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) < mvallemil...@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) < > mvallemil...@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) < > mvallemil...@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.shey...@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) < > >>> mvallemil...@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 > > > > > > >
