Hadoop Datanode, namenode, secondary-namenode, job-tracker and task-tracker

Question

I am new in hadoop so I have some doubts. If the master-node fails what happened the hadoop cluster? Can we recover that node without any loss? Is it possible to keep a secondar

Answer 1

Although, It is too late to answer your question but just It may help others..

First of all let me Introduce you with Secondary Name Node:

It Contains the name space image, edit log files' back up for past one hour (configurable). And its work is to merge latest Name Node NameSpaceImage and edit logs files to upload back to Name Node as replacement of the old one. To have a Secondary NN in a cluster is not mandatory.

Now coming to your concerns..

• If the master-node fails what happened the hadoop cluster?

Supporting Frail's answer, Yes hadoop has single point of failure so whole of your currently running task like Map-Reduce or any other that is using the failed master node will stop. The whole cluster including client will stop working.

• Can we recover that node without any loss?

That is hypothetical, Without loss it is least possible, as all the data (block reports) will lost which has sent by Data nodes to Name node after last back up taken by secondary name node. Why I mentioned least, because If name node fails just after a successful back up run by secondary name node then it is in safe state.

• Is it possible to keep a secondary master-node to switch automatically to the master when the current one fails?

It is staright possible by an Administrator (User). And to switch it automatically you have to write a native code out of the cluster, Code to moniter the cluster that will cofigure the secondary name node smartly and restart the cluster with new name node address.

• We have the backup of the namenode (Secondary namenode), so we can restore the namenode from Secondary namenode when it fails. Like this, How can we restore the data's in datanode when the datanode fails?

It is about replication factor, We have 3 (default as best practice, configurable) replicas of each file block all in different data nodes. So in case of failure for time being we have 2 back up data nodes. Later Name node will create one more replica of the data that failed data node contained.

• The secondary namenode is the backup of namenode only not to datenode, right?

Right. It just contains all the metadata of data nodes like data node address,properties including block report of each data node.

• If a node is failed before completion of a job, so there is job pending in job tracker, is that job continue or restart from the first in the free node?

HDFS will forcely try to continue the job. But again it depends on replication factor, rack awareness and other configuration made by admin. But if following Hadoop's best practices about HDFS then it will not get failed. JobTracker will get replicated node address to continnue.

• How can we restore the entire cluster data if anything happens?

By Restarting it.

• And my final question, can we use C program in Mapreduce (For example, Bubble sort in mapreduce)?

yes, you can use any programming language which support Standard file read write operations.

I Just gave a try. Hope it will help you as well as others.

*Suggestions/Improvements are welcome.*