hadoop 1 can have one named node. Hadoop2 we can have multiple named nodes.
This technique is called as named node federation. For each primary node there will be
secondary node. Federation is for load sharing. secondary node is for fault tolerance.
What is speculative execution?
if all the nodes except few nodes are small from where job is getting executed.
After certain level of waiting, the same code will be executed in differnt machines.
Both of the code will be keep on executing, whichever gives the result first then that
results are considered rest other machine gets ignored.
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