graph convolutional network with sequential attention for

Graph Convolutional Network with Sequential Attention forGoal-Oriented Dialogue Systems

Suman Banerjee and Mitesh M. Khapra

Department of Computer Science and Engineering,Robert Bosch Centre for Data Science and Artificial Intelligence (RBC-DSAI),

Indian Institute of Technology Madras, India{suman, miteshk}@cse.iitm.ac.in

AbstractDomain-specific goal-oriented dialogue sys-tems typically require modeling three types ofinputs, namely, (i) the knowledge-base asso-ciated with the domain, (ii) the history of theconversation, which is a sequence of utter-ances, and (iii) the current utterance for whichthe response needs to be generated. Whilemodeling these inputs, current state-of-the-artmodels such as Mem2Seq typically ignorethe rich structure inherent in the knowledgegraph and the sentences in the conversa-tion context. Inspired by the recent successof structure-aware Graph Convolutional Net-works (GCNs) for various NLP tasks suchas machine translation, semantic role labeling,and document dating, we propose a memory-augmented GCN for goal-oriented dialogues.Our model exploits (i) the entity relation graphin a knowledge-base and (ii) the dependencygraph associated with an utterance to computericher representations for words and entities.Further, we take cognizance of the fact that incertain situations, such as when the conversa-tion is in a code-mixed language, dependencyparsers may not be available. We show that insuch situations we could use the global wordco-occurrence graph to enrich the representa-tions of utterances. We experiment with fourdatasets: (i) the modified DSTC2 dataset, (ii)recently released code-mixed versions of DSTC2dataset in four languages, (iii) Wizard-of-Ozstyle CAM676 dataset, and (iv) Wizard-of-Ozstyle MultiWOZ dataset. On all four datasetsour method outperforms existing methods, ona wide range of evaluation metrics.

1 Introduction

Goal-oriented dialogue systems that can assisthumans in various day-to-day activities have

widespread applications in several domains suchas e-commerce, entertainment, healthcare, and soforth. For example, such systems can help humansin scheduling medical appointments or reservingrestaurants, booking tickets. From a modelingperspective, one clear advantage of dealing withdomain-specific goal-oriented dialogues is thatthe vocabulary is typically limited, the utteranceslargely follow a fixed set of templates, and thereis an associated domain knowledge that canbe exploited. More specifically, there is somestructure associated with the utterances as well asthe knowledge base (KB).

More formally, the task here is to generate thenext response given (i) the previous utterancesin the conversation history, (ii) the current userutterance (known as the query), and (iii) theentities and their relationships in the associatedknowledge base. Current state-of-the-art methods(Seo et al., 2017; Eric and Manning, 2017;Madotto et al., 2018) typically use variants ofRecurrent Neural Networks (RNNs) (Elman,1990) to encode the history and current utteranceor an external memory network (Sukhbaatar et al.,2015) to encode them along with the entitiesin the knowledge base. The encodings of theutterances and memory elements are then suitablycombined using an attention network and fed tothe decoder to generate the response, one wordat a time. However, these methods do not exploitthe structure in the knowledge base as defined byentity–entity relations and the structure in theutterances as defined by a dependency parse.Such structural information can be exploitedto improve the performance of the system, asdemonstrated by recent works on syntax-awareneural machine translation (Eriguchi et al., 2016;Bastings et al., 2017; Chen et al., 2017), semanticrole labeling (Marcheggiani and Titov, 2017),and document dating (Vashishth et al., 2018),which use Graph Convolutional Networks (GCNs)

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Transactions of the Association for Computational Linguistics, vol. 7, pp. 485–500, 2019. https://doi.org/10.1162/tacl a 00284Action Editor: Asli Celikyilmaz. Submission batch: 1/2019; Revision batch: 5/2019; Published 9/2019.

c© 2019 Association for Computational Linguistics. Distributed under a CC-BY 4.0 license.

https://doi.org/10.1162/tacl_a_00284

(Defferrard et al., 2016; Duvenaud et al., 2015;Kipf and Welling, 2017) to exploit sentencestructure.

In this work, we propose to use such graphstructures for goal-oriented dialogues. In partic-ular, we compute the dependency parse tree foreach utterance in the conversation and use aGCN to capture the interactions between words.This allows us to capture interactions betweendistant words in the sentence as long as they areconnected by a dependency relation. We also useGCNs to encode the entities of the KB where theentities are treated as nodes and their relationsas edges of the graph. Once we have a richerstructure aware representation for the utterancesand the entities, we use a sequential attentionmechanism to compute an aggregated contextrepresentation from the GCN node vectors of thequery, history, and entities. Further, we note that incertain situations, such as when the conversationis in a code-mixed language or a language forwhich parsers are not available, then it may notbe possible to construct a dependency parse forthe utterances. To overcome this, we construct aco-occurrence matrix from the entire corpus anduse this matrix to impose a graph structure onthe utterances. More specifically, we add an edgebetween two words in a sentence if they co-occurfrequently in the corpus. Our experiments suggestthat this simple strategy acts as a reasonablesubstitute for dependency parse trees.

