Explain to me the difference between Name Node and Secondary Name Node

Question

I am a newbie to Big Data thing. I have a doubt related to the Hadoop Architecture. Is it like the Secondary Name Node stand up as a backup to the Name Node?

If Yes,

Then, Why does Hadoop go to "Hadoop not available" state when the Name Node fails?

Please help me learn this.

Answer 1

First of all, 

Let us begin with the architecture.

Now the above image represents the architecture of Hadoop.

Let me explain u in detail.

Here you have the following

• Name Node 
• Data Node
• Secondary Name Node

Secondary Name Node acts as a buffer to the Name Node, Not as its Back up. It stores the intermediate updates of the FS in a file called edit-log and updates them to the Final FS Image when Name Node is inactive. 

We are currently updated to Hadoop Version 2.x and it is no more related to the Secondary Name Node. It is replaced by the Standby Name node Which acts as a stand by to the actual name node and handles the Name Node Failures.

Answer 2

Secondary namenode is just a helper for Namenode. It gets the edit logs from the namenode in regular intervals and applies to fsimage. Once it has new fsimage, it copies back to namenode. Namenode will use this fsimage for the next restart, which will reduce the startup time. Secondary Namenode's whole purpose is to have a checkpoint in HDFS. Its just a helper node for namenode. That’s why it also known as checkpoint node. But, It cant replace namenode on namenode's failure.

So, Namenode still is Single-Point-of-Failure. To overcome this issue; STANDBY-NAMENODE comes into picture.

It does three things:

1. merging fsimage and edits-log files. (Secondary-namenode's work)

2. receive online updates of the file system meta-data, apply them to its memory state and persist them on disks just like the name-node does.

Thus at any time the Backup node contains an up-to-date image of the namespace both in memory and on local disk(s).

      3. Cluster will switch over to the new name-node (this standby-node) if the active namenode dies.