Hi all,

thanks for reporting the issue! I just looked into it. The VertexGroupReduce outputs two semantically different tuples: one for each vertex without the value (null) and one tuple representing the whole group including the value. As Till pointed out, this crashes if the value has no serializer for null values.


Since we cannot have two different output types for the same GroupReduce function, I propose using a Either<NullValue, VV> here. If there are no objections I will take the issue and fix it.

Again, thx for pointing it out.

Best,

Martin

On 15.09.2016 20:10, Vasiliki Kalavri wrote:
Hi,

thanks for looking into this Till! I'm not quite sure what the algorithm
behavior should be when the vertex value is null (probably skip the
record?). Let's wait for Martin's input.

Cheers,
-V.

On 15 September 2016 at 19:19, Olga Golovneva <melcha...@gmail.com> wrote:

Hi Till,

Thanks a lot for your help! I'll try to use another variable type in the
meantime.

Best regards,
Olga


Best regards,
Olga Golovneva

On Thu, Sep 15, 2016 at 1:03 PM, Till Rohrmann <trohrm...@apache.org>
wrote:

Hi Olga,

it’s indeed an error in Flink’s Summarization algorithm. The problem is
the
following: The vertex group value of the VertexGroupItem is null in the
VertexGroupReducer. This works in the SummarizationIT case because the
vertex value is of type String and the StringSerializer can deal with
null
values.

However, in your case where you use longs, it fails, because the
LongSerializer cannot handle null values. You can verify this behaviour
by
changing the vertex value type to String. Then everything should work
without a problem.

I’ve cc’ed Martin who can tell you probably more about the Summarization
algorithm. I’ve also opened a JIRA ticket [1] to fix this problem.

Thanks for reporting this bug.

[1] https://issues.apache.org/jira/browse/FLINK-4624

Cheers,
Till
​

On Thu, Sep 15, 2016 at 5:05 PM, Olga Golovneva <melcha...@gmail.com>
wrote:

Hi Till,

I've created a simple (Java) example to show you what's going on. The
code
is in attachment and shown below. This example creates simple graph
with
Double EV and Long VV. Then it runs Summarization, that should compute
a
condensed version of the input graph by grouping vertices and edges
based
on their values. I run this code with IntelliJ IDEA. The code executes
fine
until you want to see what is written in resulted edges (just uncomment
line 46, edgesOut.print();). Then it throws the following Exception:

_________EXCEPTION START_____________
Exception in thread "main" org.apache.flink.runtime.
client.JobExecutionException:
Job execution failed.
at org.apache.flink.runtime.jobmanager.JobManager$$
anonfun$handleMessage$1$$anonfun$applyOrElse$8.apply$
mcV$sp(JobManager.scala:830)
at org.apache.flink.runtime.jobmanager.JobManager$$
anonfun$handleMessage$1$$anonfun$applyOrElse$8.apply(
JobManager.scala:773)
at org.apache.flink.runtime.jobmanager.JobManager$$
anonfun$handleMessage$1$$anonfun$applyOrElse$8.apply(
JobManager.scala:773)
at scala.concurrent.impl.Future$PromiseCompletingRunnable.
liftedTree1$1(Future.scala:24)
at scala.concurrent.impl.Future$PromiseCompletingRunnable.run(
Future.scala:24)
at akka.dispatch.TaskInvocation.run(AbstractDispatcher.scala:41)
at akka.dispatch.ForkJoinExecutorConfigurator$AkkaForkJoinTask.exec(
AbstractDispatcher.scala:401)
at scala.concurrent.forkjoin.ForkJoinTask.doExec(
ForkJoinTask.java:260)
at scala.concurrent.forkjoin.ForkJoinPool$WorkQueue.
pollAndExecAll(ForkJoinPool.java:1253)
at scala.concurrent.forkjoin.ForkJoinPool$WorkQueue.
runTask(ForkJoinPool.java:1346)
at scala.concurrent.forkjoin.ForkJoinPool.runWorker(
ForkJoinPool.java:1979)
at scala.concurrent.forkjoin.ForkJoinWorkerThread.run(
ForkJoinWorkerThread.java:107)
Caused by: org.apache.flink.types.NullFieldException: Field 2 is null,
but expected to hold a value.
at org.apache.flink.api.java.typeutils.runtime.
TupleSerializer.serialize(
TupleSerializer.java:126)
at org.apache.flink.api.java.typeutils.runtime.
TupleSerializer.serialize(
TupleSerializer.java:30)
at org.apache.flink.runtime.plugable.SerializationDelegate.write(
SerializationDelegate.java:56)
at org.apache.flink.runtime.io.network.api.serialization.
SpanningRecordSerializer.addRecord(SpanningRecordSerializer.java:83)
at org.apache.flink.runtime.io.network.api.writer.RecordWriter.emit(
RecordWriter.java:85)
at org.apache.flink.runtime.operators.shipping.
OutputCollector.collect(
OutputCollector.java:65)
at org.apache.flink.runtime.operators.util.metrics.
CountingCollector.collect(CountingCollector.java:35)
at org.apache.flink.api.java.operators.translation.PlanFilterOperator$
FlatMapFilter.flatMap(PlanFilterOperator.java:51)
at org.apache.flink.runtime.operators.chaining.
ChainedFlatMapDriver.collect(ChainedFlatMapDriver.java:80)
at org.apache.flink.runtime.operators.util.metrics.
CountingCollector.collect(CountingCollector.java:35)
at org.apache.flink.graph.library.Summarization$
VertexGroupReducer.reduce(
Summarization.java:323)
at org.apache.flink.runtime.operators.GroupReduceDriver.
run(GroupReduceDriver.java:131)
at org.apache.flink.runtime.operators.BatchTask.run(
BatchTask.java:486)
at org.apache.flink.runtime.operators.BatchTask.invoke(
BatchTask.java:351)
at org.apache.flink.runtime.taskmanager.Task.run(Task.java:590)
at java.lang.Thread.run(Thread.java:745)
Caused by: java.lang.NullPointerException
at org.apache.flink.api.common.typeutils.base.LongSerializer.
serialize(LongSerializer.java:64)
at org.apache.flink.api.common.typeutils.base.LongSerializer.
serialize(LongSerializer.java:27)
at org.apache.flink.api.java.typeutils.runtime.
TupleSerializer.serialize(
TupleSerializer.java:124)
... 15 more

