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I started to learn hbase and I don't understand how it scales linearly.

The problem is that before you install hbase you have to have an hdfs cluster. The HDFS cluster have a master node which can be only one in the whole cluster, so it is a bottleneck. Ofcourse we can run 1 more master node (it is possible to run only 1 more master node) but it will be in the standby state. As I understand hbase uses the HDFS cluster to store data. So, for me it is logically that it have no sense to run more than one Hmaster because all requests will go to the hdfs active master which performance can suffer if we have too much requests.

Also I don't understand properly do we need to install hbase on the same nodes with hdfs or separately. What are the benefits if we run hbase separately from HDFS. As for me it is logically to install hbase cluster on the same nodes with hdfs as in the following example:

HDFS active master - HMaster
HDFS standby master - HMaster backup
HDFS Data node - HRegion server

for me it is the most logically structure because if we separate hdfs master from hmaster then probability to loose hbase cluster will be two times bigger.

I will be very happy if someone can share information about all these stuff. Because I really don't understand how hbase can linearly scales and how it works with hdfs.

Oleksandr
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  • You can federate HDFS, which eliminates the single NN problem. Keep in mind that the NN is only used for meta operations (eg create a partition/WAL), which hbase is not doing much of if you configure it properly (only rotate on big enough WALs, split and compact big partitions). – Thomas Jungblut Aug 02 '16 at 18:27
  • I tried to read data from data node in the same moment when I cutted off the hdfs master server and it showed me an error. As I understand when we read / write something from the data nodes they first ask the master node for some information. If the master node is dead then we can't work with the cluster. I may be wrong but it is my assumption – Oleksandr Aug 02 '16 at 18:58
  • yes you are right, but that doesn't impair scalability, only fault tolerance. When you use HA HDFS you won't have that problem. See https://hadoop.apache.org/docs/r2.7.1/hadoop-project-dist/hadoop-hdfs/HDFSHighAvailabilityWithQJM.html – Thomas Jungblut Aug 02 '16 at 19:01
  • I know about HA but I talk about bottleneck. If all data nodes ask master node for some sort of information (doesn't matter what they ask) every read and write operation then our whole throughput and performance will be limited by the master node. If we have slow connection between data nodes and the master node then all requests will be very slow. – Oleksandr Aug 02 '16 at 19:18
  • why would the datanodes request so many information? it is just meta ops. – Thomas Jungblut Aug 02 '16 at 19:32
  • As i understand if we have 100 ms latency (very rarely but can be) between data nodes and the master server then each write / read request will always be more or equal to 100 ms – Oleksandr Aug 02 '16 at 19:39
  • you don't understand that hbase rarely communicates with the namenode. All appends and flushes go to the local datanode, with replication going to another. NN communication is only done on rotations for WAL and compactions / splits. – Thomas Jungblut Aug 02 '16 at 19:46
  • Let us [continue this discussion in chat](http://chat.stackoverflow.com/rooms/118952/discussion-between-alexandr-and-thomas-jungblut). – Oleksandr Aug 02 '16 at 19:48

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First if you want you can install HBase over any supported file system. It is not mandatory to use it over Hdfs but using it with Hdfs give advantage to it like Fault taulrence , Data replication, checksums etc. That's why it is recommended to use HBase over hdfs

Moreover although there is a bottleneck of namenode in hdfs but it does not effect HBase efficiency because it is not that every operation internal working is dependent on namenode of hdfs for instance Region servers serve data for reads and writes. When accessing data, clients communicate with HBase RegionServers directly while Region assignment, DDL (create, delete tables) operations are handled by the HBase Master process. Which means that reading and writing of data is independent of creating and deleting of table.

You can refer https://www.mapr.com/blog/in-depth-look-hbase-architecture for more details about hdfs.

Also see this webinar on HBase by lars george. https://m.youtube.com/watch?v=_HLoH_PgrLk

Hope this will clear your doubts.

siddhartha jain
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