Prompt-augmented Temporal Point Process for Streaming Event Sequence

Neural Temporal Point Processes (TPPs) are the prevalent paradigm for modeling continuous-time event sequences, such as user activities on the web and financial transactions. In real-world applications, event data is typically received in a \emph{streaming} manner, where the distribution of patterns may shift over time. Additionally, \emph{privacy and memory constraints} are commonly observed in practical scenarios, further compounding the challenges. Therefore, the continuous monitoring of a TPP to learn the streaming event sequence is an important yet under-explored problem. Our work paper addresses this challenge by adopting Continual Learning (CL), which makes the model capable of continuously learning a sequence of tasks without catastrophic forgetting under realistic constraints. Correspondingly, we propose a simple yet effective framework, PromptTPP\footnote{Our code is available at {\small \url{ https://github.com/yanyanSann/PromptTPP}}}, by integrating the base TPP with a continuous-time retrieval prompt pool. The prompts, small learnable parameters, are stored in a memory space and jointly optimized with the base TPP, ensuring that the model learns event streams sequentially without buffering past examples or task-specific attributes. We present a novel and realistic experimental setup for modeling event streams, where PromptTPP consistently achieves state-of-the-art performance across three real user behavior datasets.

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Peer review

Reviewer GihP6/10 · confidence 4/52023-07-02

Summary

The author introduces PromptTPP, a method that integrates classical neural temporal point processes with a continuous-time retrieval prompt pool. This approach enables the model to learn event streams sequentially, eliminating the need to buffer previous examples or task-specific features in the context of continual learning.

Strengths

The paper's motivation for introducing this method is clear, and the experiments conducted to validate its effectiveness are relatively comprehensive.

Weaknesses

The method description is not clear. The author needs to present the prompt-learning-based task in more detail. For example, the prompt length Lp, selection size N, and prompt pool size M are hyperparameters, but under common sense, the buffer of continual learning is dynamically changing. You can show the case study of the Taobao dataset and the Amazon dataset. The combination of PPT, CL, and PL is somewhat unnatural. For example, I hardly see the idea of CL in the method. Explain how the prompt learning method can help CL in terms of theoretical or technical details, rather than just experimental accuracy. Instead of a decline in catastrophic forgetting, it's possible that the decrease in average error is caused by an improvement in accuracy at each time point. Thus, the author also needs to show how the model performs on previous events after learning new events. If it is a part of the experiment, kindly explain it in detail. Do the prompt settings need to be related to downstream tasks? If I understand correctly, your setting only concerns time, and it does not change depending on the process's intended goal. But in classical prompt learning, the prompt is customized, in NLP, adapting language model using downstream tasks. As you show in experiments, the task of each dataset is known beforehand, so will custom methods improve the results more than general methods? Can we investigate temporal prompt + event prompt?

Questions

see above

Rating

6: Weak Accept: Technically solid, moderate-to-high impact paper, with no major concerns with respect to evaluation, resources, reproducibility, ethical considerations.

Confidence

4: You are confident in your assessment, but not absolutely certain. It is unlikely, but not impossible, that you did not understand some parts of the submission or that you are unfamiliar with some pieces of related work.

Soundness

3 good

Presentation

2 fair

Contribution

3 good

Limitations

see above

Reviewer nDRf6/10 · confidence 2/52023-07-04

Summary

This paper addresses the problem of Temporal Point Processes (TPP), which involves modeling continuous-time events. Specifically, the authors focus on developing a model for streaming data, where the model needs to be improved in a Continual Learning (CL) setting. To tackle this challenge, the authors propose a TPP model that combines the concepts of Prompt (which is modified for continuous time) and CL. The proposed method has demonstrated superior performance compared to alternative approaches on two datasets.

Strengths

The paper is easy to read and is well-structured. The experiments conducted effectively demonstrate how each idea (Prompt and CL) contributes to the improvement over the alternative design.

Weaknesses

I am not familiar with the related works on Neural TPP. However, I noticed that the experiment in this paper only utilizes two datasets, which might be considered somewhat limited. It is unclear if this is a common approach for papers working on this specific problem.

Questions

1. Can the authors provide justification on using only two datasets in the experiment section? 2. In the abstract, the authors talk about privacy constraints associated with the TPP problem. Can authors provide more discussion on this with different TPP methods?

Rating

6: Weak Accept: Technically solid, moderate-to-high impact paper, with no major concerns with respect to evaluation, resources, reproducibility, ethical considerations.

Confidence

2: You are willing to defend your assessment, but it is quite likely that you did not understand the central parts of the submission or that you are unfamiliar with some pieces of related work. Math/other details were not carefully checked.

Soundness

3 good

Presentation

3 good

Contribution

3 good

Limitations

The paper includes a section discussing the limitations and potential societal impact.

