Patent Publication Number: US-2023134742-A1

Title: Deep neural network system for similarity-based graph representations

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 16/416,070, filed on May 17, 2019, which claims priority to U.S. Provisional Application No. 62/673,841, filed on May 18, 2018. The disclosures of the prior applications are considered part of and are incorporated by reference in the disclosure of this application. 
    
    
     BACKGROUND 
     This specification relates to a neural network system for determining graph similarity and for indexing and retrieving data associated with graphs. 
     Neural networks are machine learning models that employ one or more layers of nonlinear units to predict an output for a received input. Some neural networks include one or more hidden layers in addition to an output layer. The output of each hidden layer is used as input to the next layer in the network, i.e., the next hidden layer or the output layer. Each layer of the network generates an output from a received input in accordance with current values of a respective set of parameters. 
     Some neural networks are recurrent neural networks. A recurrent neural network is a neural network that receives an input sequence and generates an output sequence from the input sequence. In particular, a recurrent neural network can use some or all of the internal state of the network from a previous time step in computing an output at a current time step. An example of a recurrent neural network is a long short term memory (LSTM) neural network that includes one or more LSTM memory blocks. Each LSTM memory block can include one or more cells that each include an input gate, a forget gate, and an output gate that allow the cell to store previous states for the cell, e.g., for use in generating a current activation or to be provided to other components of the LSTM neural network. 
     SUMMARY 
     This specification describes a system implemented as computer programs on one or more computers in one or more locations that determines graph similarity. 
     Graph structures may be used to represent many different types of physical systems and entities. A system capable of determining graph similarity provides a system capable of solving many technical problems involving real-world physical entities by comparing graphs representing such entities. For example, a graph may represent a drug molecule, a computer network, a transportation network, a computer chip, a warehouse layout for control of a robotic system in the warehouse; an item of manufacture which may be used to control a robotic system to assemble, disassemble or repair the item; a graph may represent a physical location and may be used by an autonomous agent such as a self-driving vehicle to navigate through the physical location. It will be appreciated that there exists many other technical applications for generated graph structures. 
     In general, a graph comprises a set of nodes and a set of edges that connect two nodes. It is possible however, for graphs to have only unconnected nodes with no edges or in a special case, for a graph to contain no nodes and no edges. 
     A node of a graph may be used to represent a particular entity in a physical system and may have a type based upon a type of the entity. For example, a node may represent a particular atom of a molecule, the atom being of a particular type of chemical element. A feature vector may be associated with a node. The feature vector may represent the type of node and other properties of the entity represented by the node. 
     An edge of a graph may be used to represent a relationship between two nodes connected by the edge. For example, an edge may represent a chemical bond between two atoms or may represent a communication link between two nodes of a network. Edges may also have a type associated with, for example, if a communication link is a wired or wireless link. A feature vector may be associated with an edge. The feature vector may represent the type of edge and other properties associated with the edge. Edges may be directed or undirected. An undirected edge may be represented using a pair of directed edges having opposing directions. 
     The neural network system for determining graph similarity comprises one or more neural networks configured to: process an input graph to generate a vector representation of the input graph. The neural network system further comprises one or more processors configured to: receive a first graph; receive a second graph; generate a vector representation of the first graph using the one or more neural networks; generate a vector representation of the second graph using the one or more neural networks; and determine a similarity score for the first graph and the second graph based upon the vector representations of the first graph and the second graph. Processing an input graph to generate a vector representation of the input graph may comprise processing the input graph to generate a node state representation vector for each node of the input graph and an edge representation vector for each edge of the input graph; and processing the node state representation vectors and the edge representation vectors to generate the vector representation of the input graph. 
     By using one or more neural networks for determining the vector representation of a graph, the pertinent features for determining similarity between graphs may be learned automatically as compared to prior art methods that require hand-crafted features or similarity measures. In addition, by determining a similarity score based upon vector representations of the first and second graphs, a similarity between the graphs can be determined efficiently. For example, fast nearest neighbour search data structures such as KD-trees or locality sensitive hashing may then be used to index and store a large number of graphs and to efficiently retrieve a specific graph or graphs having similar properties. In addition, the vector representation of a graph provides a compact yet expressive representation of the graph and avoids the need to store an entire graph, resulting in reduced memory and storage requirements. This is particularly advantageous when operating on larger datasets and databases. 
     Aspects may include one or more of the following features. The one or more neural networks may be configured to process a pair of input graphs to generate a vector representation of each input graph of the pair of input graphs, the vector representation of each input graph being based upon both input graphs of the pair of graphs; and the vector representation of the first graph and the vector representation of the second graph may be generated based upon inputting the first graph and the second graph as a pair of input graphs to the one or more neural networks. As such, the vector representation of a first graph depends on the other graph that the first graph is compared with and vice versa. In this way, the one or more neural networks can model the differences and similarities between graphs more effectively as compared to a neural network system that models graphs independently. 
     The one or more neural networks may comprise an encoder neural network subsystem configured to process an input graph to generate the node state representation vector for each node of the input graph and the edge representation vector for each edge of the input graph. 
     The one or more neural networks may comprise a propagation neural network subsystem configured to update the node state representation vector associated with a particular node of the input graph based upon the node state representation vectors associated with one or more adjacent nodes in the input graph and the edge representation vectors associated with the edges connecting the particular node to the one or more adjacent nodes in the input graph. An adjacent node is a neighbouring node in which there exists an edge between the particular node and the adjacent node. The node state representation vector for a particular node may therefore be updated based upon information from a local neighbourhood of the particular node. 
     The one or more neural networks may comprise an aggregator neural network subsystem configured to process the node state representation vectors associated with each node of the input graph to generate the vector representation of the input graph. 
     The encoder neural network subsystem may comprise a node encoder neural network configured to generate the node state representation vector; and an edge encoder neural network configured to generate the edge representation vector. 
     The node state representation vector may comprise a plurality of node state representation vectors for each node of the input graph. For example, the node encoder neural network may comprise a plurality of hidden layers and an output layer; and the plurality of node state representation vectors for each node of the input graph comprises a vector corresponding to each of the plurality of hidden layers and the output layer. Alternatively, there may be a vector for each of a selection of the hidden layers and output layer. In a similar manner, the edge representation vector may also comprise a plurality of edge representation vectors for each edge of the input graph which may also correspond to each or a selection of the hidden layers and output layer of the edge encoder neural network. 
     An input graph may comprise a set of nodes and edges. The input graph may comprise an initial vector representation associated with each respective node and respective edge of the input graph. The initial vector representation may comprise a feature vector. The feature vector may be based upon a type of the node or edge, a direction of the edge, or any other information as appropriate based upon the domain being modelled by the graph. The initial vector representation may be initialized to a vector of all ones. 
     The encoder neural network subsystem may be configured to generate the node state representation vector for a node of the input graph based upon the initial vector representation associated with the node in the input graph. Likewise, the encoder neural network subsystem may be configured to generate the edge representation vector for an edge of the input graph based upon the initial vector representation associated with the edge in the input graph. 
     The propagation neural network subsystem may comprise a message generation neural network configured to process the node state representation vector associated with a particular node and the node state representation vector associated with an adjacent node in the input graph and the edge representation vector associated with the edge connecting the particular node and the adjacent node to generate a message vector associated with the adjacent node in the input graph. The propagation neural network subsystem may comprise a node update neural network configured to generate an updated node state representation vector associated with the particular node based upon the current node state representation vector associated with the particular node and message vectors generated for each adjacent node adjacent the particular node in the input graph. In this way, information may be propagated through the graph and the node state representation vector for a particular node may include information from the local neighbourhood of the particular node. For example, the node state representation vector may be indicative of the local structure of the input graph around the particular node. 
