This is a frequent asked question:
In hadoop 2, Secondary Name Node can be implemented in two ways:
1. With HA (High Availability Cluster): if you are setting up HA cluster then you may not need to use Secondary namenode because standby namenode keep its state synchronized with the Active namenode.
The HDFS NameNode High Availability feature enables you to run redundant NameNodes in the same cluster in an Active/Passive configuration with a hot standby.Both NameNode require the same type of hardware configuration.In HA hadoop cluster Active NameNode reads and write metadata information in Separate JournalNode.
In the event of failover, standby NameNode will ensure that its namespace is completely updated according to edit logs before it is changes to active state. So there is no need of Secondary NameNode in this Cluster Setup.
2. Without HA: you can have a hadoop setup without standby node. Then the secondary NameNode will act as you already mentioned in Hadoop 1.x