We perform experiments with the modifiedDSTC2 (Bordes et al., 2017) dataset, whichcontains goal-oriented conversations for makingrestaurant reservations. We also use its recentlyreleased code-mixed versions (Banerjee et al.,2018), which contain code-mixed conversations infour different languages: Hindi, Bengali, Gujarati,and Tamil. We compare with recent state-of-the-art methods and show that on average, the pro-posed model gives an improvement of 2.8 BLEUpoints and 2 ROUGE points. We also performexperiments on two human–human dialoguedatasets of different sizes: (i) Cam676 (Wenet al., 2017): a small scale dataset containing676 dialogues from the restaurant domain; and (ii)MultiWOZ (Budzianowski et al., 2018): a large-scale dataset containing around 10k dialogues andspanning multiple domains for each dialogue. Onthese two datasets as well, we observe a similartrend, wherein our model outperforms existingmethods.

Our contributions can be summarized as fol-lows: (i) We use GCNs to incorporate structuralinformation for encoding query, history, andKB entities in goal-oriented dialogues; (ii) Weuse a sequential attention mechanism to obtainquery aware and history aware context repre-sentations; (iii) We leverage co-occurrence fre-quencies and PPMI (positive-pointwise mutualinformation) values to construct contextual graphsfor code-mixed utterances; and (iv) We showthat the proposed model obtains state-of-the-artresults on four different datasets spanning fivedifferent languages.

2 Related Work

In this section, we review the previous work ingoal-oriented dialogue systems and describe theintroduction of GCNs in NLP.

Goal-Oriented Dialogue Systems: Initial goal-oriented dialogue systems (Young, 2000; Williamsand Young, 2007) were based on dialogue statetracking (Williams et al., 2013; Henderson et al.,2014a,b) and included pipelined modules fornatural language understanding, dialogue statetracking, policy management, and natural lan-guage generation. Wen et al. (2017) used neuralnetworks for these intermediate modules butstill lacked absolute end-to-end trainability. Suchpipelined modules were restricted by the fixedslot-structure assumptions on the dialogue stateand required per-module based labeling. Tomitigate this problem, Bordes et al. (2017) re-leased a version of goal-oriented dialogue datasetthat focuses on the development of end-to-endneural models. Such models need to reasonover the associated KB triples and generate re-sponses directly from the utterances without anyadditional annotations. For example, Bordes et al.(2017) proposed a Memory Network (Sukhbaataret al., 2015) based model to match the responsecandidates with the multi-hop attention weightedrepresentation of the conversation history and theKB triples in memory. Liu and Perez (2017)further added highway (Srivastava et al., 2015)and residual connections (He et al., 2016) to thememory network in order to regulate the access tothe memory blocks. Seo et al. (2017) developeda variant of RNN cell that computes a refinedrepresentation of the query over multiple iterationsbefore querying the memory. However, all theseapproaches retrieve the response from a set of

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candidate responses and such a candidate set isnot easy to obtain for any new domain of interest.To account for this, Eric and Manning (2017) andZhao et al. (2017) adapted RNN-based encoder-decoder models to generate appropriate responsesinstead of retrieving them from a candidate set.Eric et al. (2017) introduced a key-value memorynetwork based generative model that integratesthe underlying KB with RNN-based encode-attend-decode models. Madotto et al. (2018) usedmemory networks on top of the RNN decoderto tightly integrate KB entities with the decoderin order to generate more informative responses.However, as opposed to our work, all these worksignore the underlying structure of the entity–entity graph of the KB and the syntactic structureof the utterances.

GCNs in NLP: Recently, there has beenan active interest in enriching existing encode-attend-decode models (Bahdanau et al., 2015)with structural information for various NLPtasks. Such structure is typically obtained fromthe constituency and/or dependency parse ofsentences. The idea is to treat the output of aparser as a graph and use an appropriate networkto capture the interactions between the nodes ofthis graph. For example, Eriguchi et al. (2016)and Chen et al. (2017) showed that incorporatingsuch syntactical structures as Tree-LSTMs in theencoder can improve the performance of neuralmachine translation. Peng et al. (2017) use Graph-LSTMs to perform cross sentence n-ary relationextraction and show that their formulation isapplicable to any graph structure and Tree-LSTMscan be thought of as a special case of it. Inparallel, Graph Convolutional Networks (GCNs)(Duvenaud et al., 2015; Defferrard et al., 2016;Kipf and Welling, 2017) and their variants (Liet al., 2016) have emerged as state-of-the-artmethods for computing representations of entitiesin a knowledge graph. They provide a moreflexible way of encoding such graph structuresby capturing multi-hop relationships betweennodes. This has led to their adoption for variousNLP tasks such as neural machine translation(Marcheggiani et al., 2018; Bastings et al., 2017),semantic role labeling (Marcheggiani and Titov,2017), document dating (Vashishth et al., 2018),and question answering (Johnson, 2017; De Caoet al., 2019).