Reviewer 26jC6/10 · confidence 3/52023-07-07

Summary

The paper proposes a novel prompt-augmented continual learning (CL) framework for modeling streaming event sequences, called PromptTPP. The framework incorporates a continuous-time retrieval prompt pool and a retrieval mechanism to dynamically lookup a subset of task-relevant prompts based on the instance-wise input of event sequences. The main contributions of the paper are (i) the introduction of PromptTPP, a novel prompt-augmented CL framework for neural TPPs, and (ii) the applicability and generality of the approach.

Strengths

1. The use of a continuous-time retrieval prompt pool and a retrieval mechanism to dynamically look up a subset of task-relevant prompts based on the instance-wise input of event sequences is a novel approach to CL. 2. The paper is well-organized and easy to follow. The authors provide clear explanations of the proposed framework and the experiments. The figures and tables are also well-designed and easy to understand. 3. The PromptTPP addresses modelling streaming event sequences in a continual learning setting.

Weaknesses

1. The paper does not provide a detailed analysis of the computational complexity of the proposed framework, which is an important consideration when working with large datasets and complex models. 2. The paper does not provide a detailed comparison with other state-of-the-art methods for modeling streaming event sequences in a continual learning setting, which would help researchers understand the strengths and weaknesses of PromptTPP in comparison to other approaches.

Questions

1. Can the authors provide more details on the retrieval mechanism used in the proposed framework? Specifically, how does the retrieval mechanism select a subset of task-relevant prompts based on the instance-wise input of event sequences? 2. How can the proposed approach be used for unwanted tracking of individual behaviour, and what steps can be taken to mitigate this risk? 3. It would greatly enhance the understanding of the proposed framework if the authors could provide more in-depth information regarding the generative loss and its optimization. Specifically, how are the generative loss and retrieval prompt pool jointly optimized?

Rating

6: Weak Accept: Technically solid, moderate-to-high impact paper, with no major concerns with respect to evaluation, resources, reproducibility, ethical considerations.

Confidence

3: You are fairly confident in your assessment. It is possible that you did not understand some parts of the submission or that you are unfamiliar with some pieces of related work. Math/other details were not carefully checked.

Soundness

2 fair

Presentation

2 fair

Contribution

2 fair

Limitations

See the weaknesses above

Reviewer n7nj7/10 · confidence 2/52023-07-27

Summary

This paper aims to enhance the Neural Temporal Point Processes (TPPs) used in modeling continuous-time event sequences. Particularly in the real-world, the distribution of event data shifts over time, which existing TPPs fail to reflect. To address this, the authors propose PromptTPP, a continual learning framework that utilizes prompts. The effectiveness of this proposed method has been demonstrated through experiments with Taobao and Amazon datasets.

Strengths

1. The paper is reader-friendly, with comprehensive equations and illustrative figures. 2. The proposed method is well-structured and intuitively compelling. 3. The experimental results convincingly demonstrate the effectiveness of the proposed approach. 4. The availability of the code simplifies the process of reproduction, making it easily accessible for further study.

Weaknesses

While I'm not deeply familiar with this field, there don't seem to be significant weaknesses. However, I do have a concern related to the experiments conducted. The authors refer to CLTPP (Dubey et al., 2022) as the only neural TPP with continual learning (CL) capabilities. However, they did not conduct experiments on the Yelp and Meme datasets used in this paper (Dubey et al., 2022). Is there any reason for it?

Questions

1. In line 112, it appears that the latter part of the sentence is missing. (`Compared with ordinary fine-tuning, literature shows […] In our`) 2. According to Figure 3, while the type error rate shows improvement (a decrease) for later tasks in the sequence, the time RMSE demonstrates the opposite trend, worsening (increasing) over time. What is the reason for this opposing trend between the two performance metrics? 3. I am interested in understanding the specific degree of distribution variances across tasks. The authors illustrate the KL divergence of each subsequent period relative to the 0-th period in Supplementary Figure 7. However, a more comprehensive comparison across all task pairs, possibly utilizing a confusion matrix, might yield more insightful and useful information.

Rating

7: Accept: Technically solid paper, with high impact on at least one sub-area, or moderate-to-high impact on more than one areas, with good-to-excellent evaluation, resources, reproducibility, and no unaddressed ethical considerations.

Confidence

2: You are willing to defend your assessment, but it is quite likely that you did not understand the central parts of the submission or that you are unfamiliar with some pieces of related work. Math/other details were not carefully checked.

Soundness

4 excellent

Presentation

4 excellent

Contribution

3 good

Limitations

N/A

Reviewer nDRf2023-08-16

I am satisfied with the authors' response, and will update my original rating.

Reviewer GihP2023-08-18

Thanks for the authors' response. I have updated my rating.

Authorsrebuttal2023-08-18

Thank you very much for your support! We are thrilled to hear that our response has addressed your concerns. And we are committed to improving our presentation for the next version.

Authorsrebuttal2023-08-18

Thanks very much for your response. We are thrilled to know that our response has addressed your concerns. We will improve the paper presentation based on all the reviews.

Reviewer 26jC2023-08-20

After carefully reviewing the authors' responses and the comments from other reviewers, I have decided to maintain my original rating.

Program Chairsdecision2023-09-21

Decision

Accept (poster)

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