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From Avery Ching <>
Subject Re: on the semantics of the combiner
Date Fri, 13 Jan 2012 18:51:20 GMT

I'm fine with this.  If we agree to return an Iterable, then we should 
make sure to either throw if the size of the Iterable > messages.size() 
to at the very least LOG.warn("This combiner is likely to be implemented 
wrong").  I prefer an exception, since we have no use case for expanding 
the set of messages.

Also, I'd like to have something in the javadoc saying something like 
"While the number of messages returned can be equal to the same number 
of messages that was inputted, the purpose of the combiner is to reduced 
the number of messages from the input."


On 1/13/12 9:34 AM, Claudio Martella wrote:
> Ok,
> I guess we can vote then about this, what do you think?
> Shall we take 72h?
> I'm +1 for returning an iterable that can be empty.
> I'm +1 for the returned iterable to be<= messages.size()
> On Tue, Jan 10, 2012 at 9:48 PM, Sebastian Schelter<>  wrote:
>> I think we should make the combiner return a list/iterable that can
>> potentially be empty. However we should assume that the number of
>> elements returned is smaller than or equal to the number of input
>> elements (whats the use of a combiner if this is not given?). I also
>> concur that the code should not depend on the combiner being applied
>> (similar to the way combiners work in hadoop).
>> --sebastian
>> 2012/1/10 Jakob Homan<>:
>>> A composite object would essentially be a wrapper around a list and
>>> introduce the need for all vertices to be ready to extract that list
>>> at all times.  For instance, a combiner passed 10 messages may be able
>>> to combine 7 of them but do nothing with the other three, leaving four
>>> messages.  If we allow zero or one return elements, the combiner would
>>> have to create a composite object with a list of those four messages,
>>> whereas if we return a list, it just skips that step and returns the
>>> four messages.  Additionally, the receiving vertex would have to
>>> handle the possibility of a composite object every time even though
>>> the combiner may or may not have been run during the superstep, or
>>> even included in that job (since combiners are optional to the job
>>> itself).  It would be better if one could write a Giraph application
>>> that was completely agnostic of whether or not a combiner was
>>> included.
>>> On Tue, Jan 10, 2012 at 12:00 PM, Claudio Martella
>>> <>  wrote:
>>>> I believe the argument of not letting users shoot their foot doesn't
>>>> stand :) Once you give them any API they have the power to do anything
>>>> wrong, as they already can with Giraph (or anything else for what it
>>>> matters), by designing an algorithm wrongly (which would be what it
>>>> would turn out to be a wrong combiner). It's definitely true that a
>>>> composite object would make the grouping (List<Group>) but I thought
>>>> we were talking about simplifying life to users :). I think it would
>>>> be more flexible (for the present and for the future) and also more
>>>> elegant,  but not necessarily a must (although it'd come practically
>>>> for free).
>>>> Very cool discussion.
>>>> On Tue, Jan 10, 2012 at 8:30 PM, Jakob Homan<>  wrote:
>>>>>> 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)!
>>>>> Agreed.
>>>>> On Tue, Jan 10, 2012 at 11:23 AM, Avery Ching<>
>>>>>> The general idea of combiners is to reduce the number of messages
>>>>>>   Combiners are purely an optimization and the application should
>>>>>> correctly without it (since it's never guaranteed to actually be
>>>>>>   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
>>>>>> 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
>>>>>>> that letting the user send k messages in the combiner is useful
>>>>>>> the grouping behavior shown by the label propagation example
should do
>>>>>>> so).
>>>>>>> On Tue, Jan 10, 2012 at 7:04 PM, Jakob Homan<>
>>>>>>>> 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
>>>>>>>> that combiners can generate messages bound for vertices other
than the
>>>>>>>> original destination, which I don't know if that has even
>>>>>>>> discussed.
>>>>>>>> On Tue, Jan 10, 2012 at 9:49 AM, Claudio Martella
>>>>>>>> <>    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<>
>>>>>>>>>>> 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
>>>>>>>>>> <>    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<>
>>>>>>>>>>>   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
>>>>>>>>>>>>> <>  
>>>>>>>>>>>>>> Yes, what is you say is completely
reasonable, you convinced me :)
>>>>>>>>>>>>>> On Mon, Jan 9, 2012 at 11:28 PM,
Avery Ching<>
>>>>>>>>>>>>>>   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
>>>>>>>>>>>>>>>> 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<>
>>>>>>>>>>>>>>>>   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()
>>>>>>>>>>>>>>>>>>   /**
>>>>>>>>>>>>>>>>>>    * 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
>>>>>>>>>>>>>>>>>>>> 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
>>>>>>>>>>>>>>>>>>>>> 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);
                  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 Martella
>>>>>>>>> --
>>>>>>>>>     Claudio Martella
>>>> --
>>>>     Claudio Martella

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