To the best of our knowledge, ours is the firstwork that uses GCNs to incorporate dependencystructural information and the entity–entity graphstructure in a single end-to-end neural modelfor goal-oriented dialogues. This is also the firstwork that incorporates contextual co-occurrenceinformation for code-mixed utterances, for whichno dependency structures are available.

3 Background

In this section, we describe GCNs (Kipf andWelling, 2017) for undirected graphs and thendescribe their syntactic versions, which work withdirected labeled edges of dependency parse trees.

3.1 GCN for Undirected GraphsGraph convolutional networks operate on a graphstructure and compute representations for thenodes of the graph by looking at the neighborhoodof the node. We can stack k layers of GCNsto account for neighbors that are k-hops awayfrom the current node. Formally, let G = (V , E)be an undirected graph, where V is the set ofnodes (let |V| = n) and E is the set of edges.Let X ∈ R

n×m be the input feature matrix withn nodes and each node xu(u ∈ V) is representedby an m-dimensional feature vector. The outputof a 1-layer GCN is the hidden representationmatrix H ∈ R

n×d where each d-dimensionalrepresentation of a node captures the interactionswith its 1-hop neighbors. Each row of this matrixcan be computed as:

hv = ReLU

( ∑u∈N (v)

(Wxu + b)

), ∀v ∈ V

(1)HereW ∈ R

d×m is the model parameter matrix,b ∈ R

d is the bias vector, and ReLU is therectified linear unit activation function. N (v) isthe set of neighbors of node v and is assumedto also include the node v so that the previousrepresentation of the node v is also consideredwhile computing its new hidden representation. Tocapture interactions with nodes that are multiplehops away, multiple layers of GCNs can be stackedtogether. Specifically, the representation of nodev after kth GCN layer can be formulated as:

hk+1v = ReLU

( ∑u∈N (v)

(W khku + bk)

)(2)

∀v ∈ V . Here hku is the representation of the uth

node in the (k − 1)th GCN layer and h1u = xu.

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3.2 Syntactic GCN

In a directed labeled graph G = (V , E), eachedge between nodes u and v is representedby a triple (u, v, L(u, v)) where L(u, v) is theassociated edge label. Marcheggiani and Titov(2017) modified GCNs to operate over directedlabeled graphs, such as the dependency parse treeof a sentence. For such a tree, in order to allowinformation to flow from head to dependents andvice-versa, they added inverse dependency edgesfrom dependents to heads such as (v, u, L(u, v)′)to E and made the model parameters and biaseslabel specific. In their formulation,

hk+1v = ReLU

( ∑u∈N (v)

(W kL(u,v)h

ku + bk

L(u,v))

)

(3)∀v ∈ V . Notice that unlike equation 2,

equation 3 has parameters W kL(u,v) and bk

L(u,v)

which are label-specific. Suppose there are Ldifferent labels, then this formulation will requireL weights and biases per GCN layer, resulting ina large number of parameters. To avoid this, theauthors use only three sets of weights and biasesper GCN layer (as opposed to L) depending onthe direction in which the information flows. Morespecifically, W k

L(u,v) = W kdir(u,v), where dir(u, v)

indicates whether information flows from u to v,v to u or u = v. In this work, we also makebkL(u,v) = bk

dir(u,v) instead of having a separatebias per label. The final GCN formulation can thusbe described as:

hk+1v = ReLU

( ∑u∈N (v)

(W kdir(u,v)h

ku+bk

dir(u,v))

)

(4)

4 Model

We first formally define the task of end-to-endgoal-oriented dialogue generation. Each dialogueof t turns can be viewed as a succession ofuser utterances (U ) and system responses (S) andcan be represented as: (U1, S1, U2, S2, . . . , Ut, St).Along with these utterances, each dialogue is alsoaccompanied by e KB triples that are relevantto that dialogue and can be represented as:(k1, k2, k3, . . . , ke). Each triple is of the form:(entity1, relation, entity2). These triples can berepresented in the form of a graph Gk = (Vk, Ek)where Vk is the set of all entities and each edgein Ek is of the form: (entity1, entity2, relation),

where relation signifies the edge label. At anydialogue turn i, given the (i) dialogue historyH = (U1, S1, U2, . . . , Si−1), (ii) the current userutterance as the query Q = Ui and (iii) theassociated knowledge graph Gk, the task is togenerate the current response Si that leads to acompletion of the goal. As mentioned earlier, weexploit the graph structure in KB and the syntacticstructure in the utterances to generate appropriateresponses. Toward this end, we propose a modelwith the following components for encoding thesethree types of inputs. The code for the model isreleased publicly.1

4.1 Query EncoderThe query Q = Ui is the ith (current) userutterance in the dialogue and contains |Q| tokens.We denote the embedding of the ith token inthe query as qi. We first compute the contextualrepresentations of these tokens by passing themthrough a bidirectional RNN:

bt = BiRNNQ(bt−1,qt) (5)

Now, consider the dependency parse tree ofthe query sentence denoted by GQ = (VQ, EQ).We use a query-specific GCN to operate on GQ,which takes {bi}|Q|

i=1 as the input to the first GCNlayer. The node representation in the kth hop ofthe query specific GCN is computed as:

ck+1v = ReLU

( ∑u∈N (v)

(W kdir(u,v)c

ku + gk

dir(u,v))

)

(6)∀v ∈ VQ. Here W k

dir(u,v),gkdir(u,v) are edge

direction specific query-GCN weights and biasesfor the kth hop and c1u = bu.

