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From Serega Sheypak <serega.shey...@gmail.com>
Subject Re: write availability
Date Tue, 07 Apr 2015 16:38:22 GMT
Marcelo, if you are comparing with Cassandra:
1. don't think about data replication/redundancy. It's out of HBase scope.
C* thinks about it, HBase doesn't HBase uses HDFS. So assume you never-ever
can lost the data. Assume, that HDFS configured properly.

2. HBase doesn't think in terms of replica. It's out if HBase scope.
It supposes that you have several files for table. And that's all.
Each RS takes it's own range of keys from table/ There is no keyspace in
HBase. There are tables. Each table has it's own key range. Each table has
it's own splits. There are no tokes/vnodes. You can split keys in HBase
manually of HBase will do it for you.
Assume we have a table. It has 1000 rowkeys starting from 0 to 999.
We decided to split it manually on two parts (regions in HBase terms):
0...900 and 901...999.
We need at least 1 region server to serve these regions. Assume we have 2
region servers and one takes region 0...900, the second takes 901...999.
If second RS which serves region 901...999 goes down you can't access this
data using HBase, but data still exists. We need to wait until other Region
server would take care of  region 901 ... 999.
C* solves too many problems at one time, HBase doesn't.

2015-04-07 18:04 GMT+02:00 Marcelo Valle (BLOOMBERG/ LONDON) <
mvallemilita@bloomberg.net>:

> Wellington,
>
> I might be misinterpreting this:
> http://stackoverflow.com/questions/13741946/role-of-datanode-regionserver-in-hbase-hadoop-integration
>
> But aren't HBase region servers and HDFS datanodes always in the same
> server? With a replication factor of 3, what happens if all 3 datanodes
> hosting that information go down and one of them come back, but with the
> disk intact? Considering from the time they went down to the time it went
> back HBase received new writes that would go to the same data node...
>
>
> From: user@hbase.apache.org
> Subject: Re: write availability
>
> The data is stored on files on hdfs. If a RS goes down, the master knows
> which regions were on that RS and which hdfs files contain data for these
> regions, so it will just assign the regions to others RS, and these others
> RS will have access to the regions data because it's stored on HDFS. The RS
> does not "own" the disk, this is HDFS job, so the recovery on this case is
> transparent.
>
>
> On 7 Apr 2015, at 16:51, Marcelo Valle (BLOOMBERG/ LONDON) <
> mvallemilita@bloomberg.net> wrote:
>
> > So if a RS goes down, it's assumed you lost the data on it, right?
> > HBase has replications on HDFS, so if a RS goes down it doesn't mean I
> lost all the data, as I could have the replicas yet... But what happens if
> all RS hosting a specific region goes down?
> > What if one RS from this one comes back again, but with the disk intact,
> with all the data it had before crashing?
> >
> >
> > From: user@hbase.apache.org
> > Subject: Re: write availability
> >
> > When a RS goes down, the Master will try to assign the regions on the
> remaining RSes. When the RS comes back, after a while, the Master balancer
> process will re-distribute regions between RS, so the given RS will be
> hosting regions, but not necessarily the one it used to host before it went
> down.
> >
> >
> > On 7 Apr 2015, at 16:31, Marcelo Valle (BLOOMBERG/ LONDON) <
> mvallemilita@bloomberg.net> wrote:
> >
> >>> So if the cluster is up, then you can insert records in to HBase even
> though you lost a RS that was handing a specific region.
> >>
> >> What happens when the RS goes down? Writes to that region will be
> written to another region server? Another RS assumes the region "range"
> while the RS is down?
> >>
> >> What happens when the RS that was down goes up again?
> >>
> >>
> >> From: user@hbase.apache.org
> >> Subject: Re: write availability
> >>
> >> I don’t know if I would say that…
> >>
> >> I read Marcelo’s question of “if the cluster is up, even though a RS
> may be down, can I still insert records in to HBase?”
> >>
> >> So if the cluster is up, then you can insert records in to HBase even
> though you lost a RS that was handing a specific region.
> >>
> >> But because he talked about syncing nodes… I could be misreading his
> initial question…
> >>
> >>> On Apr 7, 2015, at 9:02 AM, Serega Sheypak <serega.sheypak@gmail.com>
> wrote:
> >>>
> >>>> If I have an application that writes to a HBase cluster, can I count
> that
> >>> the cluster will always available to receive writes?
> >>> No, it's CP, not AP system.
> >>>> so everything get in sync when the other nodes get up again
> >>> There is no hinted backoff, It's not Cassandra.
> >>>
> >>>
> >>>
> >>> 2015-04-07 14:48 GMT+02:00 Marcelo Valle (BLOOMBERG/ LONDON) <
> >>> mvallemilita@bloomberg.net>:
> >>>
> >>>> If I have an application that writes to a HBase cluster, can I count
> that
> >>>> the cluster will always available to receive writes?
> >>>> I might not be able to read if a region server which handles a range
> of
> >>>> keys is down, but will I be able to keep writing to other nodes, so
> >>>> everything get in sync when the other nodes get up again?
> >>>> Or I might get no write availability for a while?
> >>
> >> The opinions expressed here are mine, while they may reflect a
> cognitive thought, that is purely accidental.
> >> Use at your own risk.
> >> Michael Segel
> >> michael_segel (AT) hotmail.com
> >
> >
>
>
>

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