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From Anoop John <anoop.hb...@gmail.com>
Subject Re: [DISCUSSION] Removing the bypass semantic from the Coprocessor APIs
Date Tue, 10 Oct 2017 18:50:49 GMT
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

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