     The node update neural network may be configured to process a summation of the message vectors generated for each adjacent node adjacent the particular node in the input graph to generate the updated node state representation vector. The summation may be a weighted sum or an attention-based weighted sum. 
     The node update neural network may be a recurrent type of neural network. For example, the node update neural network may be a basic recurrent neural network, a gated recurrent unit (GRU) based recurrent neural network or a LSTM based recurrent neural network. 
     The propagation neural network subsystem may be configured to generate a cross-graph matching vector based upon a similarity between a particular node of the input graph and one or more nodes of a second input graph. The node update neural network may be configured to generate an updated node state representation vector associated with the particular node based upon the current node state representation vector associated with the particular node, the message vectors generated for each adjacent node adjacent the particular node in the input graph, and the cross-graph matching vector. In this way, additional comparisons at levels other than a final graph level comparison may be performed, enabling a more comprehensive comparison between graphs. By using the cross-graph matching vector to update the node state representation vector, the neural network system is better able to model differences and similarities between graphs as compared to a neural network system that models graphs independently. Information may be propagated between two graphs based upon the cross-graph matching vector. The model may learn to allocate capacity toward representing a graph itself and toward representing features for determining similarity as appropriate. 
     The similarity upon which the cross-graph matching vector is based may be based upon a similarity of the node state representation vector associated with the particular node of the input graph and the node state representation vector associated with each of the one or more nodes of the second input graph. The similarity may be based upon a difference between the node state representation vector associated with the particular node of the input graph and a weighted sum of the node state representation vectors associated with each of the one or more nodes of the second input graph. In this way, if two graphs are similar, the cross-graph matching vector is likely to be small and have limited effect on the node state representation vector and the vector graph representation of the two graphs. However, if the two graphs are dissimilar, the cross-graph matching vector is likely to have a larger value and has the effect of altering the representations of the two graphs to be further apart. 
     The weighted sum may be a weighted sum of all of the node state representation vectors associated with each of the nodes of the second input graph. The weight for each respective node of the one or more nodes of the second input graph may be based upon a similarity score between the node state representation vector associated with the particular node of the input graph and the node state representation vector associated with the respective node of the second input graph. In this way, the cross-graph matching vector may be a measure of how well a node of one graph matches another node in a second graph and may be a comparison between a particular node of one graph and a closest node or nodes of the second graph. The similarity score may be based upon any vector space metric. The weight for each respective node of the one or more nodes of the second input graph may be normalized. For example, by applying a softmax function to the set of similarity scores. 
     It will be appreciated that where a pair of graphs are input to the one or more neural networks, the second input graph is the other graph of the pair of graphs. It will also be appreciated that updating based upon a cross-graph matching vector may be performed for the second graph of the pair of input graphs in a similar manner to generate the vector representation of the second graph. 
     The node state representation vectors associated with each node of the input graph may undergo a plurality of updates. In this way, information in the graph may be propagated further through the graph and the node state representation vector associated with a particular node may be able to model information from beyond its immediate neighbourhood. 
     The one or more neural networks may comprise a plurality of propagation neural network subsystems. The propagation neural network subsystems may share a portion of the parameters of their respective neural networks. 
     The plurality of propagation neural network subsystems may comprise a propagation neural network subsystem for each of the plurality of hidden layers and the output layer of the node encoder neural network. Alternatively, plurality of propagation neural network subsystems may comprise a propagation neural network subsystem for a selection of the plurality of hidden layers and the output layer of the node encoder neural network. 
     The aggregator neural network subsystem may comprise an aggregator neural network configured to process a node state representation vector to generate a transformed vector associated with the node; and the vector representation of the input graph may be based upon a summation of the transformed vectors associated with each node of the input graph. The summation of the transformed vectors may be a weighted summation. The weights of the weighted summation may comprise a gating vector for each transformed vector. It will be appreciated that where there is a plurality of node state representation vectors associated with each node of the input graph, the aggregator neural network may be configured to generate a transformed vector for each respective node state representation vector of the plurality of node state representation vectors for subsequent summation or alternatively, the aggregator neural network may be configured to generate a transformed vector by processing the plurality of node state representation vectors jointly. 
     The determined similarity score for the first graph and the second graph may be based upon a Euclidean distance, cosine distance, Hamming distance or any other vector space distance metric. It will also be appreciated that where a similarity score is described as being determined, these similarity metrics may also be suitable metrics for computing a similarity score. 
     The one or more neural networks may be trained based upon optimization of a loss function based upon a similarity between a pair of graphs. In this way, the one or more neural networks may learn to model the pertinent features for determining similarity between graphs. In addition, the training data set does not require graphs to be labelled according to their respective classes as in supervised learning. The one or more neural networks are capable of generalizing and determining similarity even when no examples of a graph belonging to a particular class are present in the training data set. 
     The one or more neural networks may be implemented as an end-to-end neural network system. That is, each of the above described neural networks may be connected as appropriate and may be trained jointly. Therefore, standard gradient descent methods may be used to optimize the loss function. 
     The one or more neural networks may be trained based upon a training data set comprising a plurality of pairs of graphs, each pair of graphs labelled as similar or not similar. For example, a pair of similar graphs may have a label of +1 whilst a pair of dissimilar graphs may have a label of −1. 
     Alternatively, the one or more neural networks may be trained based upon optimization of a loss function based upon a relative similarity between a triplet of graphs. For example, each triplet of graphs may be labelled based upon whether a first graph of the triplet is closer in similarity to a second graph of the triplet or whether the first graph is closer in similarity to a third graph of the triplet. In the former case, the label may be +1 and in the latter case, the label may be −1. In this way, training does not require a data set of graphs explicitly labelled as similar or dissimilar, only whether one graph has greater similarity compared to another graph. 
     Two graphs may be considered similar if they are full-graph isomorphic, sub-graph isomorphic, or have small graph edit distances. Similarity may also be defined based upon similarity measures in the domain of the objects represented by the graphs. 
     The loss function may be a margin-based loss function. The loss function similarity may be based upon a Euclidean distance or a Hamming distance or any other suitable distance metric. 
     The vector representation of the input graph may be modified by applying a tan h function to the vector representation of the input graph. Each component of the vector representation of the input graph may take one of two possible values. That is, the vector representation may be binary but is not limited to having values of either zero or one and may, for example, have a value of +1 or −1. 
     The graph may represent one of the following: a molecule, a computer program function, a parse tree, a computer network, a vehicular traffic network, and a knowledge graph. 
     It will be appreciated that each of the described neural networks may be a deep neural network having a plurality of hidden layers. Each of neural networks may also be a recurrent type neural network. It will also be appreciated that where it is described that a summation is performed, it may be also possible to use an alternative form of aggregation such as a mean, median or mode. 
     According to another aspect, there is described a method for determining graph similarity, the method comprising, receiving, by one or more processors, a first graph; receiving, by the one or more processors, a second graph; generating, by one or more neural networks, a vector representation of the first graph; generating, by the one or more neural networks, a vector representation of the second graph using the one or more neural networks; determining, by the one or more processors, a similarity score based upon the vector representations of the first graph and the second graph. Generating, by the one or more neural networks, a vector representation of a graph may comprise: processing, by the one or more neural networks, the graph to generate a node state representation vector for each node of the graph and an edge representation vector for each edge of the graph; and processing, by the one or more neural networks, the node state representation vectors and the edge representation vectors to generate the vector representation of the graph. 
