# Gelly: Flink Graph API

Gelly is a Graph API for Flink. It contains a set of methods and utilities which aim to simplify the development of graph analysis applications in Flink. In Gelly, graphs can be transformed and modified using high-level functions similar to the ones provided by the batch processing API. Gelly provides methods to create, transform and modify graphs, as well as a library of graph algorithms.

## Using Gelly

Gelly is currently part of the _libraries_ Maven project. All relevant classes are located in the _org.apache.flink.graph_ package.

Add the following dependency to your `pom.xml` to use Gelly.



```
<dependency>
    <groupId>org.apache.flink</groupId>
    <artifactId>flink-gelly_2.11</artifactId>
    <version>1.7.1</version>
</dependency>
```





```
<dependency>
    <groupId>org.apache.flink</groupId>
    <artifactId>flink-gelly-scala_2.11</artifactId>
    <version>1.7.1</version>
</dependency>
```



Note that Gelly is not part of the binary distribution. See [linking](//ci.apache.org/projects/flink/flink-docs-release-1.7/dev/linking.html) for instructions on packaging Gelly libraries into Flink user programs.

The remaining sections provide a description of available methods and present several examples of how to use Gelly and how to mix it with the Flink DataSet API.

## Running Gelly Examples

The Gelly library jars are provided in the [Flink distribution](https://flink.apache.org/downloads.html "Apache Flink: Downloads") in the **opt** directory (for versions older than Flink 1.2 these can be manually downloaded from [Maven Central](http://search.maven.org/#search|ga|1|flink%20gelly)). To run the Gelly examples the **flink-gelly** (for Java) or **flink-gelly-scala** (for Scala) jar must be copied to Flink’s **lib** directory.



```
cp opt/flink-gelly_*.jar lib/
cp opt/flink-gelly-scala_*.jar lib/
```



Gelly’s examples jar includes drivers for each of the library methods and is provided in the **examples** directory. After configuring and starting the cluster, list the available algorithm classes:



```
./bin/start-cluster.sh
./bin/flink run examples/gelly/flink-gelly-examples_*.jar
```



The Gelly drivers can generate graph data or read the edge list from a CSV file (each node in a cluster must have access to the input file). The algorithm description, available inputs and outputs, and configuration are displayed when an algorithm is selected. Print usage for [JaccardIndex](./library_methods.html#jaccard-index):



```
./bin/flink run examples/gelly/flink-gelly-examples_*.jar --algorithm JaccardIndex
```



Display [graph metrics](./library_methods.html#metric) for a million vertex graph:



```
./bin/flink run examples/gelly/flink-gelly-examples_*.jar \
    --algorithm GraphMetrics --order directed \
    --input RMatGraph --type integer --scale 20 --simplify directed \
    --output print
```



The size of the graph is adjusted by the _--scale_ and _--edge_factor_ parameters. The [library generator](./graph_generators.html#rmat-graph) provides access to additional configuration to adjust the power-law skew and random noise.

Sample social network data is provided by the [Stanford Network Analysis Project](http://snap.stanford.edu/data/index.html). The [com-lj](http://snap.stanford.edu/data/bigdata/communities/com-lj.ungraph.txt.gz) data set is a good starter size. Run a few algorithms and monitor the job progress in Flink’s Web UI:



```
wget -O - http://snap.stanford.edu/data/bigdata/communities/com-lj.ungraph.txt.gz | gunzip -c > com-lj.ungraph.txt

./bin/flink run -q examples/gelly/flink-gelly-examples_*.jar \
    --algorithm GraphMetrics --order undirected \
    --input CSV --type integer --simplify undirected --input_filename com-lj.ungraph.txt --input_field_delimiter /figure>\t' \
    --output print

./bin/flink run -q examples/gelly/flink-gelly-examples_*.jar \
    --algorithm ClusteringCoefficient --order undirected \
    --input CSV --type integer --simplify undirected --input_filename com-lj.ungraph.txt --input_field_delimiter /figure>\t' \
    --output hash

./bin/flink run -q examples/gelly/flink-gelly-examples_*.jar \
    --algorithm JaccardIndex \
    --input CSV --type integer --simplify undirected --input_filename com-lj.ungraph.txt --input_field_delimiter /figure>\t' \
    --output hash
```



Please submit feature requests and report issues on the user [mailing list](https://flink.apache.org/community.html#mailing-lists) or [Flink Jira](https://issues.apache.org/jira/browse/FLINK). We welcome suggestions for new algorithms and features as well as [code contributions](https://flink.apache.org/contribute-code.html).