Must Know Technologies for Hadoop developers

Having knowledge on Java is an essential skill to program in Hadoop - Hadoop is an open-source, Java-based programming framework that supports the processing of large data sets in a distributed computing environment. Based on Google’s MapReduce model, Hadoop distributes computing jobs and then combines results. The MapReduce scripts used here are written in Java.

you should also be comfortable with below Hadoop frameworks tools. 

Data injection	Storage and It's API - File system/NoSQL DB	Analytical Processing
		Programing	SQL on Hadoop
Apache Flume Apache Sqoop Apache Kafka Apache NiFi	Apache HDFS Apache HBase(column family) MongoDB(Document) Redis DataBase(Key-Value)	Apache MapReduce Apache Pig Apache Spark Apache Tez	Apache Hive Apache Drill Cloudera Impala Apache Phoenix Kylin

You should also have good understanding of  Service Programming frameworks (Apache Thrift, Apache Zookeeper), Serialization tools(Apache Avro) , Scheduling and Workflow tools (Apache Oozie), Security framework (Apache Sentry) and System deployment tools like Apache Ambari, Cloudera HUE, MAPR Admin UI