     According to a further aspect, there is described a system for binary function vulnerability detection comprising: one or more processors configured to: receive a first control flow graph associated with a first binary function having a known vulnerability; receive a second control flow graph associated with a second binary function; determine a similarity score for the first control flow graph and the second control flow graph using the neural network system of as described above; determining that second binary function is vulnerable if the similarity score exceeds a threshold similarity score. It is often the case that source code is unavailable for inspection and only a compiled binary function is available. The system therefore enables determining whether binary functions are vulnerable based upon their similarity to known vulnerable binary functions. 
     A node of a control flow graph may represent a block of instructions and an edge of a control flow graph may represent control flow between blocks of instructions. A control flow graph may be produced using a disassembler and code analyzer. 
     The neural network system may be trained based upon a similarity metric such that control flow graphs associated with a binary function having the same source code have higher similarity than control flow graphs associated with a binary function having different source code. In this way, invariance to compiler type and compiler optimizations may be achieved. 
     The one or more neural networks may further comprise a neural network configured to generate a feature vector based upon the instruction types associated with the block of instructions represented by the node. This neural network may be trained jointly with the other neural networks described above. 
     According to another aspect, there is provided a neural network system implemented by one or more computers for retrieval of data associated with a graph, the neural network system comprising: one or more neural networks configured to: process an input graph to generate a vector representation of the input graph; a memory storing a database comprising a plurality of records, each record of the plurality of records being associated with a respective graph; one or more processors configured to: receive a query graph; generate a vector representation of the query graph using the one or more neural networks; for each record of the plurality of records, process the vector representation of the query graph and a vector representation associated with the respective graph associated with the record to determine a respective similarity score; and outputting data associated with one or more records based upon the determined similarity scores. In this way, the plurality of records associated with a graph may be efficiently indexed using the vector representation of the graph. As the one or more neural networks are capable of modeling graphs such that the pertinent features for determining similarity between graphs is reflected in the vector representation, the index also functions as means to efficiently store graphs and their associated records by similarity, enabling efficient means to search for records associated with similar graphs and to retrieve records associated with a specific query graph. Processing an input graph to generate a vector representation of the input graph may comprise processing the input graph to generate a node state representation vector for each node of the input graph and an edge representation vector for each edge of the input graph; and processing the node state representation vectors and the edge representation vectors to generate the vector representation of the input graph. 
     The one or more neural networks may be further configured to process a pair of input graphs to generate a vector representation of each input graph of the pair of input graphs, the vector representation of each input graph being based upon both input graphs of the pair of graphs; and the one or more processors may be further configured to: determine a set of candidate graphs based upon the determined similarity scores between the vector representations of the query graph and each respective graph associated with each of the plurality of records; determine a second vector representation for each query graph and candidate graph pair in the set of candidate graphs using the one or more neural networks; determine a similarity score for each query graph and candidate graph pair based upon the determined second vector representations; and outputting data associated with the record associated with a candidate graph based upon the determined similarity scores for each query graph and candidate graph pair. In this way, by first determining similarity of graphs based upon vector representations of graphs generated independently, a set of candidate graphs may be quickly determined. The set of candidate graphs may be then be compared more precisely by generating vector graph representations using the query graph and each respective candidate graph as an input pair to the one or more neural networks. 
     The neural network can be configured to receive any kind of digital data input and to generate any kind of score, classification, or regression output based on the input. 
     For example, if the inputs to the neural network are graphs representative of images or features that have been extracted from images, the output generated by the neural network for a given image may be used to determine an estimated likelihood that the image contains an image of an object belonging to the category based upon the determined similarity score. 
     As another example, if the inputs to the neural network are graphs representative of Internet resources (e.g., web pages), documents, or portions of documents or features extracted from Internet resources, documents, or portions of documents, the output generated by the neural network for a given Internet resource, document, or portion of a document may be used to determine for each of a set of topics, an estimated likelihood that the Internet resource, document, or document portion is about the topic based upon the determined similarity score. 
     As another example, if the inputs to the neural network are graphs representative of features of an impression context for a particular advertisement, the output generated by the neural network may be used to determine an estimated likelihood that the particular advertisement will be clicked on based upon the determined similarity score. 
     As another example, if the inputs to the neural network are graphs representative of features of a personalized recommendation for a user, e.g., features characterizing the context for the recommendation, e.g., features characterizing previous actions taken by the user, the output generated by the neural network may be used for each of a set of content items, to determine an estimated likelihood that the user will respond favorably to being recommended the content item based upon the determined similarity score. 
     As another example, if the input to the neural network is a graph representative of a sequence of text in one language, the output generated by the neural network may be used to determine for each of a set of pieces of text in another language, an estimated likelihood that the piece of text in the other language is a proper translation of the input text into the other language based upon the determined similarity score. 
     As another example, if the input to the neural network is a graph representative of a sequence representing a spoken utterance, the output generated by the neural network may be used to determine for each of a set of pieces of text, an estimated likelihood that the piece of text is the correct transcript for the utterance based upon the determined similarity score. 
     The subject matter described in this specification can be implemented in particular embodiments so as to realize one or more of the following advantages. 
     By using one or more neural networks for determining the vector representation of a graph, the pertinent features for determining similarity between graphs may be learned automatically as compared to prior art methods that require hand-crafted features or similarity measures. In addition, by determining a similarity score based upon vector representations of the first and second graphs, a similarity between the graphs can be determined efficiently. For example, fast nearest neighbour search data structures such as KD-trees or locality sensitive hashing may then be used to index and store a large number of graphs and to efficiently retrieve a specific graph or graphs having similar properties. In addition, the vector representation of a graph provides a compact yet expressive representation of the graph and avoids the need to store an entire graph, resulting in reduced memory and storage requirements. This is particularly advantageous when operating on larger datasets and databases. 
     Where both graphs being compared are input to the one or more neural networks to generate a vector representation of the graphs, the one or more neural networks are able to perform additional comparisons between the graphs, the results of which may be reflected by the vector representation of the two graphs. As such, the vector representation of the two graphs provides more information relating to their similarity and enables an efficient vector space comparison to be performed to provide an accurate similarity score. 
     In the context of data retrieval from a database, the plurality of records associated with a graph may be efficiently indexed using the vector representation of the graph. As the one or more neural networks are capable of modeling graphs such that the pertinent features for determining similarity between graphs is reflected in the vector representation, the index also functions as means to efficiently store graphs and their associated records by similarity, enabling efficient means to search for records associated with similar graphs and to retrieve records associated with a specific query graph. 
     The neural network system may be used in many other different applications, for example, searching and retrieval from large databases of molecules, generation of similar molecules to a particular molecule, for example for drug discovery, determining alternative network architectures and vulnerabilities in network architectures, determining alternative traffic routes and determining variants of computer viruses. 
     The details of one or more embodiments of the subject matter described in this specification are set forth in the accompanying drawings and the description below. Other features, aspects, and advantages of the subject matter will become apparent from the description, the drawings, and the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    shows an example neural network system for determining graph similarity. 
         FIG.  2    shows exemplary neural network subsystems of the neural network system in more detail. 
         FIG.  3    shows an exemplary configuration of the propagation neural network subsystem. 
         FIG.  4    shows another exemplary configuration of the propagation neural network subsystem. 
         FIGS.  5 A and  5 B  show visualizations of attention weights for two pairs of exemplary graphs. 
         FIG.  6    is a flow chart of an example process for determining graph similarity. 
         FIG.  7    shows two exemplary control flow graphs. 
     
