What are accumulators in Spark?

In Spark, an Accumulator is a distributed variable that can only be added to and allows for read and write operations in parallel processing. Accumulators are typically used to increment counts or sums in parallel operations, such as calculating the number of data under certain conditions. The value of an accumulator can only be read in the Driver program and not in the Executor program.

Using an accumulator can avoid the issue of data inconsistency caused by writing to shared variables in parallel operations. In Spark, an accumulator is a special shared variable whose value can only be incremented on the Executor side, not decremented, and its final value can be retrieved on the Driver side by calling the value method.

In Spark, accumulators can be created using the accumulator method from SparkContext, and operated on using the add method. The value of the accumulator can be accessed in the Driver program and reset using the reset method.