Yes, what is you say is completely reasonable, you convinced me :)

On Mon, Jan 9, 2012 at 11:28 PM, Avery Ching <ach...@apache.org> wrote:
> Combiners should be commutative and associative.  In my opinion that means
> reducing to a single message or none at all.  Can you think of a case when
> more than 1 message should be returned from a combiner?  I know that
> returning null isn't preferable in general, but I think that functionality
> (returning no messages), is nice to have and isn't a huge amount of work on
> our side.
>
> Avery
>
>
> On 1/9/12 12:13 PM, Claudio Martella wrote:
>>
>> To clarify, I was not discussing the possibility for combine to return
>> null. I see why it would be useful, given that combine returns M,
>> there's no other way to let combiner ask not to send any message,
>> although i agree with Jakob, I also believe returning null should be
>> avoided but only used, roughly, as an init value for a
>> reference/pointer.
>> Perhaps, we could, but i'm just thinking out loud here, let combine()
>> return Iterable<M>, basicallly letting it define what to combine to
>> ({0, 1, k } messages). It would be a powerful extension to the model,
>> but maybe it's too much.
>>
>> As far as the size of the messages parameter, I agree with you that 0
>> messages gives nothing to combine and it would be somehow awkward, it
>> was more a matter of synching it with the other methods getting the
>> messages parameter.
>> Probably, having a more clear javadoc will do the job here.
>>
>> What do you think?
>>
>> On Mon, Jan 9, 2012 at 8:42 PM, Jakob Homan<jgho...@gmail.com>  wrote:
>>>
>>> I'm not a big fan of returning null as it adds extra complexity to the
>>> calling code (null checks, or not, since people usually will forget
>>> them).  Avery is correct that combiners are application specific.  Is
>>> it conceivable that one would want to write a combiner that returned
>>> something for an input of no parameters, ie combining the empty list
>>> doesn't return the empty list?  I imagine for most combiners,
>>> combining a single message would result in that message.
>>>
>>> On Mon, Jan 9, 2012 at 11:28 AM, Avery Ching<ach...@apache.org>  wrote:
>>>>
>>>> The javadoc for VertexCombiner#combine() is
>>>>
>>>>  /**
>>>>   * Combines message values for a particular vertex index.
>>>>   *
>>>>   * @param vertexIndex Index of the vertex getting these messages
>>>>   * @param msgList List of the messages to be combined
>>>>   * @return Message that is combined from {@link MsgList} or null if no
>>>>   *         message it to be sent
>>>>   * @throws IOException
>>>>   */
>>>>
>>>> I think we are somewhat vague on what a combiner can return to support
>>>> various use cases.  A combiner should be particular to a particular
>>>> compute() algorithm.  I think it should be legal to return null from a
>>>> combiner, in that case, no message should be sent to that vertex.
>>>>
>>>> It seems like it would be an overhead to call a combiner when there are
>>>> 0
>>>> messages.  I can't see a case where that would be useful.  Perhaps we
>>>> should
>>>> change the javadoc to insure that msgList must contain at least one
>>>> message
>>>> to have combine() being called.
>>>>
>>>> Avery
>>>>
>>>>
>>>> On 1/9/12 5:37 AM, Claudio Martella wrote:
>>>>>
>>>>> Hi Sebastian,
>>>>>
>>>>> yes, that was my point, I agree completely with you.
>>>>> Fixing my test was not the issue, my question was whether we want to
>>>>> define explicitly the semantics of this scenario.
>>>>> Personally, I believe the combiner should be ready to receive 0
>>>>> messages, as it's the case of BasicVertex::initialize(), putMessages()
>>>>> and compute(), and act accordingly.
>>>>>
>>>>> In the particular example, I believe the SimpleSumCombiner is bugged.
>>>>> It's true that the sum of no values is 0, but it's also true that the
>>>>> null return semantics of combine() is more suitable for this exact
>>>>> situation.
>>>>>
>>>>>
>>>>> On Mon, Jan 9, 2012 at 2:21 PM, Sebastian Schelter<s...@apache.org>
>>>>>  wrote:
>>>>>>
>>>>>> I think we currently implicitly assume that there is at least one
>>>>>> element in the Iterable passed to the combiner. The messaging code
>>>>>> only
>>>>>> invokes the combiner only if at least one message for the target
>>>>>> vertex
>>>>>> has been sent.
>>>>>>
>>>>>> However, we should not rely on implicit implementation details but
>>>>>> explicitly specify the semantics of combiners.
>>>>>>
>>>>>> --sebastian
>>>>>>
>>>>>> On 09.01.2012 13:29, Claudio Martella wrote:
>>>>>>>
>>>>>>> Hello list,
>>>>>>>
>>>>>>> for GIRAPH-45 I'm touching the incoming messages and hit an
>>>>>>> interesting problem with the combiner semantics.
>>>>>>> currently, my code fails testBspCombiner for the following reason:
>>>>>>>
>>>>>>> SimpleSumCombiner::compute() returns a value even if there are no
>>>>>>> messages in the iterator (in this case it returns 0) and for this
>>>>>>> reason the vertices get activated at each superstep.
>>>>>>>
>>>>>>> At each superstep, under-the-hood, I pass the combiner for each
>>>>>>> vertex
>>>>>>> an Iterable, which can be empty:
>>>>>>>
>>>>>>>     public Iterable<M>    getMessages(I vertexId) {
>>>>>>>       Iterable<M>    messages = inMessages.getMessages(vertexId);
>>>>>>>       if (combiner != null) {
>>>>>>>               M combinedMsg;
>>>>>>>               try {
>>>>>>>                       combinedMsg = combiner.combine(vertexId,
>>>>>>> messages);
>>>>>>>               }  catch (IOException e) {
>>>>>>>                       throw new RuntimeException("could not combine",
>>>>>>> e);
>>>>>>>               }
>>>>>>>               if (combinedMsg != null) {
>>>>>>>                       List<M>    tmp = new ArrayList<M>(1);
>>>>>>>                       tmp.add(combinedMsg);
>>>>>>>                       messages = tmp;
>>>>>>>               } else {
>>>>>>>                       messages = new ArrayList<M>(0);
>>>>>>>               }
>>>>>>>       }
>>>>>>>       return messages;
>>>>>>>     }
>>>>>>>
>>>>>>> the Iterable returned by this methods is passed to
>>>>>>> basicVertex.putMessages() right before the compute().
>>>>>>> Now, the question is: who's wrong? The combiner code that returns a
>>>>>>> sum of 0 over no values, or the framework that calls the combiner
>>>>>>> with
>>>>>>> 0 messages?
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>
>>
>>
>



-- 
   Claudio Martella
   claudio.marte...@gmail.com

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