- Gelly is a graph processing library built on Apache Flink that provides APIs for Java and Scala to work with graphs and perform graph algorithms - It allows seamless integration of graph-based and record-based analysis by mixing the Gelly and Flink DataSet APIs - Common graph algorithms like connected components, PageRank, and similarity recommendations are included in the library

1. Gelly School
Vasia Kalavri
Apache Flink PMC, PhD student @KTH
kalavri@kth.se, @vkalavri

2. ● Java & Scala Graph APIs on top of Flink
● Library of common Graph algorithms
● Iteration abstractions
● Can be seamlessly mixed with the DataSet Flink API
→ easily implement applications that use both record-based
and graph-based analysis
Meet Gelly
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3. Gelly in the Flink stack
3
Scala API
(batch and streaming)
Java API
(batch and streaming)
FlinkML Gelly
Runtime and Execution Environments
Data storage
Table APIPython API
Transformations
and Utilities
Iterative Graph
Processing
Graph Library

4. Hello, Gelly!
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ExecutionEnvironment env = ExecutionEnvironment.getExecutionEnvironment();
DataSet<Edge<Long, NullValue>> edges = getEdgesDataSet(env);
Graph<Long, Long, NullValue> graph = Graph.fromDataSet(edges, env);
DataSet<Vertex<Long, Long>> verticesWithMinIds = graph.run(
new ConnectedComponents(maxIterations));
val env = ExecutionEnvironment.getExecutionEnvironment
val edges: DataSet[Edge[Long, NullValue]] = getEdgesDataSet(env)
val graph = Graph.fromDataSet(edges, env)
val components = graph.run(new ConnectedComponents(maxIterations))
Java
Scala

5. Go to http://gellyschool.com/flink-forward
● Tutorial#0: Why Graphs and Gelly Basics
● Tutorial#1: Calculate Degree Distributions
● Tutorial#2: PageRank
● Tutorial#3: People you Might Know
Skeleton: http://github.com/vasia/gelly-school/tree/ff-skeleton
Solutions: http://github.com/vasia/gelly-school/tree/ff-solutions
...or in the home folder of your VM :-)
Tasks for Today
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6. Today you will
learn how to...
Create a Graph from a file of edges
Compute simple Graph properties
Use Gelly’s neighborhood methods
Run Gelly library algorithms
Use DataSet and Gelly APIs together
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7. Today you will not
learn how to...
Use the Scala Gelly API
Write your own vertex-centric or
gather-sum-apply iterative programs
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8. Tutorial#0: Let’s get Started!

9. // create a vertex with ID=42 and value=0.8
Vertex<Integer, Double> v = new Vertex<Integer, Double>(42, 0.8);
// create an edge from 5 to 6 with value="foo"
Edge<Integer, Integer, String> e =
new Edge<Integer, Integer, String>(5, 6, "foo");
// create an edge from 5 to 6 with no value
Edge<Integer, Integer, NullValue> e =
new Edge<Integer, Integer, NullValue>(5, 6, NullValue.getInstance());
ID type value type
source ID type target ID type edge value type
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10. ExecutionEnvironment env = ExecutionEnvironment.getExecutionEnvironment();
// create a Graph from Vertex and Edge DataSets
DataSet<Vertex<String, Long>> vertices = ...
DataSet<Edge<String, Double>> edges = ...
Graph<String, Long, Double> g1 = Graph.fromDataSet(vertices, edges, env);
...
// create a Graph from a Tuple3 DataSet
DataSet<Tuple3<String, String, Double>> edges = ...
Graph<String, NullValue, Double> g2 = Graph.fromTupleDataSet(edges, env);
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11. // create a Graph from a Tuple2 DataSet
DataSet<Tuple2<String, String>> input = ...
DataSet<Edge<String, NullValue>> edges = input.map(
new MapFunction<Tuple2<String, String>, Edge<String, NullValue>>() {
public Edge<String, NullValue> map(Tuple2<String, String> in) {
return new Edge(in.f0, in.f1, NullValue.getInstance());
}
})
Graph<String, NullValue, NullValue> g3 = Graph.fromDataSet(edges, env);
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12. Tutorial#1: Degree Distributions

13. 1 2
3 4 5
vertexID in-degree out-degree degree
1 0 2 2
2 2 2 4
3 2 1 3
4 2 1 3
5 1 1 2
degree #vertices distribution
2 2 2/5
3 2 2/5
4 1 1/5
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14. Tutorial#2: PageRank
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15. vertexID out-degree transition
probability
1 2 1/2
2 2 1/2
3 0 -
4 3 1/3
5 1 1
15
1 2
43
5
PR(3) = 0.5*PR(1) + 0.33*PR(4) + PR(5)
simplified PageRank

16. Graph<Long, Double, Double> network = ...
DataSet<Tuple2<Long, Long>> vertexOutDegrees = network.outDegrees();
// assign the transition probabilities as the edge weights
Graph<Long, Double, Double> networkWithWeights =
network.joinWithEdgesOnSource(vertexOutDegrees,
new MapFunction<Tuple2<Double, Long>, Double>() {
public Double map(Tuple2<Double, Long> value) {
return value.f0 / value.f0;
}
});
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current Edge value value from degrees
new Edge value

17. Interactions as weights
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Tom RT Wendy
Tom RT Mary
Tom RT Wendy
Sarah RT James
Tom RT Jim
Tom RT Wendy
Jim RT Obama
...
Tom
Wendy
Mary
Jim
Tom
Wendy
Mary
Jim
Tom
Wendy
Mary
Jim
3
1
1
0.6
0.2
0.2

18. Tutorial#3: People you Might Know
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19. 19
Steve Wendy
Carol
Randy Shelly
Steve knows Wendy
Wendy knows Carol
→ Steve knows Carol?
Steve knows Randy
Randy and Wendy know Shelly
→ Steve knows Shelly?

20. Feeling Gelly?
Gelly programming guide: https://ci.apache.org/projects/flink/flink-docs-
master/libs/gelly_guide.html
Blog post: https://flink.apache.org/news/2015/08/24/introducing-flink-gelly.html
More Gelly School: http://gellyschool.com
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