4.2 Dialogue History EncoderThe history H of the dialogue contains |H| tokensand we denote the embedding of the ith token inthe history by pi. Once again, we first computethe hidden representations of these tokens using abidirectional RNN:

st = BiRNNH(st−1,pt) (7)

We now compute a dependency parse treefor each sentence in the history and collectivelyrepresent all the trees as a single graph GH =

1https://github.com/sumanbanerjee1/GCN-SeA.

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https://github.com/sumanbanerjee1/GCN-SeA

https://github.com/sumanbanerjee1/GCN-SeA

Figure 1: Illustration of the GCN and RNN+GCN modules which are used as encoders in our model. The notationsare specific to the dialogue history encoder but both the encoders are similar for the query. We use only the GCNencoder for the KB.

(VH , EH). Note that this graph will only containedges between words belonging to the samesentence and there will be no edges between wordsacross sentences. We then use a history-specificGCN to operate on GH which takes st as the inputto the first layer. The node representation in thekth hop of the history-specific GCN is computedas:

ak+1v = ReLU

( ∑u∈N (v)

(V kdir(u,v)a

ku + okdir(u,v))

)

(8)∀v ∈ VH . Here V k

dir(u,v) and okdir(u,v) are edgedirection-specific history-GCN weights and biasesin the kth hop and a1u = su. Such an encoder witha single hop of GCN is illustrated in Figure 1(b)and the encoder without the BiRNN is depicted inFigure 1(a).

4.3 KB EncoderAs mentioned earlier,GK = (VK , EK) is the graphcapturing the interactions between the entities inthe knowledge graph associated with the dialogue.Let there be m such entities and we denote theembedding of the node corresponding to the ith

entity as ei. We then operate a KB-specific GCN

on these entity representations to obtain refinedrepresentations that capture relations betweenentities. The node representation in the kth hop ofthe KB specific GCN is computed as:

rk+1v = ReLU

( ∑u∈N (v)

(Ukdir(u,v)r

ku + zkdir(u,v))

)

(9)∀v ∈ VK . Here Uk

dir(u,v) and zkdir(u,v) are edgedirection-specific KB-GCN weights and biases inkth hop and r1u = eu. We also add inverse edges toEK similar to the case of syntactic GCNs in orderto allow information flow in both the directionsfor an entity pair in the knowledge graph.

4.4 Sequential Attention

We use an RNN decoder to generate the tokensof the response and let the hidden states of thedecoder be denoted as: {di}Ti=1, where T is thetotal number of decoder time steps. In order toobtain a single representation of the node vectorsfrom the final layer (k = f ) of the query-GCN, we

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use an attention mechanism as described below:

μjt = vT1 tanh(W1c

fj +W2dt−1) (10)

αt = softmax(μt) (11)

hQt =

∑|Q|j ′=1 αj ′tc

fj ′ (12)

Here v1,W1 and W2 are parameters. Further,at each decoder time step, we obtain a query-aware representation from the final layer of thehistory-GCN by computing an attention score foreach node/token in the history based on the querycontext vector hQ

t as shown below:

νjt = vT2 tanh(W3a

fj +W4dt−1 +W5h

Qt )

(13)

βt = softmax(νt) (14)

hHt =

∑|H|j ′=1 βj ′ta

fj ′ (15)

Here v2,W3,W4, and W5 are parameters.Finally, we obtain a query and history awarerepresentation of the KB by computing anattention score over all the nodes in the finallayer of KB-GCN using hQ

t and hHt as shown

below:

ωjt = vT3 tanh(W6r

fj +W7dt−1 +W8h

Qt

+W9hHt ) (16)

γt = softmax(ωt) (17)

hKt =

∑mj ′=1 γj ′tr

fj ′ (18)

Here v3,W6,W7,W8 and W9 are parameters.This sequential attention mechanism is illustratedin Figure 2. For simplicity, we depict the GCNand RNN+GCN encoders as blocks. The internalstructure of these blocks are shown in Figure 1.

4.5 Decoder

The decoder is conditioned on two components:(i) the context that contains the history and the KBand (ii) the query that is the last/previous utterancein the dialogue. We use an aggregator that learnsthe overall attention to be given to the historyand KB components. These attention scores: θHtand θKt are dependent on the respective contextvectors and the previous decoder state dt−1. Thefinal context vector is obtained as:

hCt = θHt hH

t + θKt hKt (19)

hfinalt = [hC

t ;hQt ] (20)

where [; ] denotes the concatenation operator. Atevery time step, the decoder then computes aprobability distribution over the vocabulary usingthe following equations:

dt = RNN(dt−1, [hfinalt ;wt]) (21)

Pvocab = softmax(V ′dt + b′) (22)

wherewt is the decoder input at time step t, V ′ andb′ are parameters. Pvocab gives us a probabilitydistribution over the entire vocabulary and the lossfor time step t is lt = − logPvocab(w

∗t ), where w∗

t

is the tth word in the ground truth response. Thetotal loss is an average of the per-time step losses.

