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
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 <> 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 <> 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<>  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

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