. I started with the
Kafka connector's dependency on flink-table and moved the
related dependencies to flink-table-common.
This work is tracked by FLINK-9461.  [1]
I don't know if it will conflict with what you expect to do, but from the
impact I have observed,
it will involve many classes that are currently in flink-table.

*Just a statement to prevent unnecessary conflicts.*

Thanks, vino.

[1]: https://issues.apache.org/jira/browse/FLINK-9461

Hequn Cheng <chenghequn@xxxxxxxxx> 于2018年11月24日周六 下午7:20写道:

Hi Timo,

Thanks for the effort and writing up this document. I like the idea to make
flink-table scala free, so +1 for the proposal!

It's good to make Java the first-class citizen. For a long time, we have
neglected java so that many features in Table are missed in Java Test
cases, such as this one[1] I found recently. And I think we may also need
to migrate our test cases, i.e, add java tests.

This definitely is a big change and will break API compatible. In order to
bring a smaller impact on users, I think we should go fast when we migrate
APIs targeted to users. It's better to introduce the user sensitive changes
within a release. However, it may be not that easy. I can help to
contribute.

Separation of interface and implementation is a good idea. This may
introduce a minimum of dependencies or even no dependencies. I saw your
reply in the google doc. Java8 has already supported static method for
interfaces, I think we can make use of it?

Best,
Hequn

[1] https://issues.apache.org/jira/browse/FLINK-11001


On Fri, Nov 23, 2018 at 5:36 PM Timo Walther <twalthr@xxxxxxxxxx> wrote:

Hi everyone,

thanks for the great feedback so far. I updated the document with the
input I got so far

@Fabian: I moved the porting of flink-table-runtime classes up in the
list.
@Xiaowei: Could you elaborate what "interface only" means to you? Do you
mean a module containing pure Java `interface`s? Or is the validation
logic also part of the API module? Are 50+ expression classes part of
the API interface or already too implementation-specific?

@Xuefu: I extended the document by almost a page to clarify when we
should develop in Scala and when in Java. As Piotr said, every new Scala
line is instant technical debt.

Thanks,
Timo


Am 23.11.18 um 10:29 schrieb Piotr Nowojski:
Hi Timo,

Thanks for writing this down +1 from my side :)

I'm wondering that whether we can have rule in the interim when Java
and Scala coexist that dependency can only be one-way. I found that in
the
current code base there are cases where a Scala class extends Java and
vise
versa. This is quite painful. I'm thinking if we could say that extension
can only be from Java to Scala, which will help the situation. However,
I'm
not sure if this is practical.
Xuefu: I’m also not sure what’s the best approach here, probably we
will
have to work it out as we go. One thing to consider is that from now on,
every single new code line written in Scala anywhere in Flink-table
(except
of Flink-table-api-scala) is an instant technological debt. From this
perspective I would be in favour of tolerating quite big inchonvieneces
just to avoid any new Scala code.
Piotrek

On 23 Nov 2018, at 03:25, Zhang, Xuefu <xuefu.z@xxxxxxxxxxxxxxx>
wrote:
Hi Timo,

Thanks for the effort and the Google writeup. During our external
catalog rework, we found much confusion between Java and Scala, and this
Scala-free roadmap should greatly mitigate that.
I'm wondering that whether we can have rule in the interim when Java
and Scala coexist that dependency can only be one-way. I found that in
the
current code base there are cases where a Scala class extends Java and
vise
versa. This is quite painful. I'm thinking if we could say that extension
can only be from Java to Scala, which will help the situation. However,
I'm
not sure if this is practical.
Thanks,
Xuefu


------------------------------------------------------------------
Sender:jincheng sun <sunjincheng121@xxxxxxxxx>
Sent at:2018 Nov 23 (Fri) 09:49
Recipient:dev <dev@xxxxxxxxxxxxxxxx>
Subject:Re: [DISCUSS] Long-term goal of making flink-table Scala-free

Hi Timo,
Thanks for initiating this great discussion.

Currently when using SQL/TableAPI should include many dependence. In
particular, it is not necessary to introduce the specific
implementation
dependencies which users do not care about. So I am glad to see your
proposal, and hope when we consider splitting the API interface into a
separate module, so that the user can introduce minimum of
dependencies.
So, +1 to [separation of interface and implementation; e.g. `Table` &
`TableImpl`] which you mentioned in the google doc.
Best,
Jincheng

Xiaowei Jiang <xiaoweij@xxxxxxxxx> 于2018年11月22日周四 下午10:50写道:

Hi Timo, thanks for driving this! I think that this is a nice thing
to
do.
While we are doing this, can we also keep in mind that we want to
eventually have a TableAPI interface only module which users can take
dependency on, but without including any implementation details?

