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From lars hofhansl <lhofha...@yahoo.com>
Subject Re: Limited cross row transactions
Date Wed, 18 Jan 2012 05:45:23 GMT
This thread is probably getting too long...

In HBase we have to let go of "global stuff". I submit that global transactions across 1000's
of nodes that can fail will never work adequately.
For that kind of consistency you will be hit in availability.

Like Megastore the trick is in creating a local grouping of entities that can participate
in local transactions.
If you limit the (consistent) index to child entities of parent entity you can form your index
like this:
parentKey1...
parentKey1.childTableName1.indexedField1
parentKey1.childTableName1.indexedField2
...
parentKey1.childTableName2.indexedField1
parentKey1.childTableName2.indexedField2
...
(assuming . cannot be in any parent key or child table name here, but you get the idea).


When scanning the parent you'd have to skip the index rows with a filter.
Within a parentKey you can find childKeys efficiently by scanning the index rows.

Since the parent and the index entries would sort together the table can be pre-split (or
one could have a simple prefix based balancer).

-- Lars

----- Original Message -----
From: Mikael Sitruk <mikael.sitruk@gmail.com>
To: dev@hbase.apache.org
Cc: 
Sent: Tuesday, January 17, 2012 3:07 PM
Subject: Re: Limited cross row transactions

Well i understand the limitation now, asking to be in the same region is
really hard constraint.
Even if this is on the same RS this is not enough, because after a restart,
regions may be allocated differently and now part of the data may be in one
region under server A and the other part under server B.

Well perhaps we need use case for better understanding, and perhaps finding
alternative.

The first use case i was thinking of is as follow -
I need to insert data with different access criteria, but the data inserted
should be inserted in atomic way.
In RDBMS i would have two table, insert data in the first one with key#1
and then in the second one with key #2 then commit.
In HBase i need to use different column family with key #1 (for atomicity)
then to manage a kind of secondary index to map key#2 to key #1 (perhaps
via co-processor) to have quick access to the data of key#2.
Having cross row trx, i would think of sing different keys under the same
table (and probably different cf too), without the need to have secondary
index, but again with the limitation it does not seems to be easily
feasible.

Mik.

On Wed, Jan 18, 2012 at 12:22 AM, Ted Yu <yuzhihong@gmail.com> wrote:

> People rely on RDBMS for the transaction support.
>
> Consider the following example:
> A highly de-normalized schema puts related users in the same region where
> this 'limited cross row transactions' works.
> After some time, the region has to be split (maybe due to good business
> condition).
> What should the HBase user do now ?
>
> Cheers
>
> On Tue, Jan 17, 2012 at 2:13 PM, Mikael Sitruk <mikael.sitruk@gmail.com
> >wrote:
>
> > Ted - My 2 cents as a user.
> > The user should know what he is doing, this is like a 'delete' operation,
> > this is less intuitive that the original delete in RDBMS, so the same
> will
> > be for this light transaction.
> > If the transaction fails because of cross region server then the design
> of
> > the user was wrong
> > if the transaction fails because of concurrent access, then he should be
> > able to re-read and reprocess its request.
> > The only problem is how to make sure in advance that the different rows
> > will be in the same RS?
> >
> > Lars - is the limitation is at the region or at the region server? It was
> > not so clear.
> >
> > Mikael.S
> >
> > On Tue, Jan 17, 2012 at 11:52 PM, Ted Yu <yuzhihong@gmail.com> wrote:
> >
> > > Back to original proposal:
> > > If client side grouping reveals that the batch of operations cannot be
> > > supported by 'limited cross row transactions', what should the user do
> ?
> > >
> > > Cheers
> > >
> > > On Tue, Jan 17, 2012 at 1:49 PM, Ted Yu <yuzhihong@gmail.com> wrote:
> > >
> > > > Whether Omid fits the bill is open to discussion.
> > > >
> > > > We should revisit HBASE-2315 and provide the support Flavio, et al
> > need.
> > > >
> > > > Cheers
> > > >
> > > >
> > > > On Tue, Jan 17, 2012 at 1:41 PM, Lars George <lars.george@gmail.com
> > > >wrote:
> > > >
> > > >> Hi Ted,
> > > >>
> > > >> Wouldn't Omid (https://github.com/yahoo/omid) help there? Or is
> that
> > > too
> > > >> broad? Just curious.
> > > >>
> > > >> Lars
> > > >>
> > > >> On Jan 17, 2012, at 4:36 PM, Ted Yu wrote:
> > > >>
> > > >> > Can we collect use case for 'limited cross row transactions'
> first ?
> > > >> >
> > > >> > I have been thinking about (unlimited) multi-row transaction
> support
> > > in
> > > >> > HBase. It may not be a one-man task. But we should definitely
> > > implement
> > > >> it
> > > >> > someday.
> > > >> >
> > > >> > Cheers
> > > >> >
> > > >> > On Tue, Jan 17, 2012 at 1:27 PM, lars hofhansl <
> lhofhansl@yahoo.com
> > >
> > > >> wrote:
> > > >> >
> > > >> >> I just committed HBASE-5203 (together with HBASE-3584 this
> > implements
> > > >> >> atomic row operations).
> > > >> >> Although a relatively small patch it lays the groundwork
for
> > > >> heterogeneous
> > > >> >> operations in a single WALEdit.
> > > >> >>
> > > >> >> The interesting part is that even though the code enforced
the
> > atomic
> > > >> >> operation to be a for single row, this is not required.
> > > >> >> It is enough if all involved KVs reside in the same region.
> > > >> >>
> > > >> >> I am not saying that we should add any high level concept
to
> HBase
> > > >> (such
> > > >> >> as the EntityGroups of Megastore).
> > > >> >>
> > > >> >> But, with a slight addition to the API (allowing a grouping
of
> > > multiple
> > > >> >> row operations) client applications have all the building
blocks
> to
> > > do
> > > >> >> limited cross row atomic operations.
> > > >> >> The client application would be responsible for either correctly
> > > >> >> pre-splitting the table, or a custom balancer has to be provided.
> > > >> >>
> > > >> >> The operation would fail if the regionserver determines that
it
> > would
> > > >> need
> > > >> >> data from multiple region servers.
> > > >> >>
> > > >> >> I think this needs at least minimal support from HBase and
cannot
> > > >> >> (efficiently or without adding more moving parts) by a client
API
> > > only.
> > > >> >>
> > > >> >>
> > > >> >> Comments? Is this worth pursuing? If so, I'll file a jira
and
> > > provide a
> > > >> >> patch.
> > > >> >>
> > > >> >> Thanks.
> > > >> >>
> > > >> >>
> > > >> >> -- Lars
> > > >> >>
> > > >> >>
> > > >>
> > > >>
> > > >
> > >
> >
> >
> >
> > --
> > Mikael.S
> >
>



-- 
Mikael.S


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