.. _guide-minibatch-edge-classification-sampler:

6.2 Training GNN for Edge Classification with Neighborhood Sampling
----------------------------------------------------------------------

Training for edge classification/regression is somewhat similar to that
of node classification/regression with several notable differences.

Define a neighborhood sampler and data loader
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

You can use the
:ref:`same neighborhood samplers as node classification <guide-minibatch-node-classification-sampler>`.

.. code:: python

    sampler = dgl.dataloading.MultiLayerFullNeighborSampler(2)

To use the neighborhood sampler provided by DGL for edge classification,
one need to instead combine it with
:class:`~dgl.dataloading.pytorch.EdgeDataLoader`, which iterates
over a set of edges in minibatches, yielding the subgraph induced by the
edge minibatch and ``blocks`` to be consumed by the module above.

For example, the following code creates a PyTorch DataLoader that
iterates over the training edge ID array ``train_eids`` in batches,
putting the list of generated blocks onto GPU.

.. code:: python

    dataloader = dgl.dataloading.EdgeDataLoader(
        g, train_eid_dict, sampler,
        batch_size=1024,
        shuffle=True,
        drop_last=False,
        num_workers=4)

For a complete list of supported builtin samplers, please refer to the
:ref:`neighborhood sampler API reference <api-dataloading-neighbor-sampling>`.

If you wish to develop your own neighborhood sampler or you want a more
detailed explanation of the concept of blocks, please refer to
:ref:`guide-minibatch-customizing-neighborhood-sampler`.

Removing edges in the minibatch from the original graph for neighbor sampling
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

When training edge classification models, sometimes you wish to remove
the edges appearing in the training data from the computation dependency
as if they never existed. Otherwise, the model will “know” the fact that
an edge exists between the two nodes, and potentially use it for
advantage.

Therefore in edge classification you sometimes would like to exclude the
edges sampled in the minibatch from the original graph for neighborhood
sampling, as well as the reverse edges of the sampled edges on an
undirected graph. You can specify ``exclude='reverse'`` in instantiation
of :class:`~dgl.dataloading.pytorch.EdgeDataLoader`, with the mapping of the edge
IDs to their reverse edges IDs.  Usually doing so will lead to much slower
sampling process due to locating the reverse edges involving in the minibatch
and removing them.

.. code:: python

    n_edges = g.number_of_edges()
    dataloader = dgl.dataloading.EdgeDataLoader(
        g, train_eid_dict, sampler,
    
        # The following two arguments are specifically for excluding the minibatch
        # edges and their reverse edges from the original graph for neighborhood
        # sampling.
        exclude='reverse',
        reverse_eids=torch.cat([
            torch.arange(n_edges // 2, n_edges), torch.arange(0, n_edges // 2)]),
    
        batch_size=1024,
        shuffle=True,
        drop_last=False,
        num_workers=4)

Adapt your model for minibatch training
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The edge classification model usually consists of two parts:

-  One part that obtains the representation of incident nodes.
-  The other part that computes the edge score from the incident node
   representations.

The former part is exactly the same as
:ref:`that from node classification <guide-minibatch-node-classification-model>`
and we can simply reuse it. The input is still the list of
blocks generated from a data loader provided by DGL, as well as the
input features.

.. code:: python

    class StochasticTwoLayerGCN(nn.Module):
        def __init__(self, in_features, hidden_features, out_features):
            super().__init__()
            self.conv1 = dglnn.GraphConv(in_features, hidden_features)
            self.conv2 = dglnn.GraphConv(hidden_features, out_features)
    
        def forward(self, blocks, x):
            x = F.relu(self.conv1(blocks[0], x))
            x = F.relu(self.conv2(blocks[1], x))
            return x

The input to the latter part is usually the output from the
former part, as well as the subgraph of the original graph induced by the
edges in the minibatch. The subgraph is yielded from the same data
loader. One can call :meth:`dgl.DGLHeteroGraph.apply_edges` to compute the
scores on the edges with the edge subgraph.

The following code shows an example of predicting scores on the edges by
concatenating the incident node features and projecting it with a dense
layer.

.. code:: python

    class ScorePredictor(nn.Module):
        def __init__(self, num_classes, in_features):
            super().__init__()
            self.W = nn.Linear(2 * in_features, num_classes)
    
        def apply_edges(self, edges):
            data = torch.cat([edges.src['x'], edges.dst['x']])
            return {'score': self.W(data)}
    
        def forward(self, edge_subgraph, x):
            with edge_subgraph.local_scope():
                edge_subgraph.ndata['x'] = x
                edge_subgraph.apply_edges(self.apply_edges)
                return edge_subgraph.edata['score']

The entire model will take the list of blocks and the edge subgraph
generated by the data loader, as well as the input node features as
follows:

.. code:: python

    class Model(nn.Module):
        def __init__(self, in_features, hidden_features, out_features, num_classes):
            super().__init__()
            self.gcn = StochasticTwoLayerGCN(
                in_features, hidden_features, out_features)
            self.predictor = ScorePredictor(num_classes, out_features)
    
        def forward(self, edge_subgraph, blocks, x):
            x = self.gcn(blocks, x)
            return self.predictor(edge_subgraph, x)

DGL ensures that that the nodes in the edge subgraph are the same as the
output nodes of the last block in the generated list of blocks.

