reduceByKey: How does it work internally?

Let’s break it down to discrete methods and types. That usually exposes the intricacies for new devs:

pairs.reduceByKey((a, b) => a + b)

becomes

pairs.reduceByKey((a: Int, b: Int) => a + b)

and renaming the variables makes it a little more explicit

pairs.reduceByKey((accumulatedValue: Int, currentValue: Int) => accumulatedValue + currentValue)

So, we can now see that we are simply taking an accumulated value for the given key and summing it with the next value of that key. NOW, let’s break it further so we can understand the key part. So, let’s visualize the method more like this:

pairs.reduce((accumulatedValue: List[(String, Int)], currentValue: (String, Int)) => {
  //Turn the accumulated value into a true key->value mapping
  val accumAsMap = accumulatedValue.toMap   
  //Try to get the key's current value if we've already encountered it
  accumAsMap.get(currentValue._1) match { 
    //If we have encountered it, then add the new value to the existing value and overwrite the old
    case Some(value : Int) => (accumAsMap + (currentValue._1 -> (value + currentValue._2))).toList
    //If we have NOT encountered it, then simply add it to the list
    case None => currentValue :: accumulatedValue 
  }
})

So, you can see that the reduceByKey takes the boilerplate of finding the key and tracking it so that you don’t have to worry about managing that part.

Deeper, truer if you want

All that being said, that is a simplified version of what happens as there are some optimizations that are done here. This operation is associative, so the spark engine will perform these reductions locally first (often termed map-side reduce) and then once again at the driver. This saves network traffic; instead of sending all the data and performing the operation, it can reduce it as small as it can and then send that reduction over the wire.