Flexible and Scalable State Tracking Framework for Goal-Oriented Dialogue Systems

11/30/2018 ∙ by Rahul Goel, et al. ∙ Amazon 0

Goal-oriented dialogue systems typically rely on components specifically developed for a single task or domain. This limits such systems in two different ways: If there is an update in the task domain, the dialogue system usually needs to be updated or completely re-trained. It is also harder to extend such dialogue systems to different and multiple domains. The dialogue state tracker in conventional dialogue systems is one such component - it is usually designed to fit a well-defined application domain. For example, it is common for a state variable to be a categorical distribution over a manually-predefined set of entities (Henderson et al., 2013), resulting in an inflexible and hard-to-extend dialogue system. In this paper, we propose a new approach for dialogue state tracking that can generalize well over multiple domains without incorporating any domain-specific knowledge. Under this framework, discrete dialogue state variables are learned independently and the information of a predefined set of possible values for dialogue state variables is not required. Furthermore, it enables adding arbitrary dialogue context as features and allows for multiple values to be associated with a single state variable. These characteristics make it much easier to expand the dialogue state space. We evaluate our framework using the widely used dialogue state tracking challenge data set (DSTC2) and show that our framework yields competitive results with other state-of-the-art results despite incorporating little domain knowledge. We also show that this framework can benefit from widely available external resources such as pre-trained word embeddings.

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1 Introduction

With the rise of digital assistants such as Alexa, Cortana, Siri, and Google Assistant, conversational interfaces are increasingly becoming a part of everyday life. Such conversational interfaces can be broadly divided into chit-chat systems and task-oriented systems . Chit-chat systems are designed to be engaging to users so as to carry on natural and coherent conversations. Task-oriented systems, on the other hand, are designed to help users accomplish certain goals such as booking an airline ticket or making a reservation. With the advent of pervasive use of deep learning, there has been an increase in end-to-end learning of chit-chat systems 

(Serban et al., 2016; Yao et al., 2016; Shao et al., 2017) employing varieties of sequence-to-sequence networks (Sutskever et al., 2014). Compared to task-oriented dialogue, end-to-end learning has been more widely adopted for chit-chat dialogue systems due to the following reasons:

  • Availability of data: Training dialogue systems end-to-end requires large data sets, i.e, millions of dialogues of human-human conversations are available from social media sites such as Reddit and Twitter that are useful for training chit-chat systems. In comparison, there are far fewer task-oriented dialogue data sets available and their sizes are much smaller. SNIPS 111https://github.com/snipsco and ATIS (Hemphill et al., 1990) are examples of task-oriented dialogue data sets and they incldue less than a thousand dialogues.

  • Lack of hard constraints: Due to the open-ended nature of chit-chat dialogue, the system only needs to be engaging and coherent. In task-oriented dialogue systems, there are additional requirements such as being able to ask for correct information in non-repeatative manner and being able to perform correct actions that are in line with the user’s intents.

  • No connection to downstream applications: Chit-chat dialogue systems do not necessarily have to interface with downstream applications or databases, although recently, there has been a focus on knowledge grounded interactions (e.g., DSTC 6 and 7 (Hori et al., 2018)). On the other hand, goal-oriented dialogue systems must be able to interface with external systems with discrete input representations, which adds additional difficulties to end-to-end learning.

Due to these reasons, though there are only a few examples of complete end-to-end learning of goal-oriented dialogue systems (Dhingra et al., 2016; Liu et al., 2017, 2018; Zhao and Eskenazi, 2016), such approaches remain uncommon, especially in commercially deployed systems. Most traditional goal-oriented dialogue systems are built as a pipeline with modules for spoken language understanding, state tracking, and action selection. Typically, each of these modules is specifically designed for intended domains with manually chosen label spaces. Thus, scaling these components to many different domains or more complex use cases can be very challenging.

In a multi-turn dialogue, the system has to be able to reason over the dialogue states to ask necessary questions, inform the users with valid information, or perform sequences of actions needed to achieve user’s goals. To accommodate these requirements, dialogue states are generally defined as discrete variables that are symbolic so as to allow the system to reason over them. Furthermore, backend APIs used to accompish user’s goals require such discrete input, for example, SQL or SPARQL queries.

Despite these challenges, recently there has been a push for representing dialogue states with neural networks similar to chit-chat dialogue systems. One key advantage being the latent dialogue state representation 

(Liu et al., 2017). With end-to-end learning, the system can learn a distributional state representation as well as reason over it. However, there are few shortcomings to this approach:

  • Lack of annotated data: Automatically inferring dialogue states may require thousands of annotated conversations to learn simple actions such as updating a value associated with an entity.

