"""
Link Prediction using Graph Neural Networks
===========================================
In the :doc:`introduction <1_introduction>`, you have already learned
the basic workflow of using GNNs for node classification,
i.e. predicting the category of a node in a graph. This tutorial will
teach you how to train a GNN for link prediction, i.e. predicting the
existence of an edge between two arbitrary nodes in a graph.
By the end of this tutorial you will be able to
- Build a GNN-based link prediction model.
- Train and evaluate the model on a small DGL-provided dataset.
(Time estimate: 28 minutes)
"""
import itertools
import os
os.environ['DGLBACKEND'] = 'pytorch'
import numpy as np
import scipy.sparse as sp
import torch
import torch.nn as nn
import torch.nn.functional as F
import dgl
import dgl.data
######################################################################
# Overview of Link Prediction with GNN
# ------------------------------------
#
# Many applications such as social recommendation, item recommendation,
# knowledge graph completion, etc., can be formulated as link prediction,
# which predicts whether an edge exists between two particular nodes. This
# tutorial shows an example of predicting whether a citation relationship,
# either citing or being cited, between two papers exists in a citation
# network.
#
# This tutorial formulates the link prediction problem as a binary classification
# problem as follows:
#
# - Treat the edges in the graph as *positive examples*.
# - Sample a number of non-existent edges (i.e. node pairs with no edges
# between them) as *negative* examples.
# - Divide the positive examples and negative examples into a training
# set and a test set.
# - Evaluate the model with any binary classification metric such as Area
# Under Curve (AUC).
#
# .. note::
#
# The practice comes from
# `SEAL `__,
# although the model here does not use their idea of node labeling.
#
# In some domains such as large-scale recommender systems or information
# retrieval, you may favor metrics that emphasize good performance of
# top-K predictions. In these cases you may want to consider other metrics
# such as mean average precision, and use other negative sampling methods,
# which are beyond the scope of this tutorial.
#
# Loading graph and features
# --------------------------
#
# Following the :doc:`introduction <1_introduction>`, this tutorial
# first loads the Cora dataset.
#
dataset = dgl.data.CoraGraphDataset()
g = dataset[0]
######################################################################
# Prepare training and testing sets
# ---------------------------------
#
# This tutorial randomly picks 10% of the edges for positive examples in
# the test set, and leave the rest for the training set. It then samples
# the same number of edges for negative examples in both sets.
#
# Split edge set for training and testing
u, v = g.edges()
eids = np.arange(g.number_of_edges())
eids = np.random.permutation(eids)
test_size = int(len(eids) * 0.1)
train_size = g.number_of_edges() - test_size
test_pos_u, test_pos_v = u[eids[:test_size]], v[eids[:test_size]]
train_pos_u, train_pos_v = u[eids[test_size:]], v[eids[test_size:]]
# Find all negative edges and split them for training and testing
adj = sp.coo_matrix((np.ones(len(u)), (u.numpy(), v.numpy())))
adj_neg = 1 - adj.todense() - np.eye(g.number_of_nodes())
neg_u, neg_v = np.where(adj_neg != 0)
neg_eids = np.random.choice(len(neg_u), g.number_of_edges())
test_neg_u, test_neg_v = (
neg_u[neg_eids[:test_size]],
neg_v[neg_eids[:test_size]],
)
train_neg_u, train_neg_v = (
neg_u[neg_eids[test_size:]],
neg_v[neg_eids[test_size:]],
)
######################################################################
# When training, you will need to remove the edges in the test set from
# the original graph. You can do this via ``dgl.remove_edges``.
#
# .. note::
#
# ``dgl.remove_edges`` works by creating a subgraph from the
# original graph, resulting in a copy and therefore could be slow for
# large graphs. If so, you could save the training and test graph to
# disk, as you would do for preprocessing.
#
train_g = dgl.remove_edges(g, eids[:test_size])
######################################################################
# Define a GraphSAGE model
# ------------------------
#
# This tutorial builds a model consisting of two
# `GraphSAGE `__ layers, each computes
# new node representations by averaging neighbor information. DGL provides
# ``dgl.nn.SAGEConv`` that conveniently creates a GraphSAGE layer.
#
from dgl.nn import SAGEConv
# ----------- 2. create model -------------- #
# build a two-layer GraphSAGE model
class GraphSAGE(nn.Module):
def __init__(self, in_feats, h_feats):
super(GraphSAGE, self).__init__()
self.conv1 = SAGEConv(in_feats, h_feats, "mean")
self.conv2 = SAGEConv(h_feats, h_feats, "mean")
def forward(self, g, in_feat):
h = self.conv1(g, in_feat)
h = F.relu(h)
h = self.conv2(g, h)
