where does region server present and other?

Question

1. Where does region servers present? Is it with data nodes or else the region servers, regions are present in different hardwares.

2. Does WAL consists of data of a table along with operation?

3. What does memstore does? It stored data of WAL means along with data?

4. What does zookeeper actually do?

Answer 1

Hope the information below helps:

1. Technically a Region server present on a Datanode in the Hadoop cluster. Regions are stored in HDFS.

2. The client issues an HTable.put(Put) request to the HRegionServer, which hands the details to the matching HRegion instance. The first step is to write the data to the writeahead log (the WAL), represented by the HLog class.

3. Once the data is written to the WAL, it is placed in the MemStore. At the same time, it is checked to see if the MemStore is full and, if so, a flush to disk is requested. The request is served by a separate thread in the HRegionServer, which writes the data to a new HFile located in HDFS.

4. ZooKeeper co-ordinates the communication between the client and Hbase Cluster.

Answer 2

1.The Region servers are present in the same place whee the data node lies. They are responsble to store the data as 3 replicas in hdfs.

2.WAL is something like transaction log inSQL server it sores all operations and executes its operations when some thing failed while storing(inorder to lost data it will provide as backup).

3.While writing the data initially stores into WAL then to memstore when the limit of memstore(threshold level) is reached then data is flushed to hflie wich is present in same store.

4.It stores the -.ROOT.- table which consists of info of cluster and it helps to client while reading data. ie:It providing path to find data to client.