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From Andrew Purtell <apurt...@apache.org>
Subject Re: Looking for input on an alpha-4 thorny item
Date Fri, 06 Oct 2017 17:17:41 GMT
I think at least at the Store level we want to admit the possibility of
alternate implementations.

Region is pushing it. It would be ideal of course to have nice clean
interfaces which could be mocked or given to new implementations, but the
legacy aspects of the code base probably make that more work than worth the
effort unless someone is really going to try going all the way to a new
type of Region.


On Fri, Oct 6, 2017 at 9:33 AM, Stack <stack@duboce.net> wrote:

> On Thu, Oct 5, 2017 at 9:30 AM, Josh Elser <elserj@apache.org> wrote:
>
> > (I think I understand the problems enough to comment, but, admittedly, my
> > 5minute read is probably lacking)
> >
> >
> Thanks for chiming-in Josh.
>
>
>
> > I think the only argument against what you all have outlined here is if,
> > in the future, we have some intent to create new implementations of
> HRegion
> > or HStore. If that's the case, Region could still be the CP's "view" of
> > what a region is, we could introduce another interface which defines what
> > the internal/private "view" of a region is, and then we plug in the
> > implementation of choice (e.g. HRegion as we know it now would become
> > something like HRegionImpl).
> >
> >
>
> Currently our codebase entertains the 'fantasy' that it is possible to plug
> in alternate Region and Store implementations; there is a config that
> allows you stipulate another Region class. As part of the Duo/Anoop
> pare-back of CPs on RegionServer-side, it was noted that we should disabuse
> ourselves of all such notions. It just doesn't work. We've not expended the
> effort to keep clean Region/Store Interfaces (Witness this note on
> confusion around intent of Region Interface for example). There are no
> tests.
>
>
>
> > However, I'm struggling to come up with a concrete use-case as to why we
> > would need that extra level of indirection. As such, I think the below
> > proposal makes sense.
> >
> >
> Yeah. We could do the work to backfill an 'HRegion Interface', but years
> later, there have been no takers. It'd be a fun project for sure but would
> have to have good justification (Justification would have to include why
> use HBase at all and not something like Apache Helix instead).
>
> Good stuff,
> S
>
>
>
>
>
> > The distillation of a tricky issue is quite appreciated!
> >
> >
> > On 10/4/17 6:51 PM, Andrew Purtell wrote:
> >
> >> I think it is fine to rebrand these interfaces as for coprocessors and
> tag
> >> them LP(COPROC):
> >>
> >>      Region (use HRegion in internals)
> >>
> >>      Store (use HStore in internals)
> >>
> >>      MasterServices (use HMaster in internals)
> >>
> >>      RegionServerServices (use HRegionServer in internals)
> >>
> >> and pare them down. This is not a complete list, just a handful of
> >> examples.
> >>
> >> Some specific points of feedback I have had:
> >>
> >> In Region, it would be good for CPs to be able to schedule async flushes
> >> and compactions, poll or wait for completion of a specific request, or
> >> wait
> >> for all pending flushes and compactions to complete. There is a Phoenix
> >> use
> >> case for this in indexing.
> >>
> >> Security coprocessors use MasterServices to create their system tables.
> >> Maybe this can be replaced by using the normal admin API for same.
> >>
> >> When removing access to internal services, consider if there are client
> >> API
> >> equivalents that the CP can use, and if embedded calls to such client
> APIs
> >> from the coprocessor context would be a good idea. CP invocation of an
> >> internal service is simply an in-process method call. That's good and
> bad,
> >> right? The bad part, direct access, is the thing we want to restrict,
> the
> >> motivation for this work (in part). But the good thing is it avoids a
> lot
> >> of the fat client logic unnecessary for all-in-process service
> invocation,
> >> which might even not work correctly. Removing everything is as drastic
> as
> >> allowing CPs access to everything. It could be fine to drastically pare
> >> down, but please consider it.
> >>
> >> Some changes have been proposed that removes access to metrics (e.g.
> >> RegionMetrics, MasterMetrics). Right now coprocessors can bypass core
> >> function and replace it. Until and unless we remove the bypass semantic
> >> (under discussion) we should continue to allow CPs access to metrics
> >> objects so they can update metrics as expected by admins and users when
