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Title: DyRep: Learning Representations over Dynamic Graphs

Paper content:
Representation Learning over graph structured data has received significant attention recently due to its ubiquitous applicability.
However, most advancements have been made in static graph settings while efforts for jointly learning dynamic of the graph and dynamic on the graph are still in an infant stage.
Two fundamental questions arise in learning over dynamic graphs:
(i) How to elegantly model dynamical processes over graphs?
(ii) How to leverage such a model to effectively encode evolving graph information into low-dimensional representations?
We present DyRep - a novel modeling framework for dynamic graphs that posits representation learning as a latent mediation process bridging two observed processes namely -- dynamics of the network (realized as topological evolution) and dynamics on the network (realized as activities between nodes).
Concretely, we propose a two-time scale deep temporal point process model that captures the interleaved dynamics of the observed processes.
This model is further parameterized by a temporal-attentive representation network that encodes temporally evolving structural information into node representations which in turn drives the nonlinear evolution of the observed graph dynamics.
Our unified framework is trained using an efficient unsupervised procedure and has capability to generalize over unseen nodes.
We demonstrate that DyRep outperforms state-of-the-art baselines for dynamic link prediction and time prediction tasks and present extensive qualitative insights into our framework.
Representation learning over graph structured data has emerged as a keystone machine learning task due to its ubiquitous applicability in variety of domains such as social networks, bioinformatics, natural language processing, and relational knowledge bases.
Learning node representations to effectively encode high-dimensional and non-Euclidean graph information is a challenging problem but recent advances in deep learning has helped important progress towards addressing it BID4 BID17 Perozzi et al., 2014; Tang et al., 2015; Wang et al., 2016a; BID15 BID14 , with majority of the approaches focusing on advancing the state-of-the-art in static graph setting.
However, several domains now present highly dynamic data that exhibit complex temporal properties in addition to earlier cited challenges.
For instance, social network communications, financial transaction graphs or longitudinal citation data contain fine-grained temporal information on nodes and edges that characterize the dynamic evolution of a graph and its properties over time.These recent developments have created a conspicuous need for principled approaches to advance graph embedding techniques for dynamic graphs (Hamilton et al., 2017b) .
We focus on two pertinent questions fundamental to representation learning over dynamic graphs:
(i) What can serve as an elegant model for dynamic processes over graphs?
-A key modeling choice in existing representation learning techniques for dynamic graphs BID16 Zhou et al., 2018; BID14 Ngyuyen et al., 2018; Yu et al., 2018) assume that graph dynamics evolve as a single time scale process.
In contrast to these approaches, we observe that most real-world graphs exhibit at least two distinct dynamic processes that evolve at different time scales -Topological Evolution: where the number of nodes and edges are expected to grow (or shrink) over time leading to structural changes in the graph; and Node Interactions: which relates to activities between nodes that may or may not be structurally connected.
Modeling interleaved dependencies between these non-linearly evolving dynamic processes is a crucial next step for advancing the formal models of dynamic graphs.
(c) Communication Events (k=1) where nodes interact with each other.
For both these processes, t p,k=0 < (t 1 , t 2 , t 3 , t 4 , t 5 ) k=1 < t q,k=0 < (t 6 , t 7 ) k=1 < t r,k=0 .
(b) Evolving Representations.(ii
) How can one leverage such a model to learn dynamic node representations that are effectively able to capture evolving graph information over time? -Existing
techniques in this direction can be divided into two approaches: a.) Discrete-Time
Approach, where the evolution of a dynamic graph is observed as collection of static graph snapshots over time (Zhu et al., 2016; BID16 Zhou et al., 2018) . These approaches
tend to preserve (encode) very limited structural information and capture temporal information at a very coarse level which leads to loss of information between snapshots and lack of ability to capture fine-grained temporal dynamics. Another challenge
in such approaches is the selection of appropriate aggregation granularity which is often misspecified. b.) Continuous-Time
Approach, where evolution is modeled at finer time granularity in order to address the above challenges. While existing approaches
have demonstrated to be very effective in specific settings, they either model simple structural and complex temporal properties in a decoupled fashion BID14 or use simple temporal models (exponential family in (Ngyuyen et al., 2018) ). But several domains exhibit
highly nonlinear evolution of structural properties coupled with complex temporal dynamics and it remains an open problem to effectively model and learn informative representations capturing various dynamical properties of such complex systems.As noted in BID5 , an important requirement to effectively learn over such dynamical systems is the ability to express the dynamical processes at different scales. We propose that any dynamic
graph must be minimally expressed as a result of two fundamental processes evolving at different time scales: Association Process (dynamics of the network), that brings change in the graph structure and leads to long lasting information exchange between nodes; and Communication Process (dynamics on the network), that relates to activities between (not necessarily connected) nodes which leads to temporary information flow between them BID15 BID1 . We, then, posit our goal of
learning node representations as modeling a latent mediation process that bridges the above two observed processes such that learned representations drive the complex temporal dynamics of both processes and these processes subsequently lead to the nonlinear evolution of node representations. Further, the information propagated
across the graph is governed by the temporal dynamics of communication and association histories of nodes with its neighborhood. For instance, in a social network,
when a node's neighborhood grows, it changes that node's representation which in turn affects her social interactions (association → embedding → communication). Similarly, when node's interaction
behavior changes, it affects the representation of her neighbors and herself which in turn changes the structure and strength of her connections due to link addition or deletion (communication → embedding → association). We call this phenomenon -evolution
through mediation and illustrate it graphically in FIG0 .In this work, we propose a novel representation
learning framework for dynamic graphs, DyRep, to model interleaved evolution of two observed processes through latent mediation process expressed above and effectively learn richer node representations over time. Our framework ingests dynamic graph information
in the form of association and communication events over time and updates the node representations as they appear in these events. We build a two-time scale deep temporal point process
approach to capture the continuous-time fine-grained temporal dynamics of the two observed processes. We further parameterize the conditional intensity function
of the temporal point process with a deep inductive representation network that learns functions to compute node representations. Finally, we couple the structural and temporal components
of our framework by designing a novel Temporal Attention Mechanism, which induces temporal attentiveness over neighborhood nodes using the learned intensity function. This allows to capture highly interleaved and nonlinear dynamics
governing node representations over time. We design an efficient unsupervised training procedure for end-to-end
training of our framework. We demonstrate consistent and significant improvement over state-of-the-art
representative baselines on two real-world dynamic graphs for the tasks of dynamic link prediction and time prediction. We further present an extensive qualitative analysis through embedding visualization
and ablation studies to discern the effectiveness of our framework.