Training Loop
~~~~~~~~~~~~~

The training loop is very similar to node classification. You can
iterate over the dataloader and get a subgraph induced by the edges in
the minibatch, as well as the list of blocks necessary for computing
their incident node representations.

.. code:: python

    model = Model(in_features, hidden_features, out_features, num_classes)
    model = model.cuda()
    opt = torch.optim.Adam(model.parameters())
    
    for input_nodes, edge_subgraph, blocks in dataloader:
        blocks = [b.to(torch.device('cuda')) for b in blocks]
        edge_subgraph = edge_subgraph.to(torch.device('cuda'))
        input_features = blocks[0].srcdata['features']
        edge_labels = edge_subgraph.edata['labels']
        edge_predictions = model(edge_subgraph, blocks, input_features)
        loss = compute_loss(edge_labels, edge_predictions)
        opt.zero_grad()
        loss.backward()
        opt.step()

For heterogeneous graphs
~~~~~~~~~~~~~~~~~~~~~~~~

The models computing the node representations on heterogeneous graphs
can also be used for computing incident node representations for edge
classification/regression.

.. code:: python

    class StochasticTwoLayerRGCN(nn.Module):
        def __init__(self, in_feat, hidden_feat, out_feat):
            super().__init__()
            self.conv1 = dglnn.HeteroGraphConv({
                    rel : dglnn.GraphConv(in_feat, hidden_feat, norm='right')
                    for rel in rel_names
                })
            self.conv2 = dglnn.HeteroGraphConv({
                    rel : dglnn.GraphConv(hidden_feat, out_feat, norm='right')
                    for rel in rel_names
                })
    
        def forward(self, blocks, x):
            x = self.conv1(blocks[0], x)
            x = self.conv2(blocks[1], x)
            return x

For score prediction, the only implementation difference between the
homogeneous graph and the heterogeneous graph is that we are looping
over the edge types for :meth:`~dgl.DGLHeteroGraph.apply_edges`.

.. code:: python

    class ScorePredictor(nn.Module):
        def __init__(self, num_classes, in_features):
            super().__init__()
            self.W = nn.Linear(2 * in_features, num_classes)
    
        def apply_edges(self, edges):
            data = torch.cat([edges.src['x'], edges.dst['x']])
            return {'score': self.W(data)}
    
        def forward(self, edge_subgraph, x):
            with edge_subgraph.local_scope():
                edge_subgraph.ndata['x'] = x
                for etype in edge_subgraph.canonical_etypes:
                    edge_subgraph.apply_edges(self.apply_edges, etype=etype)
                return edge_subgraph.edata['score']

Data loader definition is also very similar to that of node
classification. The only difference is that you need
:class:`~dgl.dataloading.pytorch.EdgeDataLoader` instead of
:class:`~dgl.dataloading.pytorch.NodeDataLoader`, and you will be supplying a
dictionary of edge types and edge ID tensors instead of a dictionary of
node types and node ID tensors.

.. code:: python

    sampler = dgl.dataloading.MultiLayerFullNeighborSampler(2)
    dataloader = dgl.dataloading.EdgeDataLoader(
        g, train_eid_dict, sampler,
        batch_size=1024,
        shuffle=True,
        drop_last=False,
        num_workers=4)

Things become a little different if you wish to exclude the reverse
edges on heterogeneous graphs. On heterogeneous graphs, reverse edges
usually have a different edge type from the edges themselves, in order
to differentiate the “forward” and “backward” relationships (e.g.
``follow`` and ``followed by`` are reverse relations of each other,
``purchase`` and ``purchased by`` are reverse relations of each other,
etc.).

If each edge in a type has a reverse edge with the same ID in another
type, you can specify the mapping between edge types and their reverse
types. The way to exclude the edges in the minibatch as well as their
reverse edges then goes as follows.

.. code:: python

    dataloader = dgl.dataloading.EdgeDataLoader(
        g, train_eid_dict, sampler,
    
        # The following two arguments are specifically for excluding the minibatch
        # edges and their reverse edges from the original graph for neighborhood
        # sampling.
        exclude='reverse_types',
        reverse_etypes={'follow': 'followed by', 'followed by': 'follow',
                        'purchase': 'purchased by', 'purchased by': 'purchase'}
    
        batch_size=1024,
        shuffle=True,
        drop_last=False,
        num_workers=4)

The training loop is again almost the same as that on homogeneous graph,
except for the implementation of ``compute_loss`` that will take in two
dictionaries of node types and predictions here.

.. code:: python

    model = Model(in_features, hidden_features, out_features, num_classes)
    model = model.cuda()
    opt = torch.optim.Adam(model.parameters())
    
    for input_nodes, edge_subgraph, blocks in dataloader:
        blocks = [b.to(torch.device('cuda')) for b in blocks]
        edge_subgraph = edge_subgraph.to(torch.device('cuda'))
        input_features = blocks[0].srcdata['features']
        edge_labels = edge_subgraph.edata['labels']
        edge_predictions = model(edge_subgraph, blocks, input_features)
        loss = compute_loss(edge_labels, edge_predictions)
        opt.zero_grad()
        loss.backward()
        opt.step()

`GCMC <https://github.com/dmlc/dgl/tree/master/examples/pytorch/gcmc>`__
is an example of edge classification on a bipartite graph.