    
    
     DETAILED DESCRIPTION 
       FIG.  1    shows an example neural network system  100  for determining graph similarity. The neural network system  100  is an example of a system implemented as computer programs on one or more computers in one or more locations, in which the systems, components, and techniques described below can be implemented. As such, the system comprises one or more processors. 
     The one or more processors are configured to receive a first graph  101  and a second graph  102  of which it is desired to determine a similarity between. The one or more processors are further configured to generate a vector representation  103  of the first graph and a vector representation  104  of the second graph using one or more neural networks  105 . The one or more processors are also configured to determine a similarity score  106  for the first graph  101  and the second graph  102  based upon the generated vector representations  103 ,  104  of the first and second graphs. The similarity score  106  may be determined using a similarity scorer subsystem  107 . The computation of a similarity score  106  is described in more detail below. 
     The system  100  also comprises the one or more neural networks  105  for generating a vector representation of an input graph. In this regard, the one or more neural networks  105  are configured to process an input graph to generate a node state representation vector for each node of the input graph and an edge representation vector for each edge of the input graph. The one or more neural networks  105  are further configured to process the node state representation vectors and the edge representation vectors to generate a vector representation of the input graph. 
     It is also possible for the one or more neural networks  105  to process a pair of input graphs together to generate a vector representation of each input graph of the pair of input graphs. Thus, the vector representation of each input graph is based upon both input graphs of the pair of input graphs rather than processing each input graph separately and generating the vector representation of the input graphs independently. 
     That is, the one or more neural networks  105  may be configured to generate the vector representation of the first graph  103  and the vector representation of the second graph  105  by processing the first graph  101  and the second graph  102  individually. Where a vector representation of an input graph is obtained by processing an individual input graph, this is referred to as a graph embedding model. Alternatively, the one or more neural networks  105  may be configured to generate the vector representation of the first graph  103  and the vector representation of the second graph  105  by processing the first graph  101  and the second graph  102  together. This case is referred to as a graph matching model. Both embedding models and matching models will be described in more detail below. 
     Referring now to  FIG.  2   , in both graph embedding and graph matching models, the one or more neural networks  105  may comprise an encoder neural network subsystem  201 , a propagation neural network subsystem  202  and an aggregator neural network subsystem  203 . 
     The encoder neural network subsystem  201  may be configured to process an input graph  204  to generate the node state representation vector  205  for each node of the input graph and the edge representation vector  206  for each edge of the input graph. For both the graph embedding and graph matching models, input graphs are typically processed individually by the encoder neural network subsystem  201  and thus the node state representation vectors  205  and the edge representation vectors  206  for an input graph  204  are generated based upon the input graph  204  independent of any second input graph. In the graph matching model, the first and second input graphs may be processed in parallel to generate the node state representation vectors and edge representation vectors for the first and second input graphs at the same time. 
     The encoder neural network subsystem  201  may further comprise a node encoder neural network configured to generate the node state representation vector and an edge encoder neural network configured to generate the edge representation vector. For example, the node encoder neural network may be a multi-layer perceptron network and the node state representation vector for a node may be obtained as follows: 
         h   i   (0) =MLP node ( x   i ),∀ i∈V   (1)
 
     where V is the set of nodes of the graph, i is an index over the set of nodes, x i  is a feature vector associated with node i, MLP node  is the node encoder neural network and h i   (0)  is the node state representation vector for node i. Where nodes do not have associated feature vectors, x i  may, for example, be initialized to a vector of 1 s. 
     In a similar manner, the edge encoder neural network may also be a multi-layer perceptron network and the edge representation vector for an edge may be obtained as follows: 
         e   ij =MLP edge ( x   ij ),∀( i,j )∈ E   (2)
 
     where E is the set of edges of the graph, (i,j) is an edge in the set connecting a node i and node j of the graph, x ij  is feature vector associated with edge (i,j), MLP edge  is the edge encoder neural network and e ij  is the edge representation vector for edge (i,j). Where edges do not have associated feature vectors, x ij  may, for example, be initialized to a vector of 1 s. 
     The node encoder neural network may comprise a plurality of hidden layers and an output layer. The node state representation vector may be a concatenation of the hidden layer activations and the output layer activations. As such, the node state representation vector may comprise a plurality of node state representation vectors for each node of the input graph and may comprise a vector corresponding to each of the plurality of hidden layers and the output layers. Alternatively, the node state representation vector may comprise any subset of the hidden layer and output layer vectors concatenated. 
     The propagation neural network subsystem  202  may be configured to update the node state representation vector  205  associated with a particular node of the input graph based upon the node state representation vectors associated with one or more adjacent nodes in the input graph and the edge representation vectors associated with the edges connecting the particular node to the one or more adjacent nodes in the input graph. That is, for a particular node, the node state representation vector  205  may be updated based upon the nodes and edges that the particular node is connected to. In this way, the node state representation vector  205  for a particular node is updated based upon information from the particular node&#39;s local neighbourhood. The node state representation vector is therefore also based upon the structure of the graph rather than treating a graph as just an independent set of nodes. Further details of this updating are provided below with reference to  FIGS.  3  and  4   . 
     The aggregator neural network subsystem  203  may be configured to process the node state representation vectors associated with each node of the input graph to generate the vector representation  208  of the input graph  204 . The node state representation vectors processed by the aggregator neural network subsystem  203  may be the updated node state representation vectors  207  output by the propagation neural network subsystem  202 . Alternatively, the node state representation vectors  205  may be those output by the encoder neural network subsystem  201  or may be a combination of updated node state representation vectors for a subset of nodes and initial node state representation vectors for the rest of the nodes of the input graph  204 . 
     As an example, the vector representation  208  of the input graph  204  may be obtained as follows: 
     
       
         
           
             
               
                 
                   
                     h 
                     G 
                   
                   = 
                   
                     
                       MLP 
                       G 
                     
                     ( 
                     
                       
                         ∑ 
                         
                           i 
                           ∈ 
                           V 
                         
                       
                       
                         
                           σ 
                           ⁡ 
                           ( 
                           
                             
                               MLP 
                               gate 
                             
                             ( 
                             
                               h 
                               i 
                               
                                 ( 
                                 T 
                                 ) 
                               
                             
                             ) 
                           
                           ) 
                         
                         ⊙ 
                         
                           MLP 
                           ⁡ 
                           ( 
                           
                             h 
                             i 
                             
                               ( 
                               T 
                               ) 
                             
                           
                           ) 
                         
                       
                     
                     ) 
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
     where h i   (T)  is the node state representation vector for node i after T rounds of updates, ⊙ is an element-wise vector multiplication, MLP is an aggregator neural network configured to process a node state representation vector to generate a transformed vector associated with the node, MLP gate  is a multi-layer perceptron configured to output a gating vector for node i based upon its node state representation vector, MLP G  is a further multi-layer perceptron and h G  is the vector representation of the input graph. In general terms, the above equation (3) computes a vector representation of the input graph by processing a weighted sum of the node state representation vectors of each node of the input graph through a neural network. MLP gate  and MLP may comprise a single linear layer each having the same dimensionality as the required dimensionality of the vector representation of the graph. MLP G  may comprise one hidden layer with ReLU non-linearity. It will be appreciated that other architectures and other non-linearities may be chosen as deemed appropriate by a person skilled in the art. 
     Equation (3) may be used in both the graph embedding and graph matching models. In the graph matching model, once the node state representation vectors have been obtained and undergone a desired number of rounds of updating, the node state representation vectors for the first and second input graphs may be processed independently to obtain vector representations of the first and second input graphs respectively. 
     The vector representation of a graph may be a vector of real-values or may be a vector of binary values. It will be appreciated that where the vector is binary valued, the binary values may be two values other than 0 and 1, and may, for example, be −1 and 1. A vector of binary values may be advantageous in cases where it is necessary to search through a large database of graphs with low latency where efficient nearest neighbor search algorithms may be applied. 
     A similarity score  106  for any two graphs may be determined based upon a comparison of the vector representations of the two respective graphs. For example, the comparison may be based upon any vector space metric such as a Euclidean distance, cosine similarity or Hamming distance. 
     Referring now to  FIG.  3   , an exemplary configuration of the propagation neural network subsystem  202  will be described in more detail. The propagation neural network subsystem  202  may comprise a message generation neural network  301  and a node update neural network  302 . 
     The message generation neural network  301  may be configured to process the node state representation vector associated with a particular node and the node state representation vector associated with an adjacent node in the input graph and the edge representation vector associated with the edge connecting the particular node and the adjacent node to generate a message vector associated with the adjacent node in the input graph. That is, for each node that a particular node is connected to, a message vector can be generated using the message generation neural network  301  as exemplified below: 
         m   j→i =ƒ message ( h   i   (t)   ,h   j   (t)   ,e   ij )  (4)
 