Xiaowei

On Thu, Nov 22, 2018 at 6:37 PM Fabian Hueske <fhueske@xxxxxxxxx>
wrote:
Hi Timo,

Thanks for writing up this document.
I like the new structure and agree to prioritize the porting of the
flink-table-common classes.
Since flink-table-runtime is (or should be) independent of the API
and
planner modules, we could start porting these classes once the code
is
split into the new module structure.
The benefits of a Scala-free flink-table-runtime would be a
Scala-free
execution Jar.

Best, Fabian


Am Do., 22. Nov. 2018 um 10:54 Uhr schrieb Timo Walther <
twalthr@xxxxxxxxxx
:
Hi everyone,

I would like to continue this discussion thread and convert the
outcome
into a FLIP such that users and contributors know what to expect in
the
upcoming releases.

I created a design document [1] that clarifies our motivation why
we
want to do this, how a Maven module structure could look like, and
a
suggestion for a migration plan.

It would be great to start with the efforts for the 1.8 release
such
that new features can be developed in Java and major refactorings
such
as improvements to the connectors and external catalog support are
not
blocked.

Please let me know what you think.

Regards,
Timo

[1]


https://docs.google.com/document/d/1PPo6goW7tOwxmpFuvLSjFnx7BF8IVz0w3dcmPPyqvoY/edit?usp=sharing
Am 02.07.18 um 17:08 schrieb Fabian Hueske:
Hi Piotr,

thanks for bumping this thread and thanks for Xingcan for the
comments.
I think the first step would be to separate the flink-table module
into
multiple sub modules. These could be:

- flink-table-api: All API facing classes. Can be later divided
further
into Java/Scala Table API/SQL
- flink-table-planning: involves all planning (basically
everything
we
do
with Calcite)
- flink-table-runtime: the runtime code

IMO, a realistic mid-term goal is to have the runtime module and
certain
parts of the planning module ported to Java.
The api module will be much harder to port because of several
dependencies
to Scala core classes (the parser framework, tree iterations,
etc.).
I'm
not saying we should not port this to Java, but it is not clear to
me
(yet)
how to do it.

I think flink-table-runtime should not be too hard to port. The
code
does
not make use of many Scala features, i.e., it's writing very
Java-like.
Also, there are not many dependencies and operators can be
individually
ported step-by-step.
For flink-table-planning, we can have certain packages that we
port
to
Java
like planning rules or plan nodes. The related classes mostly
extend
Calcite's Java interfaces/classes and would be natural choices for
being
ported. The code generation classes will require more effort to
port.
There
are also some dependencies in planning on the api module that we
would
need
to resolve somehow.

For SQL most work when adding new features is done in the planning
and
runtime modules. So, this separation should already reduce
"technological
dept" quite a lot.
The Table API depends much more on Scala than SQL.

Cheers, Fabian



2018-07-02 16:26 GMT+02:00 Xingcan Cui <xingcanc@xxxxxxxxx>:

Hi all,

I also think about this problem these days and here are my
thoughts.
1) We must admit that it’s really a tough task to interoperate
with
Java
and Scala. E.g., they have different collection types (Scala
collections
v.s. java.util.*) and in Java, it's hard to implement a method
which
takes
Scala functions as parameters. Considering the major part of the
code
base
is implemented in Java, +1 for this goal from a long-term view.

2) The ideal solution would be to just expose a Scala API and
make
all
the
other parts Scala-free. But I am not sure if it could be achieved
even
in a
long-term. Thus as Timo suggested, keep the Scala codes in
"flink-table-core" would be a compromise solution.

3) If the community makes the final decision, maybe any new
features
should be added in Java (regardless of the modules), in order to
prevent
the Scala codes from growing.

Best,
Xingcan


On Jul 2, 2018, at 9:30 PM, Piotr Nowojski <
piotr@xxxxxxxxxxxxxxxxx>
wrote:
Bumping the topic.

