[jira] [Commented] (BEAM-659) WindowFn#isCompatible should provide a meaningful reason

["ASF GitHub Bot (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/BEAM-659?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16001972#comment-16001972 ] 

ASF GitHub Bot commented on BEAM-659:
-------------------------------------

GitHub user huafengw opened a pull request:

    https://github.com/apache/beam/pull/2985

    [BEAM-659] WindowFn#isCompatible should provide a meaningful reason

    Be sure to do all of the following to help us incorporate your contribution
    quickly and easily:
    
     - [ ] Make sure the PR title is formatted like:
       `[BEAM-<Jira issue #>] Description of pull request`
     - [ ] Make sure tests pass via `mvn clean verify`.
     - [ ] Replace `<Jira issue #>` in the title with the actual Jira issue
           number, if there is one.
     - [ ] If this contribution is large, please file an Apache
           [Individual Contributor License Agreement](https://www.apache.org/licenses/icla.pdf).
    
    ---


You can merge this pull request into a Git repository by running:

    $ git pull https://github.com/huafengw/beam BEAM-659

Alternatively you can review and apply these changes as the patch at:

    https://github.com/apache/beam/pull/2985.patch

To close this pull request, make a commit to your master/trunk branch
with (at least) the following in the commit message:

    This closes #2985
    
----
commit 12addb1bea6d620050b96fdf4e30602c7a710183
Author: huafengw <fv...@gmail.com>
Date:   2017-05-09T03:21:44Z

    [BEAM-659] WindowFn#isCompatible should provide a meaningful reason

----


> WindowFn#isCompatible should provide a meaningful reason
> --------------------------------------------------------
>
>                 Key: BEAM-659
>                 URL: https://issues.apache.org/jira/browse/BEAM-659
>             Project: Beam
>          Issue Type: Bug
>          Components: sdk-java-core
>            Reporter: Kenneth Knowles
>            Assignee: Huafeng Wang
>            Priority: Minor
>              Labels: easy, easyfix, starter
>
> {{WindowFn#isCompatible(...)}} returns a true/false answer without any explanation of why; it suffers from a form of [boolean blindness|https://existentialtype.wordpress.com/2011/03/15/boolean-blindness/].
> We alleviated this for {{Coder#isDeterministic()}} by switching to {{Coder#verifyDeterministic()}} which can raise an exception describing why it is not deterministic (it could just as easily have been a return value).
> We should do the same for {{WindowFn#isCompatible(...)}}.
> See here: http://stackoverflow.com/questions/39617897/inputs-to-flatten-had-incompatible-window-windowfns-when-cogroupbykey-with-calen
> (a decent {{toString}} would go a long way, for immediate and trivial improvement)



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