Why does sortBy transformation trigger a Spark job

Question

As far as I know only spark Actions can trigger a spark job, not any transformation as Spark follows lazy evaluation.

So when I execute the sortBy command, the spark web UI shows it as a spark job trigger.

Any idea why it's happening? 

Answer 1

Actually, sortBy/sortByKey depends on RangePartitioner (JVM). So when you run a sortBy/sortByKey, the partitioner is initialized which samples input RDD to compute partition boundaries. The actual sorting will happen only when an action is invoked.

The job in Web UI reflects this process.