     where m j→i  is a generated message vector for the connection between node j and node i, h i   (t)  is the current node state representation vector for node i after the latest update round t, h j   (t)  is the current node state representation vector for node j after the latest update round t, e ij  is the edge representation vector associated with the edge connecting nodes i and j, and ƒ message  is the message generation neural network  301 . As an example, the message generation neural network  301  may be a multi-layer perceptron having one hidden layer with rectified linear units (ReLU). However, it will be appreciated that other forms of neural network may be suitable for implementing the message generation neural network  301 . The inputs, h i   (t) , h j   (t) , e ij  to the message generation neural network  301  may be concatenated together to form a single vector input. 
     The node update neural network  302  may be configured to generate an updated node state representation vector associated with the particular node based upon the current node state representation vector associated with the particular node and the message vectors  303  generated for each adjacent node adjacent the particular node in the input graph. That is, a particular node&#39;s state vector representation may be updated based upon information obtained from adjacent nodes as exemplified below 
     
       
         
           
             
               
                 
                   
                     h 
                     i 
                     
                       ( 
                       
                         t 
                         + 
                         1 
                       
                       ) 
                     
                   
                   = 
                   
                     
                       f 
                       node 
                     
                     ( 
                     
                       
                         h 
                         i 
                         
                           ( 
                           t 
                           ) 
                         
                       
                       , 
                       
                         
                           ∑ 
                           
                             
                               j 
                               : 
                                  
                               
                                 ( 
                                 
                                   j 
                                   , 
                                   i 
                                 
                                 ) 
                               
                             
                             ∈ 
                             E 
                           
                         
                         
                           m 
                           
                             j 
                             → 
                             i 
                           
                         
                       
                     
                     ) 
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
     where Σ j:(j,i)∈E m j→i  is a sum of the message vectors associated with all nodes connected to node i, h i   (t)  is the current node state representation vector for node i after the latest update round t, h i   (t+1)  is the updated node state representation vector for node i, and ƒ node  is the node update neural network  302 . As an example, the node update neural network  302  may be a multi-layer perceptron or a recurrent neural network comprising conventional recurrent neural network units (RNN), gated recurrent units (GRU), or long-term short term memory units (LSTM). It will be appreciated that the sum in the above equation (5) may be replaced by an alternative aggregation operator, for example, a mean, a maximum, or an attention based weighted sum amongst others. 
     The system  100  may be configured to perform multiple rounds of node state vector representation updates. That is, as shown by the dashed line  304  in  FIG.  3   , an updated node state representation vector  207  may be fed back into the message generation neural network  301  for further processing to generate further message vectors in order to perform a second round of updating by the node update neural network  302 . By performing multiple rounds of propagation, nodes may accumulate information in relation to the structure of the graph outside of the node&#39;s local neighbourhood. The number of rounds of updates may be chosen as deemed appropriate by a person skilled in the art. As an example, the number of rounds of updates may be between 1 and 5. 
     Referring now to  FIG.  4   , an alternative configuration of the propagation neural network subsystem  202  will be described. In particular, this alternative configuration is particularly suitable for use as part of the graph matching model. This configuration also comprises a message generation neural network  401  and a node update neural network  402  along with an additional matching subsystem  403 . 
     The message generation neural network  401  may be configured to process the node state representation vectors  404 ,  406  and the edge representation vectors  405 ,  407  of a first and second input graph  101 ,  102  respectively to generate message vectors  408 . The message generation neural network  401  is configured to generate message vectors  408  in a similar manner to the message generation neural network  301  of  FIG.  3   . For example, a message vector may be generated as follows: 
         m   j→i =ƒ message ( h   i   (t)   ,h   j   (t)   ,e   ij ),∀( i,j )∈ E   1   ∪E   2   (6)
 
     where E 1  and E 2  are the set of edges of the first input graph and the second input graph respectively with the other terms having the same meaning as equation (4) above. As there are no edges connecting nodes of the first and second graphs, the message generation neural network  401  of  FIG.  4    effectively operates in the same way as the message generation neural network  301  of  FIG.  3    when applied to two graphs individually. 
     The matching subsystem  403  may be configured to generate a cross-graph matching vector  409  based upon a similarity between a particular node of the first input graph  101  and one or more nodes of the second input graph  102 . In general terms, the cross-graph matching vector provides a measure of how well a node in one graph can be matched to one or more nodes in a second graph. 
     As an example, the cross-graph matching vector may be obtained as follows: 
       μ j→i =ƒ match ( h   i   (t)   ,h   j   (t) ),∀ i∈V   1   ,j∈V   2 , or  i∈V   2   ,j∈V   1   (7)
 
     where V 1  and V 2  are the set of nodes in the first and second graphs respectively, i and j are a pair of nodes, each node of the pair from different graphs for which a matching is to be performed, h i   (t)  and h j   (t)  are the respective current node state representation vectors of nodes i and j, ƒ match  is a chosen matching function, and μ j→i  is the cross-graph matching vector for nodes i and j. 
     The matching function, ƒ match , (and hence the similarity between a node of one input graph and a node of another input graph) may be based upon a difference between the node state representation vector associated with the particular node of the input graph and a weighted sum of the node state representation vectors associated with each of the one or more nodes of the second input graph. For example, an attention based weighted sum may be used as follows: 
     
       
         
           
             
               
                 
                   
                     a 
                     
                       j 
                         
                       → 
                       i 
                     
                   
                   = 
                   
                     
                       exp 
                       ⁡ 
                       ( 
                       
                         
                           s 
                           h 
                         
                         ( 
                         
                           
                             h 
                             i 
                             
                               ( 
                               t 
                               ) 
                             
                           
                           , 
                           
                             h 
                             j 
                             
                               ( 
                               t 
                               ) 
                             
                           
                         
                         ) 
                       
                       ) 
                     
                     
                       
                         ∑ 
                         
                           j 
                           ′ 
                         
                       
                       
                         exp 
                         ⁡ 
                         ( 
                         
                           
                             s 
                             h 
                           
                           ( 
                           
                             
                               h 
                               i 
                               
                                 ( 
                                 t 
                                 ) 
                               
                             
                             , 
                             
                               h 
                               
                                 j 
                                 ′ 
                               
                               
                                 ( 
                                 t 
                                 ) 
                               
                             
                           
                           ) 
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   8 
                   ) 
                 
               
             
           
         
       
       
         
           
             
               
                 
                   
                     μ 
                     
                       j 
                       → 
                       i 
                     
                   
                   = 
                   
                     
                       a 
                       
                         j 
                         → 
                         i 
                       
                     
                     ( 
                     
                       
                         h 
                         i 
                         
                           ( 
                           t 
                           ) 
                         
                       
                       - 
                       
                         h 
                         j 
                         
                           ( 
                           t 
                           ) 
                         
