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From Andrew Purtell <apurt...@apache.org>
Subject Re: [DISCUSSION] Removing the bypass semantic from the Coprocessor APIs
Date Wed, 11 Oct 2017 17:59:25 GMT
Rather than continue to support a weird bypass() which works in some places
and not in others, perhaps we can substitute it with an exception? So if
the coprocessor throws this exception in the pre hook then where it is
allowed we catch it and do the right thing, and where it is not allowed we
don't catch it and the server aborts. This will at least improve the silent
bypass() failure problem. I also don't like, in retrospect, that calling
this environment method has magic side effects. Everyone understands how
exceptions work, so it will be clearer.

In any case we should try to address the Tephra and Phoenix cases brought
up in this discussion. They look like we can find alternatives. Shall I
file JIRAs to follow up?


On Wed, Oct 11, 2017 at 6:00 AM, 张铎(Duo Zhang) <palomino219@gmail.com>
wrote:

> These examples are great.
>
> And I think for normal region operations such as get, put, delete,
> checkAndXXX, increment, it is OK to bypass the real operation after preXXX
> as the semantic is clear enough. Instead of calling env.bypass, maybe just
> let these preXXX methods return a boolean is enough to tell the HBase
> framework that we have already done the real operation so just give up and
> return?
>
> Thanks.
>
> 2017-10-11 3:19 GMT+08:00 Gary Helmling <ghelmling@gmail.com>:
>
> > The Tephra TransactionProcessor CP makes use of bypass() in preDelete()
> to
> > override handling of delete tombstones in a transactional way:
> > https://github.com/apache/incubator-tephra/blob/master/
> > tephra-hbase-compat-1.3/src/main/java/org/apache/tephra/
> hbase/coprocessor/
> > TransactionProcessor.java#L244
> >
> > The CDAP IncrementHandler CP also makes use of bypass() in preGetOp() and
> > preIncrementAfterRRowLock() to provide a transaction implementation of
> > readless increments:
> > https://github.com/caskdata/cdap/blob/develop/cdap-hbase-
> > compat-1.1/src/main/java/co/cask/cdap/data2/increment/
> > hbase11/IncrementHandler.java#L121
> >
> > What would be the alternate approach for these applications?  In both
> cases
> > they need to impose their own semantics on the underlying KeyValue
> > storage.  Is there a different way this can be done?
> >
> >
> > On Tue, Oct 10, 2017 at 11:58 AM Anoop John <anoop.hbase@gmail.com>
> wrote:
> >
> > > Wrap core scanners is different right?  That can be done in post
> > > hooks.  I have seen many use cases for this..  Its the question abt
> > > the pre hooks where we have not yet created the core object (like
> > > scanner).  The CP pre code itself doing the work of object creation
> > > and so the core code is been bypassed.    Well the wrapping thing can
> > > be done in pre hook also. First create the core object by CP code
> > > itself and then do the wrapped object and return.. I have seen in one
> > > jira issue where the usage was this way..   The wrapping can be done
> > > in post also in such cases I believe.
> > >
> > > -Anoop-
> > >
> > > On Wed, Oct 11, 2017 at 12:23 AM, Andrew Purtell <apurtell@apache.org>
> > > wrote:
> > > > I think we should continue to support overriding function by object
> > > > inheritance. I didn't mention this and am not proposing more than
> > > removing
> > > > the bypass() sematic. No more no less. Phoenix absolutely depends on
> > > being
> > > > able to wrap core scanners and return the wrappers.
> > > >
> > > >
> > > > On Tue, Oct 10, 2017 at 11:50 AM, Anoop John <anoop.hbase@gmail.com>
> > > wrote:
> > > >
> > > >> When we say bypass the core code, it can be done today not only by
> > > >> calling bypass but by returning a not null object for some of the
> pre
> > > >> hooks.  Like preScannerOpen() if it return a scanner object, we will
> > > >> avoid the remaining core code execution for creation of the
> > > >> scanner(s).  So this proposal include this aspect also and remove
> any
> > > >> possible way of bypassing the core code by the CP hook code
> execution
> > > >> ?   Am +1.
> > > >>
> > > >> -Anoop-
> > > >>
> > > >> On Tue, Oct 10, 2017 at 11:40 PM, Andrew Purtell <
> apurtell@apache.org
> > >
> > > >> wrote:
> > > >> > The coprocessor API provides an environment method, bypass(),
that
> > > when
> > > >> > called from a preXXX hook will cause the core code to skip all
> > > remaining
> > > >> > processing. This capability was introduced on HBASE-3348. Since
> this
> > > >> time I
> > > >> > think we are more enlightened about the complications of this
> > feature.
> > > >> (Or,
> > > >> > anyway, speaking for myself:)
> > > >> >
> > > >> > Not all hooks provide the bypass semantic. Where this is the
case
> > the
> > > >> > javadoc for the hook says so, but it can be missed. If you call
> > > bypass()
> > > >> in
> > > >> > a hook where it is not supported it is a no-op. This can lead
to a
> > > poor
> > > >> > developer experience.
> > > >> >
> > > >> > Where bypass is supported what is being bypassed is all of the
> core
> > > code
> > > >> > implementing the remainder of the operation. In order to
> understand
> > > what
> > > >> > calling bypass() will skip, a coprocessor implementer should
read
> > and
> > > >> > understand all of the remaining code and its nuances. Although
I
> > think
> > > >> this
> > > >> > is good practice for coprocessor developers in general, it
> demands a
> > > >> lot. I
> > > >> > think it would provide a much better developer experience if
we
> > didn't
> > > >> > allow bypass, even though it means - in theory - a coprocessor
> would
> > > be a
> > > >> > lot more limited in some ways than before. What is skipped is
> > > extremely
> > > >> > version dependent. That core code will vary, perhaps
> significantly,
> > > even
> > > >> > between point releases. We do not provide the promise of
> consistent
> > > >> > behavior even between point releases for the bypass semantic.
To
> > > achieve
> > > >> > that we could not change any code between hook points. Therefore
> the
> > > >> > coprocessor implementer becomes an HBase core developer in
> practice
> > as
> > > >> soon
> > > >> > as they rely on bypass(). Every release of HBase may break the
> > > assumption
> > > >> > that the replacement for the bypassed code takes care of all
> > necessary
> > > >> > skipped concerns. Because those concerns can change at any point,
> > > such an
> > > >> > assumption is never safe.
> > > >> >
> > > >> > I say "in theory" because I would be surprised if anyone is
> relying
> > on
> > > >> the
> > > >> > bypass for the above reason. I seem to recall that Phoenix might
> use
> > > it
> > > >> in
> > > >> > one place to promote a normal mutation into an atomic operation,
> by
> > > >> > substituting one for the other, but if so that objective could
be
> > > >> > reimplemented using their new locking manager.
> > > >> >
> > > >> > --
> > > >> > Best regards,
> > > >> > Andrew
> > > >>
> > > >
> > > >
> > > >
> > > > --
> > > > Best regards,
> > > > Andrew
> > > >
> > > > Words like orphans lost among the crosstalk, meaning torn from
> truth's
> > > > decrepit hands
> > > >    - A23, Crosstalk
> > >
> >
>



-- 
Best regards,
Andrew

Words like orphans lost among the crosstalk, meaning torn from truth's
decrepit hands
   - A23, Crosstalk

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