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Re: Hadoop counter

On Oct 19, 2012, at 10:27 PM, Lin Ma <[EMAIL PROTECTED]> wrote:

> Thanks for the detailed reply Mike, I learned a lot from the discussion.
>
> - I just want to confirm with you that, supposing in the same job, when a specific task completed (and counter is aggregated in JT after the task completed from our discussion?), the other running task in the same job cannot get the updated counter value from the previous completed task? I am asking this because I am thinking whether I can use counter to share a global value between tasks.

Yes that is correct.
While I haven't looked at YARN (M/R 2.0) , M/R 1.x doesn't have an easy way for a task to query the job tracker. This might have changed in YARN

> - If so, what is the traditional use case of counter, only use counter values after the whole job completes?
>
Yes the counters are used to provide data at the end of the job...
> BTW: appreciate if you could share me a few use cases from your experience about how counters are used.
>
Well you have your typical job data like the number of records processed, total number of bytes read,  bytes written...

But suppose you wanted to do some quality control on your input.
So you need to keep a track on the count of bad records.  If this job is part of a process, you may want to include business logic in your job to halt the job flow if X% of the records contain bad data.

Or your process takes input records and in processing them, they sort the records based on some characteristic and you want to count those sorted records as you processed them.

For a more concrete example, the Illinois Tollway has these 'fast pass' lanes where cars equipped with RFID tags can have the tolls automatically deducted from their accounts rather than pay the toll manually each time.

Suppose we wanted to determine how many cars in the 'Fast Pass' lanes are cheaters where they drive through the sensor and the sensor doesn't capture the RFID tag. (Note its possible that you have a false positive where the car has an RFID chip but doesn't trip the sensor.) Pushing the data in a map/reduce job would require the use of counters.

Does that help?

-Mike

> regards,
> Lin
>
> On Sat, Oct 20, 2012 at 5:05 AM, Michael Segel <[EMAIL PROTECTED]> wrote:
> Yeah, sorry...
>
> I meant that if you were dynamically creating a counter foo in the Mapper task, then each mapper would be creating their own counter foo.
> As the job runs, these counters will eventually be sent up to the JT. The job tracker would keep a separate counter for each task.
>
> At the end, the final count is aggregated from the list of counters for foo.
>
>
> I don't know how you can get a task to ask information from the Job Tracker on how things are going in other tasks.  That is what I meant that you couldn't get information about the other counters or even the status of the other tasks running in the same job.
>
> I didn't see anything in the APIs that allowed for that type of flow... Of course having said that... someone pops up with a way to do just that. ;-)
>
>
> Does that clarify things?
>
> -Mike
>
>
> On Oct 19, 2012, at 11:56 AM, Lin Ma <[EMAIL PROTECTED]> wrote:
>
>> Hi Mike,
>>
>> Sorry I am a bit lost... As you are thinking faster than me. :-P
>>
>> From your this statement "It would make sense that the JT maintains a unique counter for each task until the tasks complete." -- it seems each task cannot see counters from each other, since JT maintains a unique counter for each tasks;
>>
>> From your this comment "I meant that if a Task created and updated a counter, a different Task has access to that counter. " -- it seems different tasks could share/access the same counter.
>>
>> Appreciate if you could help to clarify a bit.
>>
>> regards,
>> Lin
>>
>> On Sat, Oct 20, 2012 at 12:42 AM, Michael Segel <[EMAIL PROTECTED]> wrote:
>>
>> On Oct 19, 2012, at 11:27 AM, Lin Ma <[EMAIL PROTECTED]> wrote:
>>
>>> Hi Mike,
>>>
>>> Thanks for the detailed reply. Two quick questions/comments,