                       
                     
                     ) 
                   
                 
               
               
                 
                   ( 
                   9 
                   ) 
                 
               
             
           
         
       
     
     where a j→i  is the attention weight for the pair of nodes i and j, the sum in the denominator runs over one or more nodes j′ in the graph comprising node j, and s h  is vector-based similarity metric such as a Euclidean distance, cosine similarity or Hamming distance. 
       FIGS.  5 A and  5 B  show visualizations of attention weights for two pairs of exemplary graphs. In  FIG.  5 A , the pair of graphs has a graph edit distance of 1. In  FIG.  5 B , the pair of graphs has a graph edit distance of 2. The left-hand drawing of both  FIGS.  5 A and  5 B  shows the attention from the left graph to the right graph of the pair of graphs. The right-hand drawing of both  FIGS.  5 A and  5 B  show the attention from the right graph to the left graph of the same pair of graphs. The darker lines indicate a larger attention weight between a node of graph and a node of the other graph. 
     The node update neural network  402  may be configured to generate an updated node state representation vector associated with a particular node based upon the current node state representation vector associated with the particular node, the message vectors generated for each adjacent node adjacent the particular node in its input graph, and the cross-graph matching vector  409 . For example, an updated node state representation vector may be generated as follows: 
     
       
         
           
             
               
                 
                   
                     h 
                     i 
                     
                       ( 
                       
                         t 
                         + 
                         1 
                       
                       ) 
                     
                   
                   = 
                   
                     
                       f 
                       node 
                     
                     ( 
                     
                       
                         h 
                         i 
                         
                           ( 
                           t 
                           ) 
                         
                       
                       , 
                       
                         
                           ∑ 
                           j 
                         
                         
                           m 
                           
                             j 
                             → 
                             i 
                           
                         
                       
                       , 
                       
                         
                           ∑ 
                           
                             j 
                             ′ 
                           
                         
                         
                           m 
                           
                             
                               j 
                               ′ 
                             
                             → 
                             i 
                           
                         
                       
                     
                     ) 
                   
                 
               
               
                 
                   ( 
                   10 
                   ) 
                 
               
             
           
         
       
     
     where h i   (t+1)  is the updated node state representation vector for node i, ƒ node  is the node update network  402 , h i   (t)  is the current node state representation vector for node i, Σ j m j→i  is a sum of the message vectors associated with all nodes connected to node i, and Σ j′ μ j′→i  is a sum of the computed cross-graph matching vectors for node i and one or more nodes j′ of the other input graph. Where the cross-graph matching vectors are computed using the attention-based weighted sum of equation (9), the sum of the computed cross-graph matching vectors may be re-written as: 
     
       
         
           
             
               
                 
                   
                     
                       ∑ 
                       
                         j 
                         ⁢ 
                         ′ 
                       
                     
                     
                       μ 
                       
                         
                           j 
                           ⁢ 
                           ′ 
                         
                         → 
                         i 
                       
                     
                   
                   = 
                   
                     
                       
                         ∑ 
                         j 
                       
                       
                         
                           a 
                           
                             
                               j 
                               ⁢ 
                               ′ 
                             
                             → 
                             i 
                           
                         
                         ( 
                         
                           
                             h 
                             i 
                             
                               ( 
                               t 
                               ) 
                             
                           
                           - 
                           
                             h 
                             
                               j 
                               ⁢ 
                               ′ 
                             
                             
                               ( 
                               t 
                               ) 
                             
                           
                         
                         ) 
                       
                     
                     = 
                     
                       
                         h 
                         i 
                         
                           ( 
                           t 
                           ) 
                         
                       
                       - 
                       
                         
                           ∑ 
                           
                             j 
                             ⁢ 
                             ′ 
                           
                         
                         
                           ( 
                           
                             
                               a 
                               
                                 
                                   j 
                                   ⁢ 
                                   ′ 
                                 
                                 → 
                                 i 
                               
                             
                             ⁢ 
                             
                               h 
                               
                                 j 
                                 ⁢ 
                                 ′ 
                               
                               
                                 ( 
                                 t 
                                 ) 
                               
                             
                           
                           ) 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   11 
                   ) 
                 
               
             
           
         
       
     
     Similar to the node update neural network  302  of  FIG.  3   , the node update neural network  402  in this configuration may also be a multi-layer perceptron or a recurrent neural network comprising conventional recurrent neural network units (RNN), gated recurrent units (GRU), or long-term short term memory units (LSTM). 
     Similar to the configuration of  FIG.  3   , multiple rounds of node state vector representation updates may be performed and as shown by the dashed lines  412 ,  413  in  FIG.  4   . Updated node state representation vectors  410 ,  411  for the first and second input graphs may be fed back into the message generation neural network  401  for further processing to generate further message vectors and into the matching subsystem  403  to generate further cross-graph matching vectors in order to perform additional rounds of updating by the node update neural network  402 . 
     The one or more neural networks of the neural network system  100  may be trained using a training dataset of graphs comprising example pairs or triplets of graphs. Where the training data comprises pairs of graphs, each pair has an associated label indicating whether the two graphs are similar or dissimilar. Where the training data comprises a triplet of graphs, the triplet may be labelled based upon a relative similarity, for example, whether a first graph of the triplet is more similar to the second or third graph of the triplet. 
     The one or more neural networks may be trained based upon optimizing a loss function using gradient descent based algorithms. The loss function may be a margin-based loss function. For example, where the training dataset comprises pairs of graphs, the loss function may be a margin-based pairwise loss having the form: 
         L   pair =   (G     1     ,G     2     ,t) [max{0,γ− t (1− d ( G   1   ,G   2 ))}]  (12)
 
     where γ&gt;0 is a margin parameter, t is the label for the pair of graphs G 1 , G 2 , for example, −1 for a pair of dissimilar graphs, 1 for a pair of similar graphs, and d(G 1 , G 2 ) is the Euclidean distance between the vector representations of the two graphs. The above loss function encourages d(G 1 , G 2 )&lt;1−γ where the pair is similar (t=1), and d(G 1 , G 2 )&lt;1+γ when the pair is dissimilar (t=−1). 
     Where the training data comprises triplets of graphs having a relative similarity, the loss function may be a margin-based triplet loss having the form: 
         L   triplet =   (G     1     ,G     2     ,G     3     ) [max{0, d ( G   1   ,G   2 )− d ( G   1   ,G   3 )+γ}]  (13)
 
     where the terms have the same meaning as in equation (12). The loss function encourages d(G 1 , G 2 ) to be smaller than d(G 1 , G 3 ) by at least a margin γ. 
     Alternatively, rather than using the Euclidean distance as part of the loss function, it is possible to use the Hamming distance and to minimize the Hamming distance between similar graphs and maximize the Hamming distance for dissimilar graphs. 
     For example, the loss functions may take the form: 
         L   pair =   (G     1     ,G     2     ,t) [( t−s ( G   1   ,G   2 )) 2 ]/4  (14)
 
         L   triplet =   (G     1     ,G     2     ,G     3)   [( s ( G   1   ,G   2 )−1) 2 +( s ( G   1   ,G   3 ))+1) 2 }]/8  (15)
 
     and where 
     
       
         
           
             
               
                 
                   
                     s 
                     ⁡ 
                     ( 
                     
                       
                         G 
                         1 
                       
                       , 
                       
                         G 
                         2 
                       
                     
                     ) 
                   
                   = 
                   
                     
                       1 
                       H 
                     
                     ⁢ 
                     
                       
                         ∑ 
                         
                           i 
                           = 
                           1 
                         
                         H 
                       
                       
                         
