Hi Jan,

I hear you.
One thing that is not clear to me is whether the global object map and
the master vertex can actually be the same thing (a centralizer). A
rationalization of these two objects could actually get us to have
only one added feature instead of two similar ones.
The one thing that doesn't convince me about the centralization of
computation would be the risk of creating a hot-spot, a computational
bottleneck that would just create a big serial computation path
before/after the parallel one.
My claim is that if the same thing can be done without a
centralization, it should be done that way because of obvious
performance issues. Centralization helps user's life, but it's often
the root for bottlenecks.
I guess that a centralizer could be defined hierarchically, like
aggregators, so once per worker and on the master by composing the
output of the workers'. This computation should be commutative and
associative, I guess I'm biased by the definition of aggregators (but
this requirement is quite obvious and therefore recurrent in
distributed systems for many reasons).

My claim is that i'd think twice before adding features because they
make user's life easier at first sight. For example, such a feature
would have been useful in mapreduce as well, but the focus on having a
simple and quite restrictive paradigm (like no communication between
mappers, which is something that everybody would want) *should* win on
the long term. These are only general concerns on the topic, it would
be very useful to discuss a more defined set of modifications to the
current model so that we could actually be more precise in the

Would you like working on such a definition?

On Tue, Apr 24, 2012 at 5:53 PM, Jan van der Lugt <janl...@gmail.com> wrote:
> Dear all,
> After having worked with Giraph for some weeks I feel like there are two
> features 'missing' in Giraph. It may be I simply missed them in the Javadoc,
> since the documentation is a work in progress at this point. In another
> Google Pregel-clone, Stanford GPS, it is possible to define a global object
> map, which can be used by all workers to share data, like the current phase
> in the algorithm. I have not been able to find such a feature in Giraph. Of
> course it would be possible to (ab)use aggregators for this, but I doubt
> this is the easiest or most efficient approach. Furthermore, it would be
> very helpful if there would be one special vertex that has the role of a
> master. This should not have to correspond to an existing vertex in the
> graph, it would be easier if it were not, actually. This master node would
> then be able to perform some centralized steps in the algorithm, of which
> the output can then be shared with other workers via the global object map.
> The master node could have the same interface as the workers (compute(),
> getAggregator(), getConf(), etc.). Again, it would be possible to solve this
> otherwise, for example in the VertexReader, but this would make code less
> elegant and would require picking a vertex id that does not exist in the
> graph, which is difficult if the input is not known in advance.
> I realize I am biased because my earlier experiences with Stanford GPS, but
> I feel these features will not be very difficult to implement or would add
> bulkiness to the API. They can make the implementation of many graph
> algorithms easier, though, because many of these algorithms have some notion
> of a centralized master node. During the next 5 months I will be working
> with Giraph for my Master's project, so I would be more than willing to help
> out implementing these features, ideally after receiving some pointers from
> more experienced Giraph developers.
> Regards,
> Jan van der Lugt

   Claudio Martella

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