If we want to do this, the sooner we decide, the less code we
will
have
to rewrite. I have some objections/counter proposals to Fabian's
proposal
of doing it module wise and one module at a time.
First, I do not see a problem of having java/scala code even
within
one
module, especially not if there are clean boundaries. Like we
could
have
API in Scala and optimizer rules/logical nodes written in Java in
the
same
module. However I haven’t previously maintained mixed scala/java
code
bases
before, so I might be missing something here.
Secondly this whole migration might and most like will take
longer
then
expected, so that creates a problem for a new code that we will
be
creating. After making a decision to migrate to Java, almost any
new
Scala
line of code will be immediately a technological debt and we will
have
to
rewrite it to Java later.
Thus I would propose first to state our end goal - modules
structure
and
which parts of modules we want to have eventually Scala-free.
Secondly
taking all steps necessary that will allow us to write new code
complaint
with our end goal. Only after that we should/could focus on
incrementally
rewriting the old code. Otherwise we could be stuck/blocked for
years
writing new code in Scala (and increasing technological debt),
because
nobody have found a time to rewrite some non important and not
actively
developed part of some module.
Piotrek

On 14 Jun 2018, at 15:34, Fabian Hueske <fhueske@xxxxxxxxx>
wrote:
Hi,

In general, I think this is a good effort. However, it won't be
easy
and I
think we have to plan this well.
I don't like the idea of having the whole code base fragmented
into
Java
and Scala code for too long.

I think we should do this one step at a time and focus on
migrating
one
module at a time.
IMO, the easiest start would be to port the runtime to Java.
Extracting the API classes into an own module, porting them to
Java,
and
removing the Scala dependency won't be possible without
breaking
the
API
since a few classes depend on the Scala Table API.

Best, Fabian


2018-06-14 10:33 GMT+02:00 Till Rohrmann <trohrmann@xxxxxxxxxx
:
I think that is a noble and honorable goal and we should
strive
for
it.
This, however, must be an iterative process given the sheer
size
of
the
code base. I like the approach to define common Java modules
which
are
used
by more specific Scala modules and slowly moving classes from
Scala
to
Java. Thus +1 for the proposal.

Cheers,
Till

On Wed, Jun 13, 2018 at 12:01 PM Piotr Nowojski <
piotr@xxxxxxxxxxxxxxxxx>
wrote:

Hi,

I do not have an experience with how scala and java interacts
with
each
other, so I can not fully validate your proposal, but
generally
speaking
+1
from me.

Does it also mean, that we should slowly migrate
`flink-table-core`
to
Java? How would you envision it? It would be nice to be able
to
add
new
classes/features written in Java and so that they can coexist
with
old
Scala code until we gradually switch from Scala to Java.

Piotrek

On 13 Jun 2018, at 11:32, Timo Walther <twalthr@xxxxxxxxxx>
wrote:
Hi everyone,

as you all know, currently the Table & SQL API is
implemented
in
Scala.
This decision was made a long-time ago when the initital code
base
was
created as part of a master's thesis. The community kept
Scala
because of
the nice language features that enable a fluent Table API
like
table.select('field.trim()) and because Scala allows for
quick
prototyping
(e.g. multi-line comments for code generation). The
committers
enforced
not
splitting the code-base into two programming languages.
However, nowadays the flink-table module more and more
becomes
an
important part in the Flink ecosystem. Connectors, formats,
and
SQL
client
are actually implemented in Java but need to interoperate
with
flink-table
which makes these modules dependent on Scala. As mentioned in
an
earlier
mail thread, using Scala for API classes also exposes member
variables
and
methods in Java that should not be exposed to users [1]. Java
is
still
the
most important API language and right now we treat it as a
second-class
citizen. I just noticed that you even need to add Scala if
you
just
want
to
implement a ScalarFunction because of method clashes between
`public
String
toString()` and `public scala.Predef.String toString()`.
Given the size of the current code base, reimplementing the
entire
flink-table code in Java is a goal that we might never reach.
However, we
should at least treat the symptoms and have this as a
long-term
goal
in
mind. My suggestion would be to convert user-facing and
runtime
classes
and
split the code base into multiple modules:
flink-table-java {depends on flink-table-core}
Implemented in Java. Java users can use this. This would
require
to
convert classes like TableEnvironment, Table.
flink-table-scala {depends on flink-table-core}
Implemented in Scala. Scala users can use this.

flink-table-common
Implemented in Java. Connectors, formats, and UDFs can use
this.
It
contains interface classes such as descriptors, table sink,
table
source.
flink-table-core {depends on flink-table-common and
flink-table-runtime}
Implemented in Scala. Contains the current main code base.

flink-table-runtime
Implemented in Java. This would require to convert classes
in
o.a.f.table.runtime but would improve the runtime
potentially.
What do you think?


Regards,

Timo

[1]
http://apache-flink-mailing-list-archive.1008284.n3.
nabble.com/DISCUSS-Convert-main-Table-API-classes-into-
traits-tp21335.html