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From ramkrishna vasudevan <ramkrishna.s.vasude...@gmail.com>
Subject Re: ByteBuffer Backed Cell - New APIs (HBASE-12358)
Date Fri, 05 Dec 2014 04:03:32 GMT
>>Is there example of the above usage pattern ?
Just take cases of Filters and CP where a Cell is exposed to the user in
the read path and that cell could be having hasArray - false (Cells backed
by DBB) or true (Cells that are coming from Memstore).
>>Within HBase core, we can
make sure the above pattern doesn't exist, right ?
In HBase core code we could definitely avoid the pattern and infact always
with getXXXBuffer everywhere (use getXXXOffset and getXXXLength) depends on
either (1) or (2) approach that we take. (1) could be preferable as
multiple new small objects can be avoided.
Also would help in KeyValue type Cells also to be used with getXXXBuffer
along with getXXXOffset and getXXXLength.

Regards
Ram


On Fri, Dec 5, 2014 at 9:24 AM, Ted Yu <yuzhihong@gmail.com> wrote:

> Thanks for the writeup, Ram.
>
> This feature is targeting 2.0 release, right ?
>
> bq. If one sees hasArray() as false (a DBB backed Cell) and uses the
> getXXXArray()
> API along with offset and length
>
> Is there example of the above usage pattern ? Within HBase core, we can
> make sure the above pattern doesn't exist, right ?
>
> Cheers
>
> On Thu, Dec 4, 2014 at 7:24 PM, ramkrishna vasudevan <
> ramkrishna.s.vasudevan@gmail.com> wrote:
>
> > Hi Devs
> >
> > This write up is to provide a brief idea  on why we need a BB backed cell
> > and what are the items that we need to take care before introducing new
> > APIs in Cell that are BB backed.
> >
> > Pls refer to https://issues.apache.org/jira/browse/HBASE-12358 also and
> > its
> > parent JIRA https://issues.apache.org/jira/browse/HBASE-11425 for the
> > history.
> >
> > Coming back to the discussion on new APIs, this discussion is based on
> > supporting BB in the read path (write path is not targeted now) so that
> we
> > could work with offheap BBs also. This would avoid copying of data from
> > BlockCache to the read path ByteBuffer.
> >
> > Assume we will be working with BBs in the read path, We will need to
> >  introduce *getXXXBuffer() *APIs and also *hasArray()* in Cell itself
> > directly.
> > If we try to extend the cell or create a new Cell then *everywhere we
> need
> > to do instanceOf check or do type conversion *and that is why adding new
> > APIs to Cell interface itself makes sense.
> >
> > Plan is to use this *getXXXBuffer()* API through out the read path
> *instead
> > of getXXXArray()*.
> >
> > Now there are two ways to use it
> >
> > 1) Use getXXXBuffer() along with getXXXOffset(), getXXXLength() like how
> we
> > use now for getXXXArray() APIs with the offset and length. Doing so would
> > ensure that every where in the filters and CP one has to just replace the
> > getXXXArray() with getXXXBuffer() and continue to use getXXXOffset() and
> > getXXXLength(). We would do some wrapping of the byte[] with a BB incase
> of
> > KeyValue type of cells so that getXXXBuffer along with offset and length
> > holds true everywhere. Note that here if hasArray is true(for KV case)
> then
> > getXXXArray() would also work.
> >
> > 2)The other way of using this is that use only getXXXBuffer() API and
> > ensure that the BB is always duplicated/sliced and only the portion of
> the
> > total BB is returned which represents the individual component of the
> Cell.
> > In this case there is no use of getXXXOffset() (as it is going to be 0)
> and
> > getXXXLength() is any way going to be the sliced BB's limit.
> >
> > But in the 2nd approach we may end up in creating lot of small objects
> even
> > while doing comparison.
> >
> > Now the next problem that comes is what to do with the getXXXArray()
> APIs.
> > If one sees hasArray() as false (a DBB backed Cell) and uses the
> > getXXXArray() API along with offset and length - what should we do.
> Should
> > we create a byte[] from the DBB and return it? Then in that case what
> would
> > should the *getXXXOffset() return for a getXXXBuffer or getXXXArray()?*
> >
> > If we go with the 2nd approach then getXXXBuffer() should be clearly
> > documented saying that it has to be used without getXXXOffset() and
> > getXXXLength() and use getXXXOffset() and getXXXLength() only with
> > getXXXArray().
> >
> > Now if a Cell is backed by on heap BB then we could definitely return
> > getXXXArray() also - but what to return in the getXXXOffset() would be
> > determined by what approach to use for getXXXBuffer(). (based on (1) and
> > (2)).
> >
> > We wanted to open up this topic now so that to get some feedback on what
> > could be an option here. Since it is an user facing Interface we need to
> be
> > careful with this.
> >
> > I would suggest that whenever a Cell is *BB backed*(Onheap or offheap)
> > always *hasArray() would be false* in that Cell impl.
> >
> > Every where we would use getXXXBuffer() along with getXXXOffest() and
> > getXXXLength(). Even in case of KV we could wrap the byte[] with BB so
> that
> > we have uniformity through the read code and we don't have too many 'if'
> > else conditions.
> >
> > When ever *hasArray() is false* - using getXXXArray() API would throw
> > *UnSupportedOperation
> > Exception*.
> >
> > As said if we want *getXXXArray()* to be supported as per the existing
> way
> > then getXXXBuffer() and getXXXOffset(), getXXXLength() should be clearly
> > documented.
> >
> > Thoughts!!!
> >
> > Regards
> > Ram & Anoop
> >
>

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