  • Lack of an easy way to inject constraints: In practical settings, placing hard constraints on how dialogue system operates might be necessary, e.g., asking for pin-code before making a voice purchase.

  • Lack of interpretability: Due to the absence of symbolic dialogue states, it may be hard to explain why the system chose to perform a particular action. This kind of interpretability might be essential for commercial applications.

In this paper, we propose a flexible and extensible dialogue state tracking framework that still relies on updating values of discrete variables but the framework can be applied to new domains and new use cases much more easily. Our system has the following desirable properties: First, there is no explicit rules encoded for state variable updates. For example, values for state variables are not confined to pre-set possible values. Second, each state variable can be learned independently from each other. This makes it easier to add new state variables to the system thus making the extension of the framework to new use cases easier. Third, our framework allows for multiple values to be associated with a state variable resulting in richer representations. In most other frameworks, this is not possible. This makes our framework more flexible such that representing more complex state becomes easier. All these lead to increased scalability in terms of increasing domain complexity or adapting the framework to multiple different domains.

In our experiments with the DSTC2 corpus (Henderson et al., 2014a), we show that this framework yields competitive results to state-of-the-art on this data (Williams et al., 2016)

. We show that even without explicit output space definition, our results are competitive. We also show that models benefit from pre-trained word embeddings, which opens doors for incorporating dynamic word embeddings to state tracking models. Dynamic word embeddings have been shown to improve multiple natural language processing applications 

(Devlin et al., 2018). We believe this work is a step towards more scalable end-to-end learning of task-oriented dialogue systems, which we discuss in the following sections.

2 Methodology

Figure 1: Dialogue state tracking framework. The main inputs to the system are candidate values() for dialogue state variables, previous state (), and various context( and ) including the current user utterance(). The output is a binary number indicating whether a candidate value under consideration is a value for the current state variable.

The dialogue state tracking task involves inferring the current state of dialogue based on the conversation that has happened so far. A state could be represented as a set of variables (slot-value pairs or API name with its arguments) associated with users’ goals and the system capabilities. In this setting, the goal of the system becomes predicting either the right dialogue act and slot value combinations that form the system’s response or calling the correct APIs with correct arguments to help the user achieve his or her goal.

More concretely, the dialogue state consists of user goals and slots associated with the goal. The goals and slots depend on the domain ontology and are typically handcrafted. The DSTC2 dataset belongs to the restaurant domain. It consists of only one goal which is to find a restaurant and there are three slot types: cuisine, area, and pricerange. In this case, the dialogue state is represented by a triple with corresponding values for each slot type. When a user says “I want a cheap indian restaurant,” the dialogue state can be represented as a value triple: (indian, none, cheap) corresponding to the slots cuisine, area, and pricerange.

Given a user goal, we adopt an open-vocabulary scoring model for each slot type that needs to be tracked for the goal. Similar to (Rastogi et al., 2018), the input to our model is a set of candidates that could be a value for each slot type, a slot type , and conversation context . Using

-grams present in an utterance as the candidate values makes either spoken language understanding (SLU) system or the named entity recognition (NER) system unnecessary and we can do a joint SLU and state tracking. However, if there is SLU or NER for the domain available, we can use it to reduce the search space. Our model produces a binary classification decisions for each combination of a candidate value and a slot type. A positive class corresponds to the slot value being the correct value for the slot type. Note that we do not place any constraints on how many values can be associated with the slot type, hence we can represent slots with multiple values. For example, food={chinese, thai} in the user utterance: “I am looking for chinese or thai restaurants”.

Figure 1 shows the overall framework of this approach. Note the following characteristic of this model. First, each decision, i.e., whether a candidate value can be used to update the current state, is made completely independently from all other decisions. Second, each decision is made independently from other slot types. Third, the model only makes a binary decisions per candidate value and it does not rely on a pre-defined output space. Fourth, the model can be configured to consider varieties of dialogue context.

With the available input, we build a fully-connected neural network classifier. For a given candidate and a slot type, we minimize the cross entropy loss as shown below:

(1)

where denotes the candidate set of values, denotes the set of slot types and denotes the conversation context (Equation 2).

Given a user turn , we construct a candidate set for that turn. A candidate set is an open set consisting of possible slot values for each slot type. In a typical dialogue system this could be constructed from the output of SLU system augmented with additional values obtained using simple rules (such as business logic). For our experiments, we create two kinds of candidate sets for each turn:

  1. -gram candidate set: This consists of the -grams obtained from the user utterance. We experiment with unigrams () or bigrams () in this paper.

