> Combiners can only modify the messages sent to a single vertex, so they can't 
> send messages to other vertices.
Yeah, the more I've thought about this, the more problematic it would
be.  These new messages may be generated upon arrival at the
destination vertex (since combiners can be run on the receiving vertex
before processing as well).  When would they be forwarded to their new
destinations at that point?  It would be possible to get into a
feedback loop of messages jumping around before a superstep could ever
actually be done.

That being said, our inability to think of a good application doesn't
mean there won't be one in the future, and it's probably better to be
more flexible than try to impose what appears optimal now.  The
benefit of forcing 0 or 1 message from a combiner seems less than the
flexibility of allowing another list of messages (which may or may not
be the same number of elements as the original, less than, or even
more than).

>Good discussion (it's making me really think about this)!

On Tue, Jan 10, 2012 at 11:23 AM, Avery Ching <ach...@apache.org> wrote:
> The general idea of combiners is to reduce the number of messages sent.
>  Combiners are purely an optimization and the application should work
> correctly without it (since it's never guaranteed to actually be called).
>  Combiners can only modify the messages sent to a single vertex, so they
> can't send messages to other vertices.  Any other work (i.e. sending
> messages) should be done by the vertex in the compute() method.
> While I think that grouping behavior could actually be implemented within a
> message object (still reducing the number of messages to 1 or 0) I suppose
> that in some simple cases (i.e. grouping), it might be easier by doing it in
> the combiner as you both have mentioned?  The only thing I suppose I'm
> concerned about is letting users do something that is not optimal.
>  Generally, expanding messages is not what you want your combiner to do.
>  Also, since grouping behavior can be implemented in the message object, it
> forces users to avoid shooting themselves in the foot.
> Good discussion (it's making me really think about this)!
> Avery
> On 1/10/12 10:32 AM, Claudio Martella wrote:
>> Ok, now i see where you're going. I guess that the thing here is that
>> the combiner would "act" like (on its behalf) D, and to do so
>> concretely it would probably need some local data related to D (edges
>> values? vertexvalue?).
>> I also think that k>  n is also possible in principle and we could let
>> the user decide whether to use this power or not, once/if we agree
>> that letting the user send k messages in the combiner is useful (and
>> the grouping behavior shown by the label propagation example should do
>> so).
>> On Tue, Jan 10, 2012 at 7:04 PM, Jakob Homan<jgho...@gmail.com>  wrote:
>>> Those two messages would have gone to D, been expanded to, say, 4,
>>> which would have then then been sent to, say, M.  This would save the
>>> sending of the two to D and send the 4 directly to M.  I'm not saying
>>> it's a great example, but it is legal.  This is of course assuming
>>> that combiners can generate messages bound for vertices other than the
>>> original destination, which I don't know if that has even been
>>> discussed.
>>> On Tue, Jan 10, 2012 at 9:49 AM, Claudio Martella
>>> <claudio.marte...@gmail.com>  wrote:
>>>> i'm not sure i understand what you'd save here. if the two messages
>>>> were going to be expanded to k messages on the destination worker D,
>>>> but you expand them on W, you end up sending k messages instead of 2.
>>>> right?
>>>> On Tue, Jan 10, 2012 at 6:26 PM, Jakob Homan<jgho...@gmail.com>  wrote:
>>>>>> it doesn't have to be expand, k, the number of elements returned by
>>>>>> the combiner, can still be smaller than n,
>>>>> Right.  Grouping would be the most common case.  It would be possible
>>>>> to be great than k, as well.  For instance, consider two messages,
>>>>> both generated on the same worker (W) by two two different vertices,
>>>>> both bound for another vertex, Z.  A combiner on W could get both of
>>>>> these messages, do some work on them, as it would have knowledge of
>>>>> both, and generate some arbitrary number of messages bound for other
>>>>> vertices (thus saving the shuffle/transfer of the original messages).
>>>>> On Tue, Jan 10, 2012 at 12:08 AM, Claudio Martella
>>>>> <claudio.marte...@gmail.com>  wrote:
>>>>>> it doesn't have to be expand, k, the number of elements returned by
>>>>>> the combiner, can still be smaller than n, the size of the messages
>>>>>> parameter. as a first example, you can imagine your vertex receiving
>>>>>> semantically-different classes/types of messages, and you can imagine
>>>>>> willing to be summarizing them in different messages, i.e. if your
>>>>>> messages come along with labels or just simply by the source vertex,
>>>>>> if required by the algorithm, think of label propagation to have just
>>>>>> an example, or some sort of labeled-pagerank.
>>>>>> On Tue, Jan 10, 2012 at 3:05 AM, Avery Ching<ach...@apache.org>
>>>>>>  wrote:
>>>>>>> I agree that C&A doesn't require it, however, I can't think of why I
>>>>>>> would
>>>>>>> want to use a combiner to expand the number of messages.  Can you?
>>>>>>> Avery
>>>>>>> On 1/9/12 3:57 PM, Jakob Homan wrote:
>>>>>>>>> In my opinion that means reducing to a single message or none at
>>>>>>>>> all.
>>>>>>>> C&A doesn't require this, however.  Hadoop's combiner interface, for
>>>>>>>> instance, doesn't require a single  or no value to be returned; it
>>>>>>>> has
>>>>>>>> the same interface as a reducer, zero or more values.  Would
>>>>>>>> adapting
>>>>>>>> the semantics of Giraph's combiner to return a list of messages
>>>>>>>> (possibly empty) make it more useful?
>>>>>>>> On Mon, Jan 9, 2012 at 3:21 PM, Claudio Martella
>>>>>>>> <claudio.marte...@gmail.com>    wrote:
>>>>>>>>> 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
>>>>>> --
>>>>>>    Claudio Martella
>>>>>>    claudio.marte...@gmail.com
>>>> --
>>>>    Claudio Martella
>>>>    claudio.marte...@gmail.com

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