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HBase, mail # dev - HBase Replication use cases


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Re: HBase Replication use cases
Himanshu Vashishtha 2012-04-14, 00:18
@Lars H:
WalEdits are per transaction (aka per row). And we do ship waledits,
so at the slave, it will be end of some transaction the master have
seen?

Idea is that the atomicity is a waledit. Can we have such a scenario
where end state does not correspond to the state at the end of any of
the row transactions in the master? It will be good to know.

Thanks.
On Fri, Apr 13, 2012 at 4:54 PM, lars hofhansl <[EMAIL PROTECTED]> wrote:
> Hey Lars,
>
> in a DR scenario (i.e. a DC falls into the ocean) we SLAs that allow for a certain amount of data loss.
> The main concern here would be that "rows" could be in a state that does not correspond to the state at the end of any of the row transactions in the source system, right?
>
> Or are you referring to even cross table consistency?
>
>
> -- Lars
>
>
> ----- Original Message -----
> From: Lars George <[EMAIL PROTECTED]>
> To: [EMAIL PROTECTED]; lars hofhansl <[EMAIL PROTECTED]>
> Cc:
> Sent: Thursday, April 12, 2012 11:13 PM
> Subject: Re: HBase Replication use cases
>
> Hi Lars,
>
> I am really curious how you will handle the possible (or say likely) inconsistencies between regions of the same table in case of a DR situation. This seems to be solely applications layer logic but on the other hand a lot of people will need something here. So the question is, could this be added to the code? The idea is, could we hint to the replication what schema we are using and it can therefore handle shipping the logs somewhat "transactional" on the receiving end? For example, it could record sequence IDs or even timestamps and when the originating cluster fails there is a mechanism on the receiving end that deletes all inconsistent changes, bringing it back to a well known checkpoint. The replication does ship the WAL edits so, this might be all that is needed, and some ZooKeeper magic there to synchronize the checkpoint across the region servers?
>
> Maybe I am seeing this wrong here, but how else would you recover in the case of a DR situation?
>
> Cheers,
> Lars
>
> On Apr 12, 2012, at 11:50 PM, lars hofhansl wrote:
>
>> Thanks Himanshu,
>>
>> we're planning to use Replication for cross DC replication for DR (and we added a bunch of stuff and fixed bugs in replication).
>>
>>
>> We'll have it always on (and only use stop/start_peer, which is new in 0.94+ to temporarily stop replication, rather than stop/start_replication)
>> HBASE-2611 is a problem. We did not have time recently to work on this.
>>
>> i) and ii) can be worked around by forcing a log roll on all region servers after replication was enabled. Replication would be considered started after the logs were
>> rolled... But that is quite annoying.
>>
>> Is iii) still a problem in 0.92+? I thought we fixed that together with a).
>>
>> -- Lars
>>
>> ________________________________
>> From: Himanshu Vashishtha <[EMAIL PROTECTED]>
>> To: [EMAIL PROTECTED]
>> Sent: Thursday, April 12, 2012 12:11 PM
>> Subject: HBase Replication use cases
>>
>> Hello All,
>>
>> I have been doing testing on the HBase replication (0.90.4, and 0.92 variants).
>>
>> Here are some of the findings:
>>
>> a) 0.90+ is not that great in handling out znode changes; in an
>> ongoing replication, if I delete a peer and a region server goes to
>> the znode to update the log status, the region server aborts itself
>> when it sees a missing znode.
>>
>> Recoverable Zookeeper seems to have fix this in 0.92+?
>>
>> 0.92 has lot of new features (start/stop handle, master master, cyclic).
>>
>> But there are corner cases with the start/stop switches.
>> i)  A log is en-queue when the replication state is set to true. When we
>> start the cluster, it is true and the starting region server takes the
>> new log into the queue. If I do a stop_replication, and there is a log
>> roll, and then I do a start_replication, the current log will not be
>> replicated, as it has missed the opportunity of being added to the queue.
>>
>> ii) If I _start_ a region server when the replication state is set to