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From ramkrishna vasudevan <ramkrishna.s.vasude...@gmail.com>
Subject Re: Where is code in hbase that physically delete a record?
Date Sat, 20 Oct 2012 05:50:46 GMT
Hi
Always any KV that comes from scanning are sorted lexographically and the
recent timestamps will come out first.
So even if your data writes col qualifier c2 first and then c1 because of
lexographical ordering c1 will be coming first.  Also recent versions of a
row will be coming out first and the no of versions can be user controlled.
Have fun !!!

Regards
Ram

On Sat, Oct 20, 2012 at 1:11 AM, PG <pengyunmomo@gmail.com> wrote:

> Hi, Anoop and Ram,
> As I have coded the idea,  the detailed instructions are very helpful. One
> minor thing to add is that coming out from scanner are the KeyValues which
> are already sorted by column qualifier and time stamps. though i did not
> find it mentioned in java doc, but i found it very useful feature to do
> filtering.
>
> Thanks.
> Yun
>
> On Oct 18, 2012, at 12:20 AM, "Ramkrishna.S.Vasudevan" <
> ramkrishna.vasudevan@huawei.com> wrote:
>
> > Hi Yun
> >
> > Hope Anoop's clear explanation will help you.
> > Just to add on, after you wrap the StoreScanner in your Custome Scanner
> Impl
> > you will invoke the next(List<KeyValue>) on the delegator(here the
> delegator
> > is the actual StoreScanner).
> > The delegator will give you the KV list that it has fetched from
> underlying
> > Scanners (Memstore and StoreFileScanner).
> > Now on the returned kv you can do a check say if the KV has a column C1
> and
> > its value is 'a', just skip it so that this scanner does not send the kv
> to
> > the actual Scanner on the outside of the custom Scanner which the
> compaction
> > tries to use.
> >
> > The Code may look lik this
> > Class CustomScanner implements InternalScanner{
> > StoreScanner delegate;
> > Public CustomScanner(){
> > Delegate = new SToreScanner();
> >
> > Public boolean next(List<KeyValue>kv)
> > {
> >    delegate.next(kv);
> >    foreach(kv){
> >        //Do necessary filtering here.
> >    }
> >
> > }
> > }
> >
> > Regards
> > Ram
> >
> >> -----Original Message-----
> >> From: Anoop Sam John [mailto:anoopsj@huawei.com]
> >> Sent: Thursday, October 18, 2012 9:02 AM
> >> To: user@hbase.apache.org
> >> Subject: RE: Where is code in hbase that physically delete a record?
> >>
> >> Hi Yun,
> >>             We have preCompactScannerOpen() and preCompact() hooks..
> >> As we said, for compaction, a scanner for reading all corresponding
> >> HFiles ( all HFiles in major compaction) will be created and scan via
> >> that scanner.. ( calling next() methods).. The kernel will do this
> >> way..
> >> Now using these hooks you can create a wrapper over the actual
> >> scanner... In fact you can use preCompact() hook(I think that is fine
> >> for you).. By the time this is being called,  the actual scanner is
> >> made and will get that object passed to your hook... You can create  a
> >> custom scanner impl and wrap the actual scanner within that and return
> >> the new wrapper scanner from your post hook.. [Yes its return type is
> >> InternalScanner]  The actual scanner you can use as a delegator to do
> >> the actual scanning purpose... Now all the KVs ( which the underlying
> >> scanner passed) will flow via ur new wrapper scanner where you can
> >> avoid certain KVs based on your condition or logic
> >>
> >> Core                 WrapperScannerImpl                      Actual
> >> Scanner (created by core)
> >>        ->                 next(List<KeyValue>)     ->
> >> next(List<KeyValue>)
> >>                                                                    <-
> >> Do the real scan from HFiles
> >>                          See List KVs and remove
> >>                           those u dont want
> >>                                 <-
> >> Only the passed
> >> KVs come in final
> >> merged file
> >>
> >> Hope I make it clear for you :)
> >>
> >> Note : - preCompactScannerOpen() will be called before even creating
> >> the actual scanner while preCompact() after this scanner creation.. You
> >> can see the code in Store#compactStore()
> >>
> >> -Anoop-
> >> ________________________________________
> >> From: yun peng [pengyunmomo@gmail.com]
> >> Sent: Wednesday, October 17, 2012 9:04 PM
> >> To: user@hbase.apache.org
> >> Subject: Re: Where is code in hbase that physically delete a record?
> >>
> >> Hi, Ram and Anoop, Thanks for the nice reference on the java file,
> >> which I
> >> will check through.
> >>
> >> It is interesting to know about the recent feature on
> >> preCompactScannerOpen() hook. Ram, it would be nice if I can know how
> >> to
> >> specify conditions like c1 = 'a'.  I have also checked the example code
> >> in
> >> hbase 6496 link <https://issues.apache.org/jira/browse/HBASE-6496>.
> >> which
> >> show how to delete data before time as in a on-demand specification...
> >> Cheers,
> >> Yun
> >>
> >> On Wed, Oct 17, 2012 at 8:46 AM, Ramkrishna.S.Vasudevan <
> >> ramkrishna.vasudevan@huawei.com> wrote:
> >>
> >>> Also to see the code how the delete happens pls refer to
> >> StoreScanner.java
> >>> and how the ScanQueryMatcher.match() works.
> >>>
> >>> That is where we decide if any kv has to be avoided due to already
> >> deleted
> >>> tombstone marker.
> >>>
> >>> Forgot to tell you about this.
> >>>
> >>> Regards
> >>> Ram
> >>>
> >>>> -----Original Message-----
> >>>> From: yun peng [mailto:pengyunmomo@gmail.com]
> >>>> Sent: Wednesday, October 17, 2012 5:54 PM
> >>>> To: user@hbase.apache.org
> >>>> Subject: Where is code in hbase that physically delete a record?
> >>>>
> >>>> Hi, All,
> >>>> I want to find internal code in hbase where physical deleting a
> >> record
> >>>> occurs.
> >>>>
> >>>> -some of my understanding.
> >>>> Correct me if I am wrong. (It is largely based on my experience and
> >>>> even
> >>>> speculation.) Logically deleting a KeyValue data in hbase is
> >> performed
> >>>> by
> >>>> marking tombmarker (by Delete() per records) or setting
> >> TTL/max_version
> >>>> (per Store). After these actions, however, the physical data are
> >> still
> >>>> there, somewhere in the system. Physically deleting a record in
> >> hbase
> >>>> is
> >>>> realised by *a scanner to discard a keyvalue data record* during
> >> the
> >>>> major_compact.
> >>>>
> >>>> -what I need
> >>>> I want to extend hbase to associate some actions with physically
> >>>> deleting a
> >>>> record. Does hbase provide such hook (or coprocessor API) to inject
> >>>> code
> >>>> for each KV record that is skipped by hbase storescanner in
> >>>> major_compact.
> >>>> If not, anyone knows where should I look into in hbase (-0.94.2)
> >> for
> >>>> such
> >>>> code modification?
> >>>>
> >>>> Thanks.
> >>>> Yun
> >>>
> >>> =
> >
>

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