4.6 Contextual Graph CreationFor the dialogue history and query encoder, weused the dependency parse tree for capturingstructural information in the encodings. However,if the conversations occur in a language for whichno dependency parsers exist, for example: code-mixed languages like Hinglish (Hindi–English)(Banerjee et al., 2018), then we need an alternateway of extracting a graph structure from theutterances. One simple solution that has workedwell in practice was to create a word co-occurrencematrix from the entire corpus where the contextwindow is an entire sentence. Once we have sucha co-occurrence matrix, for a given sentence wecan connect an edge between two words if theirco-occurrence frequency is above a thresholdvalue. The co-occurrence matrix can either con-tain co-occurrence frequency counts or positive-pointwise mutual information (PPMI) values(Church and Hanks, 1990; Dagan et al., 1993;Niwa and Nitta, 1994).

5 Experimental Setup

In this section, we describe the datasets used inour experiments, the various hyperparameters thatwe considered, and the models that we compared.

5.1 DatasetsThe original DSTC2 dataset (Henderson et al.,2014a) was based on the task of restauranttable reservation and contains transcripts of realconversations between humans and bots. Theutterances were labeled with the dialogue stateannotations like the semantic intent representation,requested slots, and the constraints on the slotvalues. We report our results on the modifiedDSTC2 dataset of Bordes et al. (2017), where

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Figure 2: Illustration of sequential attention mechanism in RNN+GCN-SeA.

such annotations are removed and only the rawutterance–response pairs are present with anassociated set of KB triples for each dialogue.It contains around 1,618 training dialogues, 500validation dialogues, and 1,117 test dialogues.For our experiments with contextual graphs wereport our results on the code-mixed versions ofmodified DSTC2, which was recently releasedby Banerjee et al. (2018). This dataset has beencollected by code-mixing the utterances of the

English version of modified DSTC2 (En-DSTC2)in four languages: Hindi (Hi-DSTC2), Bengali(Be-DSTC2), Gujarati (Gu-DSTC2), and Tamil(Ta-DSTC2), via crowdsourcing. We also performexperiments on two goal-oriented dialogue datasetsthat contain conversations between humans whereinthe conversations were collected in a Wizard-of-Oz (WOZ) manner. Specifically, we use the Cam676dataset (Wen et al., 2017), which contains 676 KB-grounded dialogues from the restaurant domain

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Model per-resp.acc BLEU ROUGE Entity F1

1 2 LRule-Based (Bordes et al., 2017) 33.3 − − − − −MEMNN (Bordes et al., 2017) 41.1 − − − − −QRN (Seo et al., 2017) 50.7 − − − − −GMEMNN (Liu and Perez, 2017) 48.7 − − − − −Seq2Seq-Attn (Bahdanau et al., 2015) 46.0 57.3 67.2 56.0 64.9 67.1Seq2Seq-Attn+Copy (Eric and Manning, 2017) 47.3 55.4 − − − 71.6HRED (Serban et al., 2016) 48.9 58.4 67.9 57.6 65.7 75.6Mem2Seq (Madotto et al., 2018) 45.0 55.3 − − − 75.3GCN-SeA 47.1 59.0 67.4 57.1 65.0 71.9RNN+CROSS-GCN-SeA 51.2 60.9 69.4 59.9 67.2 78.1RNN+GCN-SeA 51.4 61.2 69.6 60.2 67.4 77.9

Table 1: Comparison of RNN+GCN-SeA with other models on the English version of modified DSTC2.

and the MultiWOZ (Budzianowski et al., 2018)dataset, which contains 10,438 dialogues.

5.2 Hyperparameters

We used the same train, test, and validation splitsas provided in the original versions of the datasets.We minimized the cross entropy loss using theAdam optimizer (Kingma and Ba, 2015) andtuned the initial learning rates in the range of0.0006 to 0.001. For regularization we used anL2 penalty of 0.001 in addition to a dropout(Srivastava et al., 2014) of 0.1. We used randomlyinitialized word embeddings of size 300. The RNNand GCN hidden dimensions were also chosen tobe 300. We used GRU (Cho et al., 2014) cells forthe RNNs. All parameters were initialized froma truncated normal distribution with a standarddeviation of 0.1.

5.3 Models Compared

We compare the performance of the followingmodels.

(i) RNN+GCN-SeA vs GCN-SeA: We useRNN+GCN-SeA to refer to the model describedin Section 4. Instead of using the hidden repre-sentations obtained from the bidirectional RNNs,we also experiment by providing the token em-beddings directly to the GCNs—that is, c1u = qu

in equation 6 and a1u = pu in equation 8. We referto this model as GCN-SeA.

