This is part of the series Apache Spark bitesize.
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Apache Spark Accumulators:

We can think about accumulators like counters, distributed counters.
Any simple value can be accumulated.
Basically, there are types that can be merged.
The result type being accumulated is the same as the types of elements being merged.

These are variables that are "added" to through an associative operation and can, therefore, be efficiently supported in parallel and distributed.
For the supported accumulator of numeric value types, Spark creates an optimized MapReduce.
However, programmers can add support for new types as well by implementing the abstract class AccumulatorV2.

Let's look at a simple example:

val accum = sc.accumulator(0) // create accumulator
val dataFrame = sc.parallelize(Array(1, 2, 3, 4)) //create DataFrame
dataFrame.foreach(x => accum += x) // distributed operation
accum.value // can be called only from driver

What happened here? We created a new accumulator variable named accum, and a DataFrame named dataFrame.
We call foreach on the DataFrame and create the logic of - foreach x in DataFrame add its value to accum.
The foreach function acts in a distributed manner.
Later we can extract the value from the accumulator by calling .value Which can be called ( and return final answer ) only from the driver.

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