  2. SLU candidate set: The DSTC2 data provides us with SLU results for each of the user utterances, which are noisy. We experiment with just using the SLU results for each slot type as our candidate set.

Furthermore, we add a value dontcare to each of these candidate sets to allow for no preference for each of the slot type. We construct our training data by assigning the candidates a value of or for each slot type based on the ground truth dialogue state. The system starts with a default state for a given action or an API. After each user utterance, we update our dialogue state with candidates that are predicted as positive. Based on the system design, various update strategies or constraints can be incorporated in the dialogue state update step. For example, if we want to enforce the constraint that one slot can have only one value, we can select the candidate with the highest score from the pool of positive candidates.

The dialogue history context features are flexible and we can easily add new context features by appending them to the existing context vector. For our experiments we use the following context features at each user turn

.

  1. Sentence Encoding () using LSTM for the current sentence

  2. Hierarchical LSTM () over past sentences to encode the dialogue context

  3. LSTM () over system dialogue acts

  4. Previous state encoding (): For each slot type we learn a dense matrix where is the number of states and is the state embedding dimension. We do an embedding lookup over the previous state and use it as a feature.

We concatenate all of these features into a context feature vector . For every candidate and slot type we have:

(2)
(3)

We also show through experiments that our performance improves when we use pretrained embeddings for the -gram candidate set, such a system could take advantage of contextual language models such as ELMO (Peters et al., 2018), which has been shown to improve many different natural language processing tasks. Our system is also computationally efficient due to the binary classification decision per slot compared to a softmax over slot-values. Due to our design we inherently support multi-word expressions and multiple values per slot which is hard to support if the output space is the set of predefined slot values.

3 Experiments

3.1 Data

We evaluate our model on the DSTC2 (Henderson et al., 2014a) dataset. Dialogues in DSTC2 are in the restaurant search domain wherein a user can search for a restaurant in multiple ways by selectively applying constraints on a set of “user-informable” slots. The values taken by these slots are either provided by the user in an utterance or are a special value — dontcare, which means the user has expressed that he/she has no preference. The slot value can also be none which indicates the user has not provided any information for this slot type yet. When representing system actions, we use the dialogue acts and slot types but do not use the slot values (except dontcare). Hence the system action inform(price = cheap), is represented as inform(price). Additionally, the dataset also contains the results obtained from the SLU system, which we use as our candidate set in some experiment configuration.

Similar to Liu et al. (2018), we combine the provided training and development datasets and then perform a 90/10 split for validation during training. The test dataset is used as it is provided in the data set. Statistics of this dataset are summarized in the Table 1.

Size of train and dev / test dialogues /
Average number of turns per dialogue
Number of values for cuisine / area / price / /
Table 1: Statistics of the DSTC2 dataset.

3.2 Configuration

We use word embeddings size of . Hidden dimension for sentence LSTM is and for the dialogue LSTM. For the dialogue acts LSTM, we use embeddings of size and LSTM hidden size of . Our state embeddings () is of size . We use ADAM for optimization (Kingma and Ba, 2014) with learning rate of for all our experiments. We use mini-batch of size during training. We limited our context to previous five utterances to make batch training more efficient. We use dropout of between fully connected layers. We also explore using FastText pretrained word embeddings (Bojanowski et al., 2016) instead of random initialization for our word embeddings. Due to the set up of our framework, our training set is very imbalanced (too few positive classes). We up-weighted the positive class by a factor of in the cross entropy loss to mitigate this issue. We use fully connected layer of size and in succession for final classification. We also do an ensemble by majority voting on the joint state for 4 runs.

3.3 Results

For the DSTC2 data set, we used a simple dialogue state update strategy, for every candidate and slot type if the candidate is classified as positive, we update the dialogue state using that candidate’s value in the order it appeared in the sentence. The goal and joint goal accuracy results on the two candidate set settings (SLU and N-gram candidates) are shown in Table 

2

. We report the standard evaluation metrics for this dataset.

Model Cuisine goal Area goal Price goal Joint goal
Word Candidates 82.5 (2.2) 87.9 (1.2) 90.8 (0.5) 68.2 (1.8)
Pretrained embeddings 81.8 (0.5) 88.1 (0.6) 91.6 (1.3) 67.8 (0.7)
SLU Candidates 77.3 (1.1) 89.4 (0.5) 91.6 (0.3) 65.3 (0.9)
Pretrained embeddings 78.3 (1.7) 89.8 (0.8) 91.0 (0.8) 65.9 (1.2)
Table 2:

Dialogue state tracking accuracy performance on the DSTC2 test set. We compare candidates obtained using unigrams and bigrams versus candidates obtained from the SLU system. In the first case, the model is doing joint state tracking and SLU. We report the per goal and joint accuracy. Our result is the average of four runs of training and the standard deviation is in the brackets.