> >> replacing functionality (via bypass). Metrics are a public facing API. I
> >> agree this is kind of dodgy. I believe we should remove the bypass
> >> semantic. Once that is done, coprocessors can only mix in additional
> >> functionality. No more cause to touch core metrics. They can export
> their
> >> own metrics if so desired.
> >> ​​
> >>
> >> On Wed, Oct 4, 2017 at 3:15 PM, Stack <stack@duboce.net> wrote:
> >> ​​
> >>
> >> A bunch of us are making good progress on the next alpha release,
> >>
> >>> hbase-2.0.0-alpha-4. The theme for this release is "Fixing the
> >>> Coprocessor
> >>> API", mostly undoing access accidentally granted Coprocessors. I'm
> >>> talking
> >>> out loud about a particularly awkward item here rather than in a
> comment
> >>> up
> >>> in JIRA so the airing sees a broader audience. Interested in any
> opinions
> >>> or input you might have.
> >>>
> >>> TL;DR MasterService/RegionServerService and Region, etc., Interfaces
> >>> were
> >>> overloaded serving two, disparate roles; a load of refactoring has to
> be
> >>> done to undo the damage. Suggestions for how to avoid making same
> mistake
> >>> in future?
> >>>
> >>> I'm working on "HBASE-12260 MasterServices - remove from coprocessor
> API
> >>> (Discuss)". MasterServices started out as a subset of the Master
> >>> functionality. The idea back then was that certain Services and
> Managers
> >>> could make do w/ less-than-full-access to the HMaster process. If so,
> we
> >>> could test the Service and Manager without having to standup a full
> >>> HMaster
> >>> instance (This usually required our putting up a cluster too). If
> >>> MasterServices had but a few methods, a Mock would be easy, making
> >>> testing
> >>> easier still. We did the same thing on the RegionServer side where we
> had
> >>> RegionServerServices.
> >>>
> >>> MasterServices (and RegionServerServices) were also exposed to
> >>> Coprocessors. The idea was that CPs could ask-for particular Master
> >>> functions via MasterService. In this role MasterServices was meant to
> >>> constrain what CPs had access to.
> >>>
> >>> MasterServices therefore had two consumers; one internal, the other
> not.
> >>>
> >>> With time, MasterServices got fat as internal Services and Managers
> >>> needed
> >>> more of HMaster. Everytime we added to MasterServices, CPs got access.
> >>>
> >>> On survey as part of the recent HBASE-12260 work, it turns out that the
> >>> bulk of the methods in MasterServices are actually annotated
> >>> InterfaceAudience.Private; i.e. for internal use only, not for
> >>> Coprocessors. A brutal purge of Private audience objects, makes for a
> >>> MasterServices we can pass Coprocessors but Coprocessors now have much
> >>> less
> >>> facility available (for parts, there are alternatives; Andy review
> >>> suggests
> >>> that CPs are severely crimped if this patch goes in). But
> MasterServices
> >>> can no longer be used for its original purpose, passing Services and
> >>> Managers a subset of HMaster. The latter brings on a substantial
> >>> refactor.
> >>>
> >>> Another example of the double-role problem outlined above was found by
> >>> Duo
> >>> and Anoop in the RegionServer Coprocessor refactor salt mine. They hit
> a
> >>> similar tangle. There was the RegionServerServices <=> MasterServices
> >>> case
> >>> but also the exposure of HRegion internals. In this latter, Region was
> >>> introduced by Andy EXPLICITLY as a subset of HRegion facility FOR
> >>> Coprocessors. Subsequently, we all confused his original intent and
> went
> >>> ahead and thought of Region (as opposed to HRegion) as an Interface for
> >>> HRegion and plumbed it throughout the code base in place of explicit
> >>> HRegion references. As Region picked up functionality, Coprocessors
> >>> gained
> >>> more access.
> >>>
> >>> The refactoring pattern that has emerged out of the RegionServer-side
> >>> refactoring (which is ahead of the Master-work), is that we move to use
> >>> the
> >>> HRegion implementation everywhere internally, undoing use of Region
> >>> Interface; Region Interface picks up a "FOR COPROCESSORS ONLY" stamp.
> I'm
> >>> following suit on the Master side moving to use HMaster in place of
> >>> MasterServices in all launched Services and Managers.
> >>>
> >>> How do we avoid this mistake in future? Should we rename Region as
> >>> CoprocessorRegion so it more plain that its consumer is Coprocessors?
> >>> Ditto
> >>> on MasterServices?
> >>>
> >>> Thanks,
> >>> S
> >>>
> >>>
> >>
> >>
> >>
>



-- 
Best regards,
Andrew

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

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