return h
######################################################################
# The model then predicts the probability of existence of an edge by
# computing a score between the representations of both incident nodes
# with a function (e.g. an MLP or a dot product), which you will see in
# the next section.
#
# .. math::
#
#
# \hat{y}_{u\sim v} = f(h_u, h_v)
#
######################################################################
# Positive graph, negative graph, and ``apply_edges``
# ---------------------------------------------------
#
# In previous tutorials you have learned how to compute node
# representations with a GNN. However, link prediction requires you to
# compute representation of *pairs of nodes*.
#
# DGL recommends you to treat the pairs of nodes as another graph, since
# you can describe a pair of nodes with an edge. In link prediction, you
# will have a *positive graph* consisting of all the positive examples as
# edges, and a *negative graph* consisting of all the negative examples.
# The *positive graph* and the *negative graph* will contain the same set
# of nodes as the original graph. This makes it easier to pass node
# features among multiple graphs for computation. As you will see later,
# you can directly feed the node representations computed on the entire
# graph to the positive and the negative graphs for computing pair-wise
# scores.
#
# The following code constructs the positive graph and the negative graph
# for the training set and the test set respectively.
#
train_pos_g = dgl.graph(
(train_pos_u, train_pos_v), num_nodes=g.number_of_nodes()
)
train_neg_g = dgl.graph(
(train_neg_u, train_neg_v), num_nodes=g.number_of_nodes()
)
test_pos_g = dgl.graph((test_pos_u, test_pos_v), num_nodes=g.number_of_nodes())
test_neg_g = dgl.graph((test_neg_u, test_neg_v), num_nodes=g.number_of_nodes())
######################################################################
# The benefit of treating the pairs of nodes as a graph is that you can
# use the ``DGLGraph.apply_edges`` method, which conveniently computes new
# edge features based on the incident nodes’ features and the original
# edge features (if applicable).
#
# DGL provides a set of optimized builtin functions to compute new
# edge features based on the original node/edge features. For example,
# ``dgl.function.u_dot_v`` computes a dot product of the incident nodes’
# representations for each edge.
#
import dgl.function as fn
class DotPredictor(nn.Module):
def forward(self, g, h):
with g.local_scope():
g.ndata["h"] = h
# Compute a new edge feature named 'score' by a dot-product between the
# source node feature 'h' and destination node feature 'h'.
g.apply_edges(fn.u_dot_v("h", "h", "score"))
# u_dot_v returns a 1-element vector for each edge so you need to squeeze it.
return g.edata["score"][:, 0]
######################################################################
# You can also write your own function if it is complex.
# For instance, the following module produces a scalar score on each edge
# by concatenating the incident nodes’ features and passing it to an MLP.
#
class MLPPredictor(nn.Module):
def __init__(self, h_feats):
super().__init__()
self.W1 = nn.Linear(h_feats * 2, h_feats)
self.W2 = nn.Linear(h_feats, 1)
def apply_edges(self, edges):
"""
Computes a scalar score for each edge of the given graph.
Parameters
----------
edges :
Has three members ``src``, ``dst`` and ``data``, each of
which is a dictionary representing the features of the
source nodes, the destination nodes, and the edges
themselves.
Returns
-------
dict
A dictionary of new edge features.
"""
h = torch.cat([edges.src["h"], edges.dst["h"]], 1)
return {"score": self.W2(F.relu(self.W1(h))).squeeze(1)}
def forward(self, g, h):
with g.local_scope():
g.ndata["h"] = h
g.apply_edges(self.apply_edges)
return g.edata["score"]
######################################################################
# .. note::
#
# The builtin functions are optimized for both speed and memory.
# We recommend using builtin functions whenever possible.
#
# .. note::
#
# If you have read the :doc:`message passing
# tutorial <3_message_passing>`, you will notice that the
# argument ``apply_edges`` takes has exactly the same form as a message
# function in ``update_all``.
#
######################################################################
# Training loop
# -------------
#
# After you defined the node representation computation and the edge score
# computation, you can go ahead and define the overall model, loss
# function, and evaluation metric.
#
# The loss function is simply binary cross entropy loss.
#
# .. math::
#
#
# \mathcal{L} = -\sum_{u\sim v\in \mathcal{D}}\left( y_{u\sim v}\log(\hat{y}_{u\sim v}) + (1-y_{u\sim v})\log(1-\hat{y}_{u\sim v})) \right)
#
# The evaluation metric in this tutorial is AUC.
#
model = GraphSAGE(train_g.ndata["feat"].shape[1], 16)
# You can replace DotPredictor with MLPPredictor.
# pred = MLPPredictor(16)
pred = DotPredictor()
def compute_loss(pos_score, neg_score):
scores = torch.cat([pos_score, neg_score])
labels = torch.cat(
[torch.ones(pos_score.shape[0]), torch.zeros(neg_score.shape[0])]
)
return F.binary_cross_entropy_with_logits(scores, labels)
def compute_auc(pos_score, neg_score):
scores = torch.cat([pos_score, neg_score]).numpy()
labels = torch.cat(
[torch.ones(pos_score.shape[0]), torch.zeros(neg_score.shape[0])]
).numpy()
return roc_auc_score(labels, scores)
######################################################################
# The training loop goes as follows:
#
# .. note::
#
# This tutorial does not include evaluation on a validation
# set. In practice you should save and evaluate the best model based on
# performance on the validation set.
#
# ----------- 3. set up loss and optimizer -------------- #
# in this case, loss will in training loop
optimizer = torch.optim.Adam(
itertools.chain(model.parameters(), pred.parameters()), lr=0.01
)
# ----------- 4. training -------------------------------- #
all_logits = []
for e in range(100):
# forward
h = model(train_g, train_g.ndata["feat"])
pos_score = pred(train_pos_g, h)
neg_score = pred(train_neg_g, h)
loss = compute_loss(pos_score, neg_score)
# backward
optimizer.zero_grad()
loss.backward()
optimizer.step()
if e % 5 == 0:
print("In epoch {}, loss: {}".format(e, loss))
# ----------- 5. check results ------------------------ #
from sklearn.metrics import roc_auc_score
with torch.no_grad():
pos_score = pred(test_pos_g, h)
neg_score = pred(test_neg_g, h)
print("AUC", compute_auc(pos_score, neg_score))
# Thumbnail credits: Link Prediction with Neo4j, Mark Needham
# sphinx_gallery_thumbnail_path = '_static/blitz_4_link_predict.png'