                           tanh 
                           ⁡ 
                           ( 
                           
                             h 
                             
                               
                                 G 
                                 1 
                               
                               ⁢ 
                               i 
                             
                           
                           ) 
                         
                         · 
                         
                           tanh 
                           ⁡ 
                           ( 
                           
                             h 
                             
                               
                                 G 
                                 2 
                               
                               ⁢ 
                               i 
                             
                           
                           ) 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   16 
                   ) 
                 
               
             
           
         
       
     
     is an approximate average Hamming distance. 
     Referring now to  FIG.  6   , processing for determining graph similarity will now be described. It will be appreciated that the processing may be performed by the neural network system  100 . At step S 601 , a first graph  101  is received by one or more processors. At step S 602 , a second graph  102  is received by the one or more processors. 
     A vector representation of the first graph is generated by one or more neural networks at step S 603  and a vector representation of the second graph is generated by the one or more neural networks at step S 604 . Generating a vector representation of a graph may comprise processing the graph to generate a node state representation vector for each node of the graph and an edge representation vector for each edge of the graph by the one or more neural networks. The node state representation vectors and the edge representation vectors are then processed by the one or more neural networks to generate the vector representation of the graph. The operations for generating a vector representation of a graph may be carried out as described above with reference to  FIGS.  2  to  4   . 
     At step S 605 , the one or more processors then determines a similarity score based upon the vector representations of the first and second graphs generated at steps S 603  and S 604 . 
     It will be appreciated that whilst the above processing is presented as being carried out in a particular order, it is not intended to limit to any particular ordering of steps and the above steps may be carried out in a different order. For example, the vector representation of the first graph may be determined prior to receipt of the second graph. It is also possible that steps are carried out in parallel rather than as a sequential process. For example, the generation of the first and second vector representations of the graph may be performed in parallel. 
     The above described neural network system may be used as part of a system for binary function vulnerability detection. For example, where a program, library or portion of binary code has a known vulnerability, it is desirable to determine whether there exists other binary code that may be susceptible to the same vulnerability or where there exists a database of code having known vulnerabilities and it is desirable to determine whether a query piece of code is susceptible to any known vulnerabilities in the database. This is particularly useful where access to source code is not available, for example, when dealing with commercial or embedded software or suspicious executables. Alternatively, where source code is available, the system may also be applied to source code and is not limited to binary functions. 
     A binary function may be represented as a control flow graph. A node in a control flow graph may be a basic block of assembly instructions with edges representing control flow, for example, a jump or return instruction used in branching, loops or function calls. A control flow graph may be obtained by processing the binary function using a disassembler and a code analyzer.  FIG.  7    shows two exemplary control flow graphs  701 ,  702  which are deemed to be similar as they have been generated based upon the same function but using different compilers.  FIG.  7    also shows a third exemplary control flow graph  703  which has been generated using a different function and is therefore dissimilar to the first and second control flow graphs  701 ,  702 . 
     This type of the problem is challenging as graphs having different structures may still be similar whilst graphs having small differences may not be similar. 
     The system for binary function vulnerability detection may comprise one or more processors configured to receive a first control flow graph associated a first binary function having a known vulnerability and a second control flow graph associated with a second binary function to be tested. Using the neural network system described above, vector representations of the first and second control flow graphs may be obtained and a similarity score between them may be generated. The second binary function may be determined as having the same vulnerability as the first binary function if the similarity score exceeds a threshold. Further binary functions may be compared with the first binary function in a similar manner to determine their susceptibility. By using the neural network system as described above, it is possible to learn a graph representation and similarity scoring that is that improves performance over classical graph theoretical matching algorithms and hand crafted features. 
     The system for binary function vulnerability detection may be trained based upon a similarity metric such that control flow graphs associated with a binary function having the same source code have higher similarity than control flow graphs associated with a binary function having different source code. For example, a training dataset may be generated by compiling source code using different compilers or different compiler optimization flags. 
     A control flow graph may also be associated with a feature vector based upon instruction types associated with the block of instructions represented by the node. These feature vectors may also be generated using a neural network and trained jointly with the neural network system for determining graph similarity. 
     In another example, the neural network system for determining graph similarity may be used as part of a system for retrieval of data. A graph may be provided as a query and a database may be searched to retrieve associated data. For example, the graph may be a representation of a molecule for which it is desired to find other molecules having similar properties. 
     The system for retrieving data associated with a graph may comprise one or more processors configured to receive a query graph and to generate a vector representation of the query graph using the one or more neural network as described above. The system may further comprise a database comprising a plurality of records, each record of the plurality of records being associated with a respective graph. For each record of the plurality of records, the one or more processors may be configured to process the vector representation of the query graph and a vector representation associated with the respective graph associated with the record to determine a respective similarity score and to output data associated with one or more records based upon the determined similarity score. 
     The above described neural network system may be used as part of a system for other technical applications. For example, a graph may represent a drug molecule and the system may be used to compare molecules e.g., for generating new potentially viable drugs. Each respective node of the graph may represent an atom of the molecule or a secondary structural element of a protein such as an alpha helix or beta sheet, i.e., a part of the molecule. Each respective edge of the graph may represent an interaction between nodes such as a chemical bond between the atoms of the molecule or one or more hydrogen bonds between the secondary structural elements, i.e., chemical bonds between the parts. The system may be trained on data representing molecules and optionally their known properties. The trained system may then be used to identify graphs representing other physically realistic molecules with the same or similar properties to those in the training data e.g., molecules that bind to a particular site and/or families of drugs. Based on the similarity score the system may be used to generate molecule such as a drug candidate, which may then be evaluated e.g., in silico or by synthesizing and testing the molecule in vitro or in vivo. 
     In another example, a graph may represent a transportation network in which the nodes represent physical locations and the edges routing paths. The system may be used to determine a route by comparing graphs representing routing paths e.g., to configure an efficient transportation network. Following on from this the system may be used for controlling an object, e.g., a vehicle on a road or a robot in a warehouse, to travel along the determined route. 
     In a similar way a graph may represent a computer network or electronic communications network in which the nodes represent entities on the network such as servers and routers and the edges routing paths. The system may then be used to determine a route for communications data e.g., a data packet, e.g., by comparing graphs representing different network configurations, and subsequently to route data e.g., the packet. 
     In a further similar way a graph may represent an electronic circuit design e.g., an integrated circuit design, in which the nodes represent elements in the circuit such as transistors or logic elements and the edges routing paths between the nodes. The system may then be used to determine routes for communicating between or connecting the elements e.g., by comparing graphs representing different routing configurations, and may subsequently be used to build an electronic circuit to the design. 
     In another similar way a graph may represent a static or moving physical structure in which the nodes represent elements of the structure and the edges connections between the elements. For example in the case of a building the nodes may represent e.g., floors, walls, roof, foundations, piers, decking and the edges connecting elements such as girders, beams, struts and ties. In another application the nodes may represent elements of a machine or deployable structure and edges their connections. The system may then be for comparing structures e.g., to design or evaluate a structure; the result may then be used to construct a structure to the design. 
     In a further example a graph may represent a visual scene and may capture semantic relationships between objects/parts of objects. Thus the nodes may represent elements of the scene e.g., increasingly larger elements of the scene and the edges relationships between the elements e.g., to which objects elements of the scene belong. The system may then be used to compare different scenes on the basis of their semantic content, e.g., for identifying of classifying a scene or for identifying a group of objects defining an object or coherent part of a scene e.g., to facilitate object/scene manipulation (editing) or information extraction (interpretation). 