_____________EXCEPTION END__________________

It looks like the problem is in the following lines in Summarization:

DataSet<Edge<K, EV>> edgesForGrouping = input.getEdges()
       .join(vertexToRepresentativeMap)
       .where(0)  // source vertex id
       .equalTo(0) // vertex id
       .with(new SourceVertexJoinFunction<K, EV>())
       .join(vertexToRepresentativeMap)
       .where(1)  // target vertex id
       .equalTo(0) // vertex id
       .with(new TargetVertexJoinFunction<K, EV>());


If you try to print edges before this step, it works fine. But after
this
step my IDE gives the same exception.

I would really appreciate any help.

Thank you,
Olga

_________EXAMPLE START_____________________

package org.apache.flink.graph.examples;

import org.apache.flink.api.common.ProgramDescription;
import org.apache.flink.api.java.DataSet;
import org.apache.flink.api.java.ExecutionEnvironment;
import org.apache.flink.graph.Edge;
import org.apache.flink.graph.Graph;
import org.apache.flink.graph.Vertex;
import org.apache.flink.graph.library.Summarization;
import java.util.LinkedList;
import java.util.List;

public class MySummarizationExample implements ProgramDescription {

     @SuppressWarnings("serial")
     public static void main(String [] args) throws Exception {

         ExecutionEnvironment env = ExecutionEnvironment.
getExecutionEnvironment();
         //Create graph
         DataSet<Edge<Long, Double>> edges = getEdgeDataSet(env);
         DataSet<Vertex<Long, Long>> vertices = getVertexDataSet(env);
         Graph<Long, Long, Double> graph = Graph.fromDataSet(vertices,
edges, env);
         //emit input
         System.out.println("Executing example with following
inputs:\n"+"Vertices:\n");
         vertices.print();
         System.out.println("Edges:\n");
         edges.print();

         Graph<Long, Summarization.VertexValue<Long>,
Summarization.EdgeValue<Double>> result = graph
                 .run(new Summarization<Long, Long, Double>());

         //now we want to read the output
         DataSet<Edge<Long, Summarization.EdgeValue<Double>>> edgesOut
=
result.getEdges();
         DataSet<Vertex<Long, Summarization.VertexValue<Long>>>
verticesOut = result.getVertices();
         // emit result
         System.out.println("Summarized graph:\n"+"Vertices:\n");
         verticesOut.print();
         System.out.println("Edges:\n");
         edgesOut.print();
     }

     @Override
     public String getDescription() {
         return "Summarization Example";
     }

     //Define edges
     private static DataSet<Edge<Long, Double>> getEdgeDataSet(
ExecutionEnvironment
env) {
         Object[][] DEFAULT_EDGES = new Object[][] {
                 new Object[]{1L, 2L, 1.0},
                 new Object[]{1L, 4L, 3.0},
                 new Object[]{2L, 3L, 6.0},
                 new Object[]{2L, 4L, 5.0},
                 new Object[]{2L, 5L, 1.0},
                 new Object[]{3L, 5L, 5.0},
                 new Object[]{3L, 6L, 2.0},
                 new Object[]{4L, 5L, 1.0},
                 new Object[]{5L, 6L, 4.0}
         };
         List<Edge<Long, Double>> edgeList = new LinkedList<Edge<Long,
Double>>();
         for (Object[] edge : DEFAULT_EDGES) {
             edgeList.add(new Edge<Long, Double>((Long) edge[0], (Long)
edge[1], (Double) edge[2]));
         }
         return env.fromCollection(edgeList);
     }
     //Define vertices
     private static DataSet<Vertex<Long, Long>> getVertexDataSet(
ExecutionEnvironment
env) {
         //We will summarize by <VV> = Long
         Object[][] DEFAULT_VERTICES = new Object[][] {
                 new Object[]{1L, 1L},
                 new Object[]{2L, 1L},
                 new Object[]{3L, 5L},
                 new Object[]{4L, 5L},
                 new Object[]{5L, 5L}
         };
         List<Vertex<Long, Long>> vertexList = new
LinkedList<Vertex<Long, Long>>();
         for (Object[] vertex : DEFAULT_VERTICES) {
             vertexList.add(new Vertex<Long, Long>((Long) vertex[0],
(Long) vertex[1]));
         }
         return env.fromCollection(vertexList);
     }
}

_________EXAMPLE END_____________________


Best regards,
Olga Golovneva

On Thu, Sep 15, 2016 at 9:16 AM, Till Rohrmann <trohrm...@apache.org>
wrote:

Hi Olga,

can you provide us with a little bit more details about the problem.
The
full stack trace of the exception and the program you're trying to run
would be helpful.

Cheers,
Till

On Wed, Sep 14, 2016 at 9:49 PM, Olga Golovneva <melcha...@gmail.com>
wrote:

Hi devs,

Do you know if there is an example (besides ITCase) of usage of
Summarization Library in Gelly? I'm having some problems trying to
use
it
in my code. Particularly, I cannot print output edges ( it throws
the
following exception: Exception in thread "main"
org.apache.flink.runtime.client.JobExecutionException: Job
execution
failed.), while vertices are printed correctly.

Best regards,
Olga



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