accumulator ID
accumulator name
partial value from a task, may be None if used on driver to describe a stage
total accumulated value so far, maybe None if used on executors to describe a task
:: DeveloperApi :: Information about an org.apache.spark.Accumulable modified during a task or stage.
Once this is JSON serialized the types of
updateandvaluewill be lost and be cast to strings. This is because the user can define an accumulator of any type and it will be difficult to preserve the type in consumers of the event log. This does not apply to internal accumulators that represent task level metrics.