(ii) Cross edges between the GCNs: Inaddition to the dependency and contextual edges,we add edges between words in the dialogue

history/query and KB entities if a history/queryword exactly matches the KB entity. Suchedges create a single connected graph that isencoded using a single GCN encoder and thenseparated into different contexts to computesequential attention. This model is referred toas RNN+CROSS-GCN-SeA.

(iii) GCN-SeA+Random vs GCN-SeA+Structure: We experiment with the modelwhere the graph is constructed by randomly con-necting edges between two words in a context.We refer to this model as GCN-SeA+Random.We refer to the model that either uses dependencyor contextual graphs instead of random graphs asGCN-SeA+Structure.

6 Results and Discussions

In this section, we discuss the results of ourexperiments as summarized in Tables 1– 5. We useBLEU (Papineni et al., 2002) and ROUGE (Lin,2004) metrics to evaluate the generation qualityof responses. We also report the per-responseaccuracy, which computes the percentage ofresponses in which the generated response exactlymatches the ground truth response. To evaluate themodel’s capability of correctly injecting entitiesin the generated response, we report the entity F1measure as defined in Eric and Manning (2017).

Results on En-DSTC2: We compare our modelwith the previous works on the English versionof modified DSTC2 in Table 1. For most of theretrieval-based models, the BLEU or ROUGEscores are not available as they select a candidate

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Dataset Model per-resp.acc BLEU ROUGE Entity F1

1 2 L

Hi-DSTC2

Seq2Seq-Bahdanau Attn 48.0 55.1 62.9 52.5 61.0 74.3HRED 47.2 55.3 63.4 52.7 61.5 71.3Mem2Seq 43.1 50.2 55.5 48.1 54.0 73.8GCN-SeA 47.0 56.0 65.0 55.3 63.0 72.4RNN+CROSS-GCN-SeA 47.2 56.4 64.7 54.9 62.6 73.5RNN+GCN-SeA 49.2 57.1 66.4 56.8 64.4 75.9

Be-DSTC2


GU-DSTC2


Ta-DSTC2


Table 2: Comparison of RNN+GCN-SeA with other models on all code-mixed datasets.

Models Match Success BLEU ROUGE-1 ROUGE-2 ROUGE-LSeq2seq-Attn 85.29 48.53 18.81 48.11 24.69 40.41HRED 83.82 44.12 19.38 48.25 24.09 39.93GCN-SeA 85.29 21.32 18.48 47.69 25.15 40.29RNN+GCN-SeA 94.12 45.59 21.62 50.49 27.69 42.35

Table 3: Comparison of our models with the baselines on the Cam676 dataset.

from a list of candidates as opposed to generatingit. Our model outperforms all of the retrieval andgeneration-based models. We obtain a gain of 0.7in the per-response accuracy compared with theprevious retrieval based state-of-the-art model ofSeo et al. (2017), which is a very strong baselinefor our generation-based model. We call this astrong baseline because the candidate selectiontask of this model is easier than the responsegeneration task of our model. We also obtaina gain of 2.8 BLEU points, 2 ROUGE, pointsand 2.5 entity F1 points compared with currentstate-of-the-art generation-based models.

Results on code-mixed datasets and effect ofusing RNNs: The results of our experiments on thecode-mixed datasets are reported in Table 2. Ourmodel outperforms the baseline models on all thecode-mixed languages. One common observationfrom the results over all the languages is thatRNN+GCN-SeA performs better than GCN-SeA.Similar observations were made by Marcheggianiand Titov (2017) for semantic role labeling.

Results on Cam676 dataset: The results ofour experiments on the Cam676 dataset arereported in Table 3. In order to evaluate goal-completeness, we use two additional metrics as

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Single Domain Dialogues (SNG)Models Match Success BLEU ROUGE-1 ROUGE-2 ROUGE-LSeq2seq-Attn 68.16 36.77 11.53 35.30 13.44 28.28HRED 84.30 52.02 10.27 38.30 14.49 30.38GCN-SeA 63.68 44.84 12.30 39.79 16.11 32.51RNN+GCN-SeA-400d 86.10 59.19 11.73 38.76 15.22 30.93RNN+GCN-SeA-100d 75.78 32.74 13.13 40.76 17.67 33.59

Multi-Domain Dialogues (MUL)Seq2seq-Attn 44.40 22.10 14.03 38.99 16.39 30.87HRED 66.40 37.70 12.75 40.57 16.83 31.98GCN-SeA 57.40 37.90 14.16 42.40 19.03 34.25RNN+GCN-SeA 62.20 40.30 15.85 43.40 19.63 35.15

Table 4: Comparison of our models with the baselines on the MultiWOZ dataset.

