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From "Marcelo Valle (BLOOMBERG/ LONDON)" <mvallemil...@bloomberg.net>
Subject Re: write availability
Date Wed, 08 Apr 2015 14:12:04 GMT
Thank you all a lot for the answers!

From: esteban@cloudera.com 
Subject: Re: write availability


--
Cloudera, Inc.


On Tue, Apr 7, 2015 at 10:36 AM, Marcelo Valle (BLOOMBERG/ LONDON) <mvallemilita@bloomberg.net>
wrote:

Sorry, there is something I asked wrongly because I was understanding it wrongly. 
1 region server correspond to 1 namenode and 1 write to 1 name node will replicate to 3 datanodes...

Not really, but I think we understood the failure mode you were curious to know more about
:)
 

So to simplify the second question, what happens to the HBase cluster when 1 region server
is down?

The simple case is something like this: The HBase Master will get a notification from ZooKeeper
that the znode for this RS has expired and will start the recovery process which will look
up into the existing WALs on HDFS for this RS and will start the distributed log splitting
of this WALs across the cluster. Once replaying the edits (writes) found in the WALs completes,
the HBase Master will open the region on other RSs and reads and writes will be available
for the clients immediately. With read replicas enabled, only writes will not be available
until the log replay completes and that can features like the distributed log replay (HBASE-7006)
can help to speed up the process. HBase provides other features like replication which can
even help you further on HA and other disaster recovery scenarios.
 
if you have more questions pelase let us know!
esteban.
 

-Marcelo


From: Marcelo Valle (BLOOMBERG/ LONDON) 
Subject: Re: write availability

Esteban,

If I understood correctly what you said:

> "For the failure mode you mention if all DNs go down (not the NN) clients will be blocked
waiting for the acknowledge of a write to the DNs and after few retries the RS will consider
there was a failure writing to the WAL, the RS will attempt to roll the WAL for a last time
and if fails at this point the RS will consider this as a fatal condition and it will shutdown
it self. At this point the client probably ran out of retries and will throw an exception
to the application."

If this scenario happens, when will my application be available to accept writes for that
region again? When I do some manual intervention on the server? 

For example: support I split data by user ids, so each user is stored in a different region.
In the scenario above, my application (and also the HBase cluster) would be working for some
users and wouldn't be working for users whose user id is in a "down region" (a region where
all corresponding DNs are down, considering 1 DN per RS). Is this right?

-Marcelo.

From: esteban@cloudera.com 
Subject: Re: write availability


Hello Marcelo,

HBase has strong durability guarantees to avoid data loss. When a write arrives to a RegionServer
data will be persisted into a Write-Ahead-Log (on HDFS) and temporarily in the RegionServer
memory until the data from this memory store is flushed (also to HDFS).

For the point of view of a client that is writing to HBase, if it  receives a response for
a successful write operation (put, delete, append, increment) then we can guarantee that data
was correctly persisted to HDFS in the WAL and in case of a catastrophic failure of a RegionServer
we will be able to recover as others have mentioned.

For the failure mode you mention if all DNs go down (not the NN) clients will be blocked waiting
for the acknowledge of a write to the DNs and after few retries the RS will consider there
was a failure writing to the WAL, the RS will attempt to roll the WAL for a last time and
if fails at this point the RS will consider this as a fatal condition and it will shutdown
it self. At this point the client probably ran out of retries and will throw an exception
to the application.

If a single DN can recover before any of the RSs goes down, the writes will recover and the
client will get the acknowledge that data has been persisted to HDFS (even with a single DN
at this point), during this period the RS logs will warn that data is getting persisted with
a lower number of replicas and data could be at risk.

If you are further interested in the write path in HBase there is a really good blog post
from Jimmy Xiang about this topic: http://blog.cloudera.com/blog/2012/06/hbase-write-path

best,
esteban.


--
Cloudera, Inc.


On Tue, Apr 7, 2015 at 9:04 AM, Marcelo Valle (BLOOMBERG/ LONDON) <mvallemilita@bloomberg.net>
wrote:

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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