When are accumulators truly reliable?

Question

I want to use an accumulator to gather some stats about the data I'm manipulating on a Spark job. Ideally, I would do that while the job computes the required transformations, but since Spark would re-compute tasks on different cases the accumulators would not reflect true metrics. Here is how the documentation describes this:

For accumulator updates performed inside actions only, Spark guarantees that each task’s update to the accumulator will only be applied once, i.e. restarted tasks will not update the value. In transformations, users should be aware of that each task’s update may be applied more than once if tasks or job stages are re-executed.

This is confusing since most actions do not allow running custom code (where accumulators can be used), they mostly take the results from previous transformations (lazily). The documentation also shows this:

val acc = sc.accumulator(0)
data.map(x => acc += x; f(x))
// Here, acc is still 0 because no actions have cause the `map` to be computed.

But if we add data.count() at the end, would this be guaranteed to be correct (have no duplicates) or not? Clearly acc is not used "inside actions only", as map is a transformation. So it should not be guaranteed.

On the other hand, discussion on related Jira tickets talk about "result tasks" rather than "actions". For instance here and here. This seems to indicate that the result would indeed be guaranteed to be correct, since we are using acc immediately before and action and thus should be computed as a single stage.

I'm guessing that this concept of a "result task" has to do with the type of operations involved, being the last one that includes an action, like in this example, which shows how several operations are divided into stages (in magenta, image taken from here):

So hypothetically, a count() action at the end of that chain would be part of the same final stage, and I would be guaranteed that accumulators used on the last map will no include any duplicates?

Clarification around this issue would be great! Thanks.

Answer 1

To answer the question "When are accumulators truly reliable ?"

Answer : When they are present in an Action operation.

As per the documentation in Action Task, even if any restarted tasks are present it will update Accumulator only once.

For accumulator updates performed inside actions only, Spark guarantees that each task’s update to the accumulator will only be applied once, i.e. restarted tasks will not update the value. In transformations, users should be aware of that each task’s update may be applied more than once if tasks or job stages are re-executed.

And Action do allow to run custom code.

For Ex.

val accNotEmpty = sc.accumulator(0)
ip.foreach(x=>{
  if(x!=""){
    accNotEmpty += 1
  }
})

But, Why Map+Action viz. Result Task operations are not reliable for an Accumulator operation?

1. Task failed due to some exception in code. Spark will try 4 times(default number of tries).If task fail every time it will give an exception.If by chance it succeeds then Spark will continue and just update the accumulator value for successful state and failed states accumulator values are ignored.
   Verdict : Handled Properly
2. Stage Failure : If an executor node crashes, no fault of user but an hardware failure - And if the node goes down in shuffle stage.As shuffle output is stored locally, if a node goes down, that shuffle output is gone.So Spark goes back to the stage that generated the shuffle output, looks at which tasks need to be rerun, and executes them on one of the nodes that is still alive.After we regenerate the missing shuffle output, the stage which generated the map output has executed some of it’s tasks multiple times.Spark counts accumulator updates from all of them.
   Verdict : Not handled in Result Task.Accumulator will give wrong output.
3. If a task is running slow then, Spark can launch a speculative copy of that task on another node.
   Verdict : Not handled.Accumulator will give wrong output.
4. RDD which is cached is huge and can't reside in Memory.So whenever the RDD is used it will re run the Map operation to get the RDD and again accumulator will be updated by it.
   Verdict : Not handled.Accumulator will give wrong output.

So it may happen same function may run multiple time on same data.So Spark does not provide any guarantee for accumulator getting updated because of the Map operation.

So it is better to use Accumulator in Action operation in Spark.

To know more about Accumulator and its issues refer this Blog Post - By Imran Rashid.

Answer 2

Accumulator updates are sent back to the driver when a task is successfully completed. So your accumulator results are guaranteed to be correct when you are certain that each task will have been executed exactly once and each task did as you expected.

I prefer relying on reduce and aggregate instead of accumulators because it is fairly hard to enumerate all the ways tasks can be executed.

• An action starts tasks.
• If an action depends on an earlier stage and the results of that stage are not (fully) cached, then tasks from the earlier stage will be started.
• Speculative execution starts duplicate tasks when a small number of slow tasks are detected.

That said, there are many simple cases where accumulators can be fully trusted.

val acc = sc.accumulator(0)
val rdd = sc.parallelize(1 to 10, 2)
val accumulating = rdd.map { x => acc += 1; x }
accumulating.count
assert(acc == 10)

Would this be guaranteed to be correct (have no duplicates)?

Yes, if speculative execution is disabled. The map and the count will be a single stage, so like you say, there is no way a task can be successfully executed more than once.

But an accumulator is updated as a side-effect. So you have to be very careful when thinking about how the code will be executed. Consider this instead of accumulating.count:

// Same setup as before.
accumulating.mapPartitions(p => Iterator(p.next)).collect
assert(acc == 2)

This will also create one task for each partition, and each task will be guaranteed to execute exactly once. But the code in map will not get executed on all elements, just the first one in each partition.

The accumulator is like a global variable. If you share a reference to the RDD that can increment the accumulator then other code (other threads) can cause it to increment too.

// Same setup as before.
val x = new X(accumulating) // We don't know what X does.
                            // It may trigger the calculation
                            // any number of times.
accumulating.count
assert(acc >= 10)

Answer 3

I think Matei answered this in the referred documentation:

As discussed on https://github.com/apache/spark/pull/2524 this is pretty hard to provide good semantics for in the general case (accumulator updates inside non-result stages), for the following reasons:

• An RDD may be computed as part of multiple stages. For example, if you update an accumulator inside a MappedRDD and then shuffle it, that might be one stage. But if you then call map() again on the MappedRDD, and shuffle the result of that, you get a second stage where that map is pipeline. Do you want to count this accumulator update twice or not?

• Entire stages may be resubmitted if shuffle files are deleted by the periodic cleaner or are lost due to a node failure, so anything that tracks RDDs would need to do so for long periods of time (as long as the RDD is referenceable in the user program), which would be pretty complicated to implement.

So I'm going to mark this as "won't fix" for now, except for the part for result stages done in SPARK-3628.