Dataset Model per-resp. BLEU ROUGE Entity F1acc 1 2 L

En-DSTC2 GCN-SeA+Random 45.9 57.8 67.1 56.5 64.8 72.2GCN-SeA+Structure 47.1 59.0 67.4 57.1 65.0 71.9

Hi-DSTC2GCN-SeA+Random 44.4 54.9 63.1 52.9 60.9 67.2GCN-SeA+Structure 47.0 56.0 65.0 55.3 63.0 72.4

Be-DSTC2GCN-SeA+Random 44.9 56.5 65.4 54.8 62.7 65.6GCN-SeA+Structure 47.1 58.4 67.4 57.3 64.9 69.6

Gu-DSTC2GCN-SeA+Random 45.0 54.0 64.1 54.0 61.9 69.1GCN-SeA+Structure 48.1 55.7 65.5 56.2 63.5 72.2

Ta-DSTC2GCN-SeA+Random 44.8 61.4 66.9 55.6 64.3 70.5GCN-SeA+Structure 46.4 62.8 68.5 57.5 66.1 71.9

Table 5: GCN-SeA with random graphs and dependency/contextual graphs on all DSTC2 datasets.

Figure 3: GCN-SeA with multiple hops on all DSTC2 datasets.

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Dataset Model per-resp.acc BLEU ROUGE Entity F1

1 2 L

Hi-DSTC2

Seq2seq-Bahdanau Attn 48.0 55.1 62.9 52.5 61.0 74.3GCN-Bahdanau Attn 38.5 50.4 58.9 47.7 56.7 59.1RNN+GCN-Bahdanau Attn 47.1 56.0 65.1 55.2 62.9 72.2RNN-SeA 45.8 55.9 65.1 55.5 63.1 71.8RNN+GCN-SeA 49.2 57.1 66.4 56.8 64.4 75.9

Be-DSTC2


Gu-DSTC2


Ta-DSTC2


En-DSTC2


Table 6: Ablation results of various models on all versions of DSTC2.

Dataset Model per resp.acc BLEU ROUGE Entity F1

1 2 L

En-DSTC2Query 22.8 38.1 53.5 37.6 50.6 18.4

Query + History 47.1 60.6 68.8 59.4 66.6 72.8Query + KB 41.4 55.8 63.7 52.4 60.9 63.5

Hi-DSTC2Query 22.5 37.5 50.9 37.8 48.4 11.1


Be-DSTC2Query 22.7 37.9 51.9 38.0 49.0 10.6


Gu-DSTC2Query 22.4 36.1 50.7 37.2 48.4 10.9


Ta-DSTC2Query 22.8 39.3 53.6 39.0 50.6 18.8


Table 7: Ablations on different parts of the encoder of RNN+GCN-SeA.

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Dataset Wins % Losses % Ties %En-DSTC2 42.17 22.83 35.00

Cam676 29.00 27.33 43.66

Table 8: Human evaluation results showing wins,losses, and ties % on En-DSTC2 and Cam676.

used in the original paper (Wen et al., 2017)which introduced this dataset, (i) match rate: thenumber of times the correct entity was suggestedby the model, and (ii) success rate: if the correctentity was suggested and the system provided allthe requestable slots then the dialogue results ina success. The results suggest that our model’sresponses are more fluent as indicated by theBLEU and ROUGE scores. It also produces thecorrect entities according to the dialogue goals butfails to provide enough requestable slots. Note thatthe model described in the original paper (Wenet al., 2017) is not directly comparable to our workas it uses an explicit belief tracker, which requiresextra supervision/annotation about the belief-state. However, for the sake of completeness wewould like to mention that their model using thisextra supervision achieves a BLEU score of 23.69and a success rate of 83.82%.

Results on MultiWOZ dataset: The results ofour experiments on two versions of the MultiWOZdataset are reported in Table 4. The first version(SNG) contains around 3K dialogues in whicheach dialogue involves only a single domainand the second version (MUL) contains all 10kdialogues. The baseline models do not use anoracle belief state as mentioned in Budzianowskiet al. (2018) and therefore are comparable to ourmodel. We observed that with a larger GCN hiddendimension (400d in Table 4) our model is able toprovide the correct entities and requestable slotsin SNG. On the other hand, with a smaller GCNhidden dimension (100d) we are able to generatefluent responses in SNG. On MUL, our model isable to generate fluent responses but strugglesin providing the correct entity mainly due tothe increased complexity of multiple domains.However, our model still provides a high num-ber of correct requestable slots, as shown by thesuccess rate. This is because multiple domains(hotel, restaurant, attraction, hospital) have thesame requestable slots (address, phone, postcode).

Effect of using hops: As we increased thenumber of hops of GCNs (Figure 3), we observed

a decrease in the performance. One reason for sucha drop in performance could be that the averageutterance length is very small (7.76 words). Thus,there is not much scope for capturing distantneighborhood information and more hops canadd noisy information. The reduction is moreprominent in contextual graphs in which multi-hop neighbors can turn out to be dissimilar wordsin different sentences.

Effect of using random graphs: GCN-SeA+Random and GCN-SeA+Structure take thetoken embeddings directly instead of passing themthough an RNN. This ensures that the difference inperformance of the two models are not influencedby the RNN encodings. The results are shown inTable 5 and we observe a drop in performancefor GCN-SeA+Random across all the languages.This shows that the dependency and contextualstructures play an important role and cannot bereplaced by random graphs.

