# GraphNeuralNetworks

This is the documentation page for GraphNeuralNetworks.jl, a graph neural network library written in Julia and based on the deep learning framework Flux.jl. GraphNeuralNetworks.jl is largely inspired by PyTorch Geometric, Deep Graph Library, and GeometricFlux.jl.

Among its features:

- Implements common graph convolutional layers.
- Supports computations on batched graphs.
- Easy to define custom layers.
- CUDA support.
- Integration with Graphs.jl.
- Examples of node, edge, and graph level machine learning tasks.

## Package overview

Let's give a brief overview of the package by solving a graph regression problem with synthetic data.

Usage examples on real datasets can be found in the examples folder.

### Data preparation

We create a dataset consisting in multiple random graphs and associated data features.

```
using GraphNeuralNetworks, Graphs, Flux, CUDA, Statistics, MLUtils
using Flux.Data: DataLoader
all_graphs = GNNGraph[]
for _ in 1:1000
g = GNNGraph(random_regular_graph(10, 4),
ndata=(; x = randn(Float32, 16,10)), # input node features
gdata=(; y = randn(Float32))) # regression target
push!(all_graphs, g)
end
```

### Model building

We concisely define our model as a `GNNChain`

containing two graph convolutional layers. If CUDA is available, our model will live on the gpu.

```
device = CUDA.functional() ? Flux.gpu : Flux.cpu;
model = GNNChain(GCNConv(16 => 64),
BatchNorm(64), # Apply batch normalization on node features (nodes dimension is batch dimension)
x -> relu.(x),
GCNConv(64 => 64, relu),
GlobalPool(mean), # aggregate node-wise features into graph-wise features
Dense(64, 1)) |> device
ps = Flux.params(model)
opt = Adam(1f-4)
```

### Training

Finally, we use a standard Flux training pipeline to fit our dataset. We use Flux's `DataLoader`

to iterate over mini-batches of graphs that are glued together into a single `GNNGraph`

using the `MLUtils.batch`

method. This is what happens under the hood when creating a `DataLoader`

with the `collate=true`

option.

```
train_graphs, test_graphs = MLUtils.split(all_graphs, at=0.8)
train_loader = DataLoader(train_graphs,
batchsize=32, shuffle=true, collate=true)
test_loader = DataLoader(test_graphs,
batchsize=32, shuffle=false, collate=true)
loss(g::GNNGraph) = mean((vec(model(g, g.ndata.x)) - g.gdata.y).^2)
loss(loader) = mean(loss(g |> device) for g in loader)
for epoch in 1:100
for g in train_loader
g = g |> device
grad = gradient(() -> loss(g), ps)
Flux.Optimise.update!(opt, ps, grad)
end
@info (; epoch, train_loss=loss(train_loader), test_loss=loss(test_loader))
end
```