Model Cuisine goal Area goal Price goal Joint goal
Rastogi et al. (2018) - - - 70.3
Liu et al. (2018) 84 90 94 72
Mrkšić et al. (2017) - - - 73.4
Word Candidates 84.0 88.8 91.7 70.7
Pretrained embeddings 83.0 89.2 91.8 70.3
SLU Candidates 78.7 90.3 92.5 67.5
Pretrained embeddings 80.5 91.0 91.6 68.6
Table 3: Accuracy result of four different training runs for ensembled system. Note that  Liu et al. (2018) also report ensemble results for their system. Our method outperforms  Rastogi et al. (2018) who use a similar candidate set ranking approach.

Our ensemble system (Table 3) is able to outperform the work by Rastogi et al. (2018). Their work, similar to us, does not have closed candidate sets for slot values but they use sequence tagging over utterances rather than making binary decisions. We are able to achieve competitive performance to the hierarchical RNN system of Liu et al. (2018) but our results are not as good as the neural belief tracker Mrkšić et al. (2017). These two approaches assume closed sets for slot values, which works for the DSTC2 data set since it does not suffer from the OOV issue.

Note that, unlike the previous work, we do not use ASR -best or in our work as our motivation is to show the feasibility of this approach. We observe that the system which does joint SLU + state tracking (-gram candidates) performs better than using the SLU results as our candidate set. This could be due the errors made by the SLU system which we are able to recover by jointly doing state stacking and SLU (SLU results in the data are noisy). We also observe that using pretrained embeddings gives us a boost in performance in some of the settings. Ensembling improves results across the board, we observe that the experiments with higher standard deviation in Table 2 have higher gains after ensembling.

4 Related Work

Dialogue state tracking (or belief tracking) aims to maintain a distribution over possible dialogue states (Bohus and Rudnicky, 2006; Williams and Young, 2007), which are often represented as a set of key-value pairs. The dialogue states are then used when interacting with the external back-end knowledge base or action sources and in determining what the next system action should be. Previous work on dialogue state tracking include rule-based approaches (Wang and Lemon, 2013)

, Bayesian networks 

(Thomson and Young, 2010), conditional random fields (CRF) (Lee and Eskenazi, 2013)

, recurrent neural networks 

(Henderson et al., 2014b), and end-to-end memory networks (Perez and Liu, 2016).

In DSTC2, many systems that rely on delexicalization, where slot values from a semantic dictionary are replaced by slot labels, outperformed systems that rely on SLU outputs. Furthermore, the set of possible states is limited. However, these approaches do not scale to real applications, where one can observe rich natural language that includes previously unseen slot value mentions and large, possibly unlimited space of dialogue states. To deal with the first issue, Mrkšić et al. (2017)

proposed the neural belief tracker approach that also eliminates the need for language understanding by directly operating on the user utterance and integrating pre-trained word embeddings to deal with the richness in natural language. However, their approach also does not scale to large dialogue state space as they iterate over all slot-value pairs in their ontology to make a decision. have proposed a candidate set ranking approach, where the candidates are generated from language understanding system’s hypotheses to deal with the scalability issues. Our approach is the most similar to this work. However, their approach does not consider multi-valued slots due to the softmax layer over all the values, whereas our approach can estimate probabilities for multiple possible values.

Previous work that investigated joint language understanding and dialogue state tracking include work by Liu et al. (2017); Rastogi et al. (2018). Liu et al. (2017) use a hierarchical recurrent neural network to represent utterances and dialogue flow, and estimates a probability for all possible values, and hence suffers from the scalability issues.

5 Conclusions

In this work we have presented a dialogue state tracking framework which is domain agnostic. Our system is independent of the number of slots and assigns state values using a flexible candidate set. Our system is scalable and our output is not limited to the slot values we have seen at the training time. We show the feasibility of our approach on the DSTC2 state tracking challenge where we achieve competitive performance to the state-of-the-art models which do state prediction over limited vocabulary. We think that such a domain agnostic component will be an important step towards general-purpose end-to-end task-oriented dialogue system. Going forward, we would like to experiment with more data sets and system set ups. One future direction of this work is to predict start and end of slot value token in utterances. This will relax the n-gram restriction and allow us to handle slot values of arbitrary length However, ultimately, we would like our system to move away with explicit state tracking and use such a system to maintain belief over values in a latent manner.

References