## Iterative graph processing

Gelly enhances Flink's iterative processing capabilities to support large scale graph processing. Currently it supports implementation of the following models:

Vertex-Centric

Scatter-Gather

Gather-Sum-Apply

Let's start by understanding these models in the context of Gelly.

### Vertex-Centric iterations

As the name suggest, these iterations are built thinking the vertex is in the center. Here each Vertex processes the same user-defined function in parallel. Each step of execution is called a **superset**. A vertex can send a message to another vertex as long as it knows its unique ID. This message would be used as input to the next superset.

To use Vertex-Centric iterations, the user needs to provide a `ComputeFunction`

. We can also define an optional `MessageCombiner`

to reduce the cost of communication. We can solve problems, such as Single Source Shortest Path in which we need to find the shortest path from source vertex to all other vertices.

### Note

Single Source Shortest Path is...