The building blocks of Hadoop

 Hadoop employs a master/slave architecture for both distributed storage and distributed computation. The distributed storage system is called the Hadoop Distributed File System (HDFS),More info visit:big data online course

On a fully configured cluster, "running Hadoop" means running a set of daemons, or resident programs, on the different servers in you network. These daemons have specific roles; some exists only on one server, some exist across multiple servers. The daemons include:

1. NameNode
2. DataNode
3. Secondary NameNode
4. JobTracker
5. TaskTracker

NameNode: The NameNode is the master of HDFS that directs the slave DataNode daemons to perform the low-level I/O tasks. It is the bookkeeper of HDFS; it keeps track of how your files are broken down into file blocks, which nodes store those blocks and the overall health of the distributed filesystem.

The server hosting the NameNode typically doesn't store any user data or perform any computations for a MapReduce program to lower the workload on the machine, hence memory & I/O intensive.

There is unfortunately a negative aspect to the importance of the NameNode - it's a single point of failure of your Hadoop cluster. For any of the other daemons, if their host fail for software or hardware reasons, the Hadoop cluster will likely continue to function smoothly or you can quickly restart it. Not so for the NameNode.

DataNode: Each slave machine in your cluster will host a DataNode daemon to perform the grunt work of the distributed filesystem - reading and writing HDFS blocks to actual files on the local file system

When you want to read or write a HDFS file, the file is broken into blocks and the NameNode will tell your client which DataNode each block resides in. Your client communicates directly with the DataNode daemons to process the local files corresponding to the blocks.

Furthermore, a DataNode may communicate with other DataNodes to replicate its data blocks for redundancy.This ensures that if any one DataNode crashes or becomes inaccessible over the network, you'll still able to read the files.

DataNodes are constantly reporting to the NameNode. Upon initialization, each of the DataNodes informs the NameNode of the blocks it's currently storing. After this mapping is complete, the DataNodes continually poll the NameNode to provide information regarding local changes as well as receive instructions to create, move, or delete from the local disk.

Secondary NameNode(SNN): The SNN is an assistant daemon for monitoring the state of the cluster HDFS. Like the NameNode, each cluster has one SNN, and it typically resides on its own machine as well. No other DataNode or TaskTracker daemons run on the same server. The SNN differs from the NameNode in that this process doesn't receive or record any real-time changes to HDFS. Instead, it communicates with the NameNode to take snapshots of the HDFS metadata at intervals defined by the cluster configuration.

As mentioned earlier, the NameNode is a single point of failure for a Hadoop cluster, and the SNN snapshots help minimize the downtime and loss of data.

There is another topic which can be covered under SNN, i.e., fsimage(filesystem image) file and edits file:

The HDFS namespace is stored by the NameNode. The NameNode uses a transaction log called the EditLog to persistently record every change that occurs to file system metadata. For example, creating a new file in HDFS causes the NameNode to insert a record into the EditLog indicating this. Similarly, changing the replication factor of a file causes a new record to be inserted into the EditLog. The NameNode uses a file in its local host OS file system to store the EditLog. The entire file system namespace, including the mapping of blocks to files and file system properties, is stored in a file called the FsImage. The FsImage is stored as a file in the NameNode’s local file system too.

JobTracker: Once you submit your code to your cluster, the JobTracker determines the execution plan by determining which files to process, assigns nodes to different tasks, and monitors all tasks as they're running. should a task fail, the JobTracker will automatically relaunch the task, possibly on a different node, up to a predefined limit of retries.

There is only one JobTracker daemon per Hadoop cluster. It's typically run on a server as a master node of the cluster.

TaskTracker: As with the storage daemons, the computing daemons also follow a master/slave architecture: the JobTracker is the master overseeing the overall execution of a MapReduce job and the TaskTracker manage the execution of individual tasks on each slave node.

Each TaskTracker is responsible for executing the individual tasks that the JobTracker assigns. Although there is a single TaskTracker per slave node, each TaskTracker can spawn multiple JVMs to handle many map or reduce tasks in parallel.

One responsibility of the TaskTracker is to constantly communicate with the JobTracker. If the JobTracker fails to receive a heartbeat from a TaskTracker within a specified amount of time, it will assume the TaskTracker has crashed and will resubmit the corresponding tasks to other nodes in the cluster.

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