     In a further example a graph may represent a parse trees or other linguistic structures for use in natural language processing and translation. For example the nodes may represent words, word pieces, or phrases, in a natural language and the edges their relations. The system may then be used to compare different pieces of natural language text on the basis of their semantic content. 
     It will be appreciated that there exists many other technical applications for generated graph structures and representations based upon similarity. 
     This specification uses the term “configured” in connection with systems and computer program components. For a system of one or more computers to be configured to perform particular operations or actions means that the system has installed on it software, firmware, hardware, or a combination of them that in operation cause the system to perform the operations or actions. For one or more computer programs to be configured to perform particular operations or actions means that the one or more programs include instructions that, when executed by data processing apparatus, cause the apparatus to perform the operations or actions. 
     Embodiments of the subject matter and the functional operations described in this specification can be implemented in digital electronic circuitry, in tangibly-embodied computer software or firmware, in computer hardware, including the structures disclosed in this specification and their structural equivalents, or in combinations of one or more of them. Embodiments of the subject matter described in this specification can be implemented as one or more computer programs, i.e., one or more modules of computer program instructions encoded on a tangible non transitory program carrier for execution by, or to control the operation of, data processing apparatus. Alternatively or in addition, the program instructions can be encoded on an artificially generated propagated signal, e.g., a machine-generated electrical, optical, or electromagnetic signal, that is generated to encode information for transmission to suitable receiver apparatus for execution by a data processing apparatus. The computer storage medium can be a machine-readable storage device, a machine-readable storage substrate, a random or serial access memory device, or a combination of one or more of them. The computer storage medium is not, however, a propagated signal. 
     The term “data processing apparatus” encompasses all kinds of apparatus, devices, and machines for processing data, including by way of example a programmable processor, a computer, or multiple processors or computers. The apparatus can include special purpose logic circuitry, e.g., an FPGA (field programmable gate array) or an ASIC (application specific integrated circuit). The apparatus can also include, in addition to hardware, code that creates an execution environment for the computer program in question, e.g., code that constitutes processor firmware, a protocol stack, a database management system, an operating system, or a combination of one or more of them. 
     A computer program (which may also be referred to or described as a program, software, a software application, a module, a software module, a script, or code) can be written in any form of programming language, including compiled or interpreted languages, or declarative or procedural languages, and it can be deployed in any form, including as a stand alone program or as a module, component, subroutine, or other unit suitable for use in a computing environment. A computer program may, but need not, correspond to a file in a file system. A program can be stored in a portion of a file that holds other programs or data, e.g., one or more scripts stored in a markup language document, in a single file dedicated to the program in question, or in multiple coordinated files, e.g., files that store one or more modules, sub programs, or portions of code. A computer program can be deployed to be executed on one computer or on multiple computers that are located at one site or distributed across multiple sites and interconnected by a communication network. 
     As used in this specification, an “engine,” or “software engine,” refers to a software implemented input/output system that provides an output that is different from the input. An engine can be an encoded block of functionality, such as a library, a platform, a software development kit (“SDK”), or an object. Each engine can be implemented on any appropriate type of computing device, e.g., servers, mobile phones, tablet computers, notebook computers, music players, e-book readers, laptop or desktop computers, PDAs, smart phones, or other stationary or portable devices, that includes one or more processors and computer readable media. Additionally, two or more of the engines may be implemented on the same computing device, or on different computing devices. 
     The processes and logic flows described in this specification can be performed by one or more programmable computers executing one or more computer programs to perform functions by operating on input data and generating output. The processes and logic flows can also be performed by, and apparatus can also be implemented as, special purpose logic circuitry, e.g., an FPGA (field programmable gate array) or an ASIC (application specific integrated circuit). For example, the processes and logic flows can be performed by and apparatus can also be implemented as a graphics processing unit (GPU). 
     Computers suitable for the execution of a computer program include, by way of example, can be based on general or special purpose microprocessors or both, or any other kind of central processing unit. Generally, a central processing unit will receive instructions and data from a read only memory or a random access memory or both. The essential elements of a computer are a central processing unit for performing or executing instructions and one or more memory devices for storing instructions and data. Generally, a computer will also include, or be operatively coupled to receive data from or transfer data to, or both, one or more mass storage devices for storing data, e.g., magnetic, magneto optical disks, or optical disks. However, a computer need not have such devices. Moreover, a computer can be embedded in another device, e.g., a mobile telephone, a personal digital assistant (PDA), a mobile audio or video player, a game console, a Global Positioning System (GPS) receiver, or a portable storage device, e.g., a universal serial bus (USB) flash drive, to name just a few. 
     Computer readable media suitable for storing computer program instructions and data include all forms of non-volatile memory, media and memory devices, including by way of example semiconductor memory devices, e.g., EPROM, EEPROM, and flash memory devices; magnetic disks, e.g., internal hard disks or removable disks; magneto optical disks; and CD ROM and DVD-ROM disks. The processor and the memory can be supplemented by, or incorporated in, special purpose logic circuitry. 
     To provide for interaction with a user, embodiments of the subject matter described in this specification can be implemented on a computer having a display device, e.g., a CRT (cathode ray tube) or LCD (liquid crystal display) monitor, for displaying information to the user and a keyboard and a pointing device, e.g., a mouse or a trackball, by which the user can provide input to the computer. Other kinds of devices can be used to provide for interaction with a user as well; for example, feedback provided to the user can be any form of sensory feedback, e.g., visual feedback, auditory feedback, or tactile feedback; and input from the user can be received in any form, including acoustic, speech, or tactile input. In addition, a computer can interact with a user by sending documents to and receiving documents from a device that is used by the user; for example, by sending web pages to a web browser on a user&#39;s client device in response to requests received from the web browser. 
     Embodiments of the subject matter described in this specification can be implemented in a computing system that includes a back end component, e.g., as a data server, or that includes a middleware component, e.g., an application server, or that includes a front end component, e.g., a client computer having a graphical user interface or a Web browser through which a user can interact with an implementation of the subject matter described in this specification, or any combination of one or more such back end, middleware, or front end components. The components of the system can be interconnected by any form or medium of digital data communication, e.g., a communication network. Examples of communication networks include a local area network (“LAN”) and a wide area network (“WAN”), e.g., the Internet. 
     The computing system can include clients and servers. A client and server are generally remote from each other and typically interact through a communication network. The relationship of client and server arises by virtue of computer programs running on the respective computers and having a client-server relationship to each other. 
     While this specification contains many specific implementation details, these should not be construed as limitations on the scope of any invention or of what may be claimed, but rather as descriptions of features that may be specific to particular embodiments of particular inventions. Certain features that are described in this specification in the context of separate embodiments can also be implemented in combination in a single embodiment. Conversely, various features that are described in the context of a single embodiment can also be implemented in multiple embodiments separately or in any suitable subcombination. Moreover, although features may be described above as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can in some cases be excised from the combination, and the claimed combination may be directed to a subcombination or variation of a subcombination. 
     Similarly, while operations are depicted in the drawings in a particular order, this should not be understood as requiring that such operations be performed in the particular order shown or in sequential order, or that all illustrated operations be performed, to achieve desirable results. In certain circumstances, multitasking and parallel processing may be advantageous. Moreover, the separation of various system modules and components in the embodiments described above should not be understood as requiring such separation in all embodiments, and it should be understood that the described program components and systems can generally be integrated together in a single software product or packaged into multiple software products. 
     Particular embodiments of the subject matter have been described. Other embodiments are within the scope of the following claims. For example, the actions recited in the claims can be performed in a different order and still achieve desirable results. As one example, the processes depicted in the accompanying figures do not necessarily require the particular order shown, or sequential order, to achieve desirable results. In certain implementations, multitasking and parallel processing may be advantageous.