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.


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
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.


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

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.


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,
               }  catch (IOException e) {
                       throw new RuntimeException("could not combine",
               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?

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