Ablations: We experiment with replacing thesequential attention by the Bahdanau attention(Bahdanau et al., 2015). We also experimentwith various combinations of RNNs and GCNsas encoders. The results are shown in Table 6.We observed that GCNs do not outperformRNNs independently. In general, RNN-Bahdanauattention performs better than GCN-Bahdanauattention. The sequential attention mechanismoutperforms Bahdanau attention as observed fromthe following comparisons: (i) GCN-Bahdanauattention vs GCN-SeA, (ii) RNN-Bahdanauattention vs RNN-SeA (in BLEU and ROUGE),and (iii) RNN+GCN-Bahdanau attention vsRNN+GCN-SeA. Overall, the best results arealways obtained by our final model, which com-bines RNN, GCN, and sequential attention. Wealso performed ablations by removing specificparts of the encoder. Specifically, we experi-ment with (i) query encoder alone, (ii) query +history encoder, and (iii) query + KB encoder.The results shown in Table 7 suggest that thequery and the KB are not enough to generatefluent responses and the previous conversationhistory is essential.

Human evaluations: In order to evaluatethe appropriateness of our model’s responsescompared to the baselines, we perform a humanevaluation of the generated responses using in-house evaluators. We evaluated randomly chosenresponses from 200 dialogues of En-DSTC2 and

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Context:Speaker 1: [silence] .Speaker 2: hello, welcome to the cambridgerestaurant system . you can ask for restaurants by area,price range or food type . how may i help you ?Speaker 1: uh cheap restaurant malaysian food .Speaker 2: i’m sorry but there is no restaurantserving malaysian food .Speaker 1: uh italian food .Speaker 2: could you please repeat that ?Speaker 1: cheap restaurant uh italian food .Speaker 2: there are restaurants servingitalian in the cheap price range . what area would you like ?Speaker 1: i dont care .

KB Triples:ask, r cuisine, italianask, r location, centreask, r phone, ask phoneask, r address, ask addressask, r price, cheapask, r rating, 0pizza hut city centre, r cuisine, italianpizza hut city centre, r location, centrepizza hut city centre, r phone,pizza hut city centre phonepizza hut city centre, r address,pizza hut city centre addresspizza hut city centre, r price, cheappizza hut city centre, r rating, 9

Seq2seq + Attn: ask is a nice place in the west of town serving tasty italian foodHRED: pizza hut city centre serves italian food in the expensive price rangeGCN-SeA: pizza hut city centre serves italian food in the cheap price rangeRNN+GCN-SeA: pizza hut city centre is a great restaurant serving cheap italian food in the centre of townRNN+CROSS-GCN-SeA:pizza hut city centre is a nice place in the centre of town serving tasty italian food

Table 9: Qualitative comparison of responses between the baselines and different versions of our model

100 dialogues of Cam676 using the methodof pairwise comparisons introduced in Serbanet al. (2017). We chose the best baseline modelfor each dataset, namely, HRED for En-DSTC2and Seq2seq+Attn for Cam676. We show eachdialogue context to three different evaluators andask them to select the most appropriate responsein that context. The evaluators were given noinformation about which model generated whichresponse. They were allowed to choose an optionfor tie if they were not able to decide whetherone model’s response was better than the othermodel. The results reported in Table 8 suggestthat our model’s responses are favorable in noisycontexts of spontaneous conversations, such asthose exhibited in the DSTC2 dataset. However,in a WOZ setting for human–human dialogues,where the conversations are less spontaneous andcontexts are properly established, both the modelsgenerate appropriate responses.

Qualitative analysis: We show the generatedresponses of the baselines and different versionsof our model in Table 9. We see that Seq2seq+Attnmodel is not able to suggest a restaurant with ahigh rating whereas HRED gets the restaurant rightbut suggests an incorrect price range. However,RNN+GCN-SeA suggests the correct restaurantwith the preferred attributes. Although GCN-SeAselects the correct restaurant, it does not providethe location in its response.

7 Conclusion

We showed that structure-aware representationsare useful in goal-oriented dialogue and ourmodel outperforms existing methods on fourdialogue datasets. We used GCNs to infuse struc-tural information of dependency graphs and con-textual graphs to enrich the representations of thedialogue context and KB. We also proposed asequential attention mechanism for combining therepresentations of (i) query (current utterance),(ii) conversation history, and (iii) the KB. Finally,we empirically showed that when dependencyparsers are not available for certain languages,such as code-mixed languages, then we can useword co-occurrence frequencies and PPMI valuesto extract a contextual graph and use such a graphwith GCNs for improved performance.

Acknowledgments

We would like to thank the anonymous reviewersand the action editor for their insightful com-ments and suggestions. We would like to thankthe Department of Computer Science and Engi-neering, IIT Madras and Robert Bosch Centrefor Data Science and Artificial Intelligence(RBC-DSAI), IIT Madras for providing thenecessary resources. We would also like to thankAccenture Technology Labs, India, for support-ing our work through their generous academicresearch grant.

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