Dear Matthew,
Yes my aggregator is commutative, and I want globally aggregated value from all
workers, and not the locally aggregated.
But I am getting locally aggregated value.
My query is :
Does Giraph call the aggregator class for the values returned by the local
aggregators or not?
- PuneetIIT Delhi, India
On Sunday, November 9, 2014 5:43 AM, Matthew Saltz <[email protected]>
wrote:
Hi Puneet,It's unclear to me what you're wanting in terms of aggregator
behavior. Are you saying you want an aggregator such that the final output is
the aggregated value just for a particular worker? With an aggregator you
should at least make sure the operations you're performing are commutative;
that is, the order in which items are aggregated should not matter unless it is
explicitly dealt with somehow. Otherwise you'll get unpredictable results. Best,
Matthew SaltzEl 08/11/2014 15:05, "Puneet Agarwal" <[email protected]>
escribió:
Hi All,In my algo, I use an Aggregator which takes a Text value. I have written
my custom aggregator class for this, as given below.
public class MyAgg extends BasicAggregator<Text> {...}
This works fine when running on my laptop with one worker.However, when running
it on the cluster, sometimes it does not return the correctly aggregated
value.It seems it is returning the locally aggregated value of one of the
workers.While it should have used my logic to decide which of the aggregated
values sent by various worker should be chosen as finally aggregated
values.(But in fact I have not written such a code anywhere, it is therefore
doing the best it could)
Following is how is my analysis about this issue.a. I guess every worker
aggregates the values locally.b. then there is a global aggregation step,
which simply compares the values sent by various aggregators.c. For global
aggregation it uses Text.compareTo() method. This method Text.compareTo() is a
default Hadoop implementation and does not include the logic of my program.d.
It seem it is because of the above the value returned by my aggregator in the
cluster is actually not globally aggregated, but the locally aggregated value
of one of the worker gets taken.
If the above analysis is correct, following is how I think I can solve this.I
should write my own class that implements Writable interface. In this class I
would also write a compareTo method as a result things will start working fine.
If it was using class MyAgg itself, to decide which of the values returned by
various workers should be taken as globally aggregated value then this problem
would not have occurred.
I seek your guidance whether my analysis is correct.
- PuneetIIT Delhi, India
