Avery Ching commented on GIRAPH-127:

I think this functionality is very useful and would actually replace a lot of 
the WorkerContext functionality. Sequential steps do need to be done between 
computations sometimes and "Pick k random initial cluster centers" is a good 

While WorkerContext allows us to do simple things, it is not as efficient for 
certain calculations (i.e. suppose all workers needed a global value from HDFS, 
it is cheaper to do once and broadcast the outcome rather than all workers 
hitting HDFS). Still, WorkerContext can be useful (say for dumping worker 
stats), so I wouldn't remove it, rather just give our users a broader choice on 
computation around supersteps. 

I see that the Master#compute() should have access to all aggregators to do its 
work.  Overall, I like the idea and would definitely like to see how we can add 
this in. Getting the interface right will be a little hard I think, but we can 
iterate over it.  

Basically, from what Semih has said is that we gain 
1)  A clean way to do sequential computation between supersteps
2)  Removing the extra superstep if we simulate this idea with a 'picked worker'
> Extending the API with a master.compute() function.
> ---------------------------------------------------
>                 Key: GIRAPH-127
>                 URL: https://issues.apache.org/jira/browse/GIRAPH-127
>             Project: Giraph
>          Issue Type: New Feature
>          Components: bsp, examples, graph
>            Reporter: Semih Salihoglu
> First of all, sorry for the long explanation to this feature.
> I want to expand the API of Giraph with a new function called 
> master.compute(), that would get called at the master before each superstep 
> and I will try to explain the purpose that it would serve with an example. 
> Let's say we want to implement the following simplified version of the 
> k-means clustering algorithm. Pseudocode below:
>  * Input G(V, E), k, numEdgesThreshold, maxIterations
>  * Algorithm:
>  * int numEdgesCrossingClusters = Integer.MAX_INT;
> *  int iterationNo = 0;
>  * while ((numEdgesCrossingCluster > numEdgesThreshold) && iterationNo < 
> maxIterations) {
>  *    iterationNo++;
>  *    int[] clusterCenters = pickKClusterCenters(k, G);
>  *    findClusterCenters(G, clusterCenters);
>  *    numEdgesCrossingClusters = countNumEdgesCrossingClusters();
>  * }
> The algorithm goes through the following steps in iterations:
> 1) Pick k random initial cluster centers
> 2) Assign each vertex to the cluster center that it's closest to (in Giraph, 
> this can be implemented in message passing similar to how ShortestPaths is 
> implemented):
> 3) Count the nuimber of edges crossing clusters
> 4) Go back to step 1, if there are a lot of edges crossing clusters and we 
> haven't exceeded maximum number of iterations yet.
> In an algorithm like this, step 2 and 3 are where most of the work happens 
> and both parts have very neat message-passing implementations. I'll try to 
> give an overview without going into the details. Let's say we define a Vertex 
> in Giraph to hold a custom Writable object that holds 2 integer values and 
> sends a message with upto 2 integer values.
> Step 2 is very similar to ShortestPaths algorithm and has two stages: In the 
> first stage, each vertex checks to see whether or not it's one of the cluster 
> centers. If so, it assigns itself the value (id, 0), otherwise it assigns 
> itself (Null, Null). In the 2nd stage, the vertices assign themselves to the 
> minimum distance cluster center by looking at their neighbors (cluster 
> centers, distance) values (received as 2 integer messages) and their current 
> values, and changing their values if they find a lower distance cluster 
> center. This happens in x number of supersteps until every vertex converges.
> Step 3, counting the number of edges crossing clusters, is also very easy to 
> implement in Giraph. Once each vertex has a cluster center, the number of 
> edges crossing clusters can be counted by an aggregator, let's say called 
> "num-edges-crossing". It would again have two stages: First stage, every 
> vertex just sends its cluster id to all its neighbors. Second stage, every 
> vertex looks at their neighbors' cluster ids in the messages, and for each 
> cluster id that is not equal to its own cluster id, it increments 
> "num-edges-crossing" by 1.
> The other 2 steps, step 1 and 4, are very simple sequential computations. 
> Step 1 just picks k random vertex ids and puts it into an aggregator. Step 4 
> just compares "num-edges-crossing" by a threshold and also checks whether or 
> not the algorithm has exceeded maxIterations (not supersteps but iterations 
> of going through Steps 1-4). With the current API, it's not clear where to do 
> these computations. There is a per worker function preSuperstep() that can be 
> implemented, but if we decide to pick a special worker, let's say worker 1,  
> to pick the k vertices then we'd waste an entire superstep where only worker 
> 1 would do work, (by picking k vertices  in preSuperstep() and put them into 
> an aggregator), and all other workers would be idle. Trying to do this in 
> worker 1 in postSuperstep() would not work either because, worker 1 needs to 
> know that all the vertices have converged to understand that it's time to 
> pick k vertices or it's time do check in step 4, which would only be 
> available to it in the beginning of the next superstep.
> A master.compute() extension would run at the master and before the superstep 
> and would modify the aggregator that would keep the k vertices before the 
> aggregators are broadcast to the workers, which are all very short sequential 
> computations, so they would not waste resources the way a preSuperstep() or 
> postSuperstep() approach would do. It would also enable running new 
> algorithms like kmeans that are composed of very vertex-centric computations 
> glued together by small sequential ones. It would basically boost Giraph with 
> sequential computation in a non-wasteful way.
> I am a phd student at Stanford and I have been working on my own BSP/Pregel 
> implementation since last year. It's called GPS. I haven't distributed it, 
> mainly because in September I learned about Giraph and I decided to slow down 
> on working on it :). We have basically been using GPS as our own research 
> platform. The source code for GPS is here if any one is interested 
> (https://subversion.assembla.com/svn/phd-projects/gps/trunk/). We have the 
> master.compute() feature in GPS, and here's an example of KMeans 
> implementation in GPS with master.compute(): 
> (https://subversion.assembla.com/svn/phd-projects/gps/trunk/src/java/gps/examples/kmeans/).
>  (Aggregators are called GlobalObjects in GPS). There is another example 
> (https://subversion.assembla.com/svn/phd-projects/gps/trunk/src/java/gps/examples/randomgraphcoarsening/),
>  which I'll skip explaining because it's very detailed and would make the 
> similar points that I am trying to make with k-means. Master.compute() in 
> general would make it possible to glue together any graph algorithm that is 
> composed of multiple stages with different message types and computations 
> that is conducive to run with vertex.compute(). There are many examples of 
> such algorithms: recursive partitioning, triangle counting, even much simpler 
> things like finding shortests paths for 100 vertices in pieces (first to 5 
> vertices, then to another 5, then to another 5, etc..), which would be good 
> because trying to find shortests paths to 100 vertices require a very large 
> messages (would need to store 100 integers per message)).
> If the Giraph team approves, I would like to take a similar approach in 
> implementing this feature in Giraph as I've done in GPS. Overall:
> Add a Master.java to org.apache.giraph.graph, that is default Master, with a 
> compute function that by default aggregates all aggregators and does the 
> check of whether or not the computation has ended (by comparining numVertices 
> with numFinishedVertices). This would be a refactoring of 
> org.apache.giraph.graph.BspServiceMaster class (as far as I can see).
> Extend GiraphJob to have a setMaster() method to set a master class (by 
> default it would be the default master above)
> The rest would be sending the custom master class to probably all workers but 
> only the master would instantiate it with reflection. I need to learn more on 
> how to do these, I am not familiar with that part of the Giraph code base yet.

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