We introduced a novel modeling framework for dynamic graphs that effectively and efficiently learns node representations by posing representation learning as latent mediation process bridging dynamic processes of topological evolution and node interactions.
We proposed a deep temporal point process model parameterized by temporally attentive representation network that models these complex and nonlinearly evolving dynamic processes and learns to encode structural-temporal information over graph into low dimensional representations.
Our superior evaluation performance demonstrates the effectiveness of our approach compared to state-of-the-art methods.
We present this work as the first generic and unified representation learning framework that adopts a novel modeling paradigm for dynamic graphs and support wide range of dynamic graph characteristics which can potentially have many exciting adaptations.
As a part of our framework, we also propose a novel temporal point process based attention mechanism that can attend over neighborhood based on the history of communications and association events in the graph.
Currently, DyRep does not support network shrinkage due to following reasons:
(i) It is difficult to procure data with fine grained deletion time stamps and
(ii) The temporal point process model requires more sophistication to support deletion.
For example, one can augment the model with a survival process formulation to account for lack of node/edge at future time.
Another interesting future direction could be to support encoding higher order dynamic structures.
contains h struct which is computed for updating each node involved in the event.
For node u, the update will come from h v struct (green flow) and for node v, the update will come from h u struct (red flow).
Please note all embeddings are dynamically evolving hence the information flow after every event is different and evolves in a complex fashion.
With this mechanism, the information is passed from neighbors of node u to node v and neighbors of node v to node u.
(i) Interaction events lead to temporary pathway -such events can occur between nodes which are not connected.
In that case, this flow will occur only once but it will not make u and v neighbors of each other (e.g. meeting at a conference).
(ii) Topological events lead to permanent pathway -in this case u and v becomes neighbor of each other and hence will contribute to structural properties moving forward (e.g. being academic friends).
The difference in number of blue arrows on each side signify different importance of each node to node u and node v respectively.
Overall Embedding Update Process.
As a starting point, neighborhood only includes nodes connected by a structural edge.
On observing an event, we update the embeddings of two nodes involved in the event using Eq 4.
For a node u, the first term of Eq 4 (Localized Embedding Propagation) requires h struct which is the information that is passed from neighborhood (N v ) of node v to node u via node v (one can visualize v as being the message passer from its neighborhood to u).
This information is used to update the embedding of node u.
However, we posit that node v does not relay equal amount of information from its neighbors to node u.
Rather, node v receives its information to be relayed based on its communication and association history with its neighbors (which relates to importance of each neighbor).
This requires to compute the attention coefficients on the structural edges between node v and its neighbors.
For any edge, we want this coefficient to be dependent on rate of events between the two nodes (thereby emulating real world phenomenon that one gains more information from people one interacts more with).
Hence, we parameterize our attention module with the temporal point process parameter S uv .
Algorithm 1 outlines the process of computing the value of this parameter.
where ∈ * ̅ is the node in neighborhood of node u.
DISPLAYFORM0 Figure 6: Temporal Point Process based Self-Attention: This figure illustrates the computation of h u struct for node u to pass to node v for the same event described before between nodes u and v at time t with any k.
h u struct is computed by aggregating information from neighbors (1,2,3) of u.
However, Nodes that are closely connected or has higher interactions tend to attend more to each other compared to nodes that are not connected or nodes between which interactions is less even in presence of connection.
Further, every node has a specific attention span for other node and therefore attention itself is a temporally evolving quantity.
DyRep computes the temporally evolving attention based on association and communication history between connected nodes.
The attention coefficient function (q's) is parameterized by S which is computed using the intensity of events between connected nodes.
Such attention mechanism allows the evolution of importance of neighbors to a particular node (u in this case) which aligns with real-world phenomenon.

assistant: text: Models Representation Learning over dynamic graphs as latent hidden process bridging two observed processes of Topological Evolution of and Interactions on dynamic graphs.