NameNode and TaskNodes

• HDFS has a master/slave architecture. HDFS is comprised of interconnected clusters of nodes where files and directories reside. An HDFS cluster consists of a single node, known as a NameNode, that manages the file system namespace and regulates client access to files,a master server that manages the file system namespace and regulates access to files by clients.. In addition, data nodes (DataNodes) store data as blocks within files.
  
  
• Internally, a file is split into one or more blocks and these blocks are stored in a set of DataNodes. Within HDFS, The NameNode executes file system namespace operations like opening, closing, and renaming files and directories. It also determines the mapping of blocks to DataNodes, which handle read and write requests from HDFS clients. Data nodes also create, delete, and replicate data blocks according to instructions from the governing name node.



• The namenode maintains two in-memory tables, one which maps the blocks to datanodes (one block maps to 3 datanodes for a replication value of 3) and a datanode to block number mapping. Whenever a datanode reports a disk corruption of a particular block, the first table gets updated and whenever a datanode is detected to be dead (because of a node/network failure) both the tables get updated.
  
  
• Data nodes continuously loop, asking the name node for instructions. A name node can't connect directly to a data node; it simply returns values from functions invoked by a data node. Each data node maintains an open server socket so that client code or other data nodes can read or write data. The host or port for this server socket is known by the name node, which provides the information to interested clients or other data nodes.
  
  

• Some interesting facts about DataNode

  
  
  • All datanodes send a heartbeat message to the namenode every 3 seconds to say that they are alive. If the namenode does not receive a heartbeat from a particular data node for 10 minutes, then it considers that data node to be dead/out of service and initiates replication of blocks which were hosted on that data node to be hosted on some other data node.
  
  
  • The data nodes can talk to each other to rebalance data, move and copy data around and keep the replication high.
  
  
  • When the datanode stores a block of information, it maintains a checksum for it as well. The data nodes update the namenode with the block information periodically and before updating verify the checksums. If the checksum is incorrect for a particular block i.e. there is a disk level corruption for that block, it skips that block while reporting the block information to the namenode. In this way, namenode is aware of the disk level corruption on that datanode and takes steps accordingly.
  
  
  
  

  Communications protocols

  
  
• All HDFS communication protocols build on the TCP/IP protocol. HDFS clients connect to a Transmission Control Protocol (TCP) port opened on the name node, and then communicate with the name node using a proprietary Remote Procedure Call (RPC)-based protocol. Data nodes talk to the name node using a proprietary block-based protocol.




The File System Namespace



• HDFS supports a traditional hierarchical file organization. A user or an application can create directories and store files inside these directories. The file system namespace hierarchy is similar to most other existing file systems; one can create and remove files, move a file from one directory to another, or rename a file. HDFS does not yet implement user quotas. HDFS does not support hard links or soft links. However, the HDFS architecture does not preclude implementing these features.
• The NameNode maintains the file system namespace. Any change to the file system namespace or its properties is recorded by the NameNode. An application can specify the number of replicas of a file that should be maintained by HDFS. The number of copies of a file is called the replication factor of that file. This information is stored by the NameNode.




Data replication



• HDFS uses an intelligent replica placement model for reliability and performance. Optimizing replica placement makes HDFS unique from most other distributed file systems, and is facilitated by a rack-aware replica placement policy that uses network bandwidth efficiently. HDFS replicates file blocks for fault tolerance. An application can specify the number of replicas of a file at the time it is created, and this number can be changed any time after that. The name node makes all decisions concerning block replication.
• Large HDFS environments typically operate across multiple installations of computers. Communication between two data nodes in different installations is typically slower than data nodes within the same installation. Therefore, the name node attempts to optimize communications between data nodes. The name node identifies the location of data nodes by their rack IDs.