Question: When A Client Communicates With The HDFS File System It Needs To Communicate With *?

What is WebHDFS?

WebHDFS provides web services access to data stored in HDFS.

At the same time, it retains the security the native Hadoop protocol offers and uses parallelism, for better throughput.

To enable WebHDFS (REST API) in the name node and data nodes, you must set the value of dfs.


How do I write an HDFS file?

HDFS follow Write once Read many models. So we cannot edit files already stored in HDFS, but we can append data by reopening the file. In Read-Write operation client first, interact with the NameNode. NameNode provides privileges so, the client can easily read and write data blocks into/from the respective datanodes.

What happens when two clients try to write into the same HDFS file?

When one client is already writing the file, the other client cannot open the file in write mode. When the client requests the NameNode to open the file for writing, NameNode provides lease to the client for writing to the file. So, if another client wants to write in the same file it will be rejected by the Namenode.

How does HDFS writing work?

HDFS write operationTo write a file inside the HDFS, the client first interacts with the NameNode. … NameNode then provides the address of all DataNodes, where the client can write its data. … If the file already exists in the HDFS, then file creation fails, and the client receives an IO Exception.

What is the default HDFS replication factor?

Each block has multiple copies in HDFS. A big file gets split into multiple blocks and each block gets stored to 3 different data nodes. The default replication factor is 3. Please note that no two copies will be on the same data node.

What is the default HDFS block size?

64 MBThe size of the data block in HDFS is 64 MB by default, which can be configured manually. In general, the data blocks of size 128MB is used in the industry.

What is Hadoop API?

This is a specification of the Hadoop FileSystem APIs, which models the contents of a filesystem as a set of paths that are either directories, symbolic links, or files.

What is edge node in Hadoop?

The interfaces between the Hadoop cluster any external network are called the edge nodes. These are also called gateway nodes as they provide access to-and-from between the Hadoop cluster and other applications. Administration tools and client-side applications are generally the primary utility of these nodes.

Does Hdfs allow a client to read a file that is already opened for writing?

Yes, the client can read the file which is already opened for writing.

How do Hadoop nodes communicate?

When you install Hadoop, you enable ssh and create ssh keys for the Hadoop user. This lets Hadoop communicate between the nodes by using RCP (remote procedure call) without having to enter a password. Formally this abstraction on top of the TCP protocol is called Client Protocol and the DataNode Protocol.

What mechanisms Hadoop uses to make NameNode resilient to failure *?

Q 17 – What mechanisms Hadoop uses to make namenode resilient to failure. A – Take backup of filesystem metadata to a local disk and a remote NFS mount.

WebHDFS REST APIGet Content Summary of a Directory.Get File Checksum.Get Home Directory.Set Permission.Set Owner.Set Replication Factor.Set Access or Modification Time.

How does Hadoop make the system more resilient?

HDFS is resilient (even in case of node failure) The file system will continue to function even if a node fails. Hadoop accomplishes this by duplicating data across nodes.

What is HDFS block in Hadoop?

Hadoop HDFS split large files into small chunks known as Blocks. Block is the physical representation of data. It contains a minimum amount of data that can be read or write. HDFS stores each file as blocks. HDFS client doesn’t have any control on the block like block location, Namenode decides all such things.

How files are stored in HDFS?

HDFS exposes a file system namespace and allows user data to be stored in files. Internally, a file is split into one or more blocks and these blocks are stored in a set of DataNodes. The NameNode executes file system namespace operations like opening, closing, and renaming files and directories.

When a client communicates with the HDFS file system it needs to communicate with?

The Client communication to HDFS happens using Hadoop HDFS API. Client applications talk to the NameNode whenever they wish to locate a file, or when they want to add/copy/move/delete a file on HDFS. The NameNode responds the successful requests by returning a list of relevant DataNode servers where the data lives.

How does a client read a file from HDFS?

Read Operation In HDFSA client initiates read request by calling ‘open()’ method of FileSystem object; it is an object of type DistributedFileSystem.This object connects to namenode using RPC and gets metadata information such as the locations of the blocks of the file.More items…•Jan 9, 2021

How do I check my HDFS replication factor?

Try to use command hadoop fs -stat %r /path/to/file , it should print the replication factor. The second column in the output signify replication factor for the file and for the folder it shows – , as shown in below pic. which shows the replication factors of all the /parent/path contents on the second column.

How do I access WebHDFS?

Steps to enable WebHDFS:Enable WebHDFS in HDFS configuration file. ( hdfs-site.xml) Set dfs. webhdfs. enabled as true.Restart HDFS daemons.We can now access HDFS with the WebHDFS API using Curl calls.Jul 17, 2014

What is difference between cluster and node?

Nodes store and process data. Nodes can be a physical computer or a virtual machine (VMs). VMs are software programs in the cloud that allow you to emulate a physical computing environment with its own operating system (OS) and applications. … A cluster is a group of servers or nodes.

Where is the HDFS replication factor controlled?

You can check the replication factor from the hdfs-site. xml fie from conf/ directory of the Hadoop installation directory. hdfs-site. xml configuration file is used to control the HDFS replication factor.