Paper: Multi-Agent Bayesian Optimization with Coupled Black-box and Affine Constraints (ICLR-2024)

Reviewer comment (Novelty):
Although the distributed multi-agent framework proposed in this paper has meaningful applications in practice, the technical contribution of the paper seems marginal. The main component of the algorithm developed in this paper relies on the method by [Zhou and Ji (NeurIPS 2022)](https://proceedings.neurips.cc/paper_files/paper/2022/hash/00295cede6e1600d344b5cd6d9fd4640-Abstract-Conference.html), and the primal-dual technique to deal with the joint affine constraint as well as the aggregated constraint function is standard. The most important part to derive the performance guarantees is basically done by the work of [Zhou and Ji (NeurIPS 2022)](https://proceedings.neurips.cc/paper_files/paper/2022/hash/00295cede6e1600d344b5cd6d9fd4640-Abstract-Conference.html) for the single-agent setting.

Author response:
Thanks for the comment. This concern is similar to that of the reviewer vwM2. So we duplicate the response here.
Indeed, our algorithm and analysis are inspired by   (Zhou \& Ji, 2022) and (Srinivas et al., 2012). However, our paper significantly differentiates from the existing works in the following three aspects.
   * **Formulation and design** We consider a multi-agent scenario with coupled black-box and affine constraints, which widely appear in all kinds of engineering areas. This is significantly harder than the single-agent case since the black-box function evaluation and the acquisition step needs to be distributed, while the coupled constraints need coordination. Primal methods, such as EIC (Gardner et al., 2014) and CONFIG (Xu et al., 2023), are not readily applicable to the multi-agent scenario. The key of our method is combining the dual-decomposition method and the GP-UCB method, which leads to a distributed primal-dual algorithm, which, as far as we know, is the first algorithm that can tackle the constrained multi-agent Bayesian optimization problem with theoretical regret/violation guarantees.       
   * **Analysis**  The additional components of affine constraints bring new analysis challenges as compared to (Zhou \& Ji, 2022). Specifically, the regret bounds in Thm. 1 (Zhou \& Ji, 2022) is highly sensitive to the Slater condition parameter $\delta$ (indeed, $\mathcal{O}(\frac{1}{\delta})$). One may rewrite the affine constraints as two linear inequality constraints equivalently.  However, for affine constraints, such Slater condition does not hold anymore. Oblivious application of the analysis pipeline in (Zhou \& Ji, 2022) fails to give a finite bound since $\frac{1}{\delta}\to\infty$ when $\delta\to0$ for affine constraints, thus necessitating the separate analysis on these two types of constraints. To remedy this issue, we introduce a critical new technique (See Lem. 1 and the discussion on case 2 in the proof of Lem. 7), which is a non-obvious extension of the analysis in (Zhou \& Ji, 2022).   

   * **Conditional bounds on strong violation** Fig. 1 (c) and Fig. 2 (c) in (Zhou \& Ji, 2022) shows an empirical sublinear growth of the strong constraint violation (that is, $\sum_{t=1}^T[g(x^t)]^+$). However, (Zhou \& Ji, 2022) did not give any theoretical insights into why this sublinear growth happens even though, in many applications, it is of more importance to manage this stronger metric of violation when the constraint can not be compensated across time (e.g., thermal discomfort in building control). Indeed, we show that in general, primal-dual method can not guarantee sublinear growth of strong violation (see a counterexample in the Appendix B). Hence, there seems to be a contradiction between theory and practice. Meanwhile, existing work that can provide theoretical bound on the strong violation, such as CONFIG (Xu et al., 2023), are not applicable to the distributed multi-agent scenario, which seems to suggest that distributed algorithm and sublinear strong violation bound can not be obtained simultaneously. To fill this gap and provide theoretical insights into the empirical observations, we provide two mild and physically meaningful conditions under which the primal-dual method can simultaneously provide the sublinear strong violation bound and the distributed algorithm. Specifically, the first condition deals with the case that $f$ and $g$ are contradictory metrics (which is common in practice) and $g(x^\star)=0$, that is, the constraint is active on the optimal solution. Our condition 1 is intuitively analogous to the concept of dissipativity in system theory. A simple example that satisfies condition 1 is when $g$ is invertible and the composed function $f\circ g^{-1}$ is convex. The second condition deals with the case that $g(x^\star)<0$, that is, the constraint is inactive on the optimal solution. Intuitively, condition 2 means infeasible solution is strictly worse than the feasible optimal solution. Interestingly, condition 2 holds with probability $\frac{1}{2}$ if the objective and the constraint are sampled from two independent Gaussian processes.

Reviewer comment (Theory):
It is a restriction that the functions of individual agents $(f_1,g_1),\ldots, (f_N, g_N)$ come from independent Gaussian processes.

Author response:
Thanks for the great suggestion. Our current setting is more like a frequentist setting, where the unknown objective/constraint is only assumed to be smooth in some sense. Independence of the functions is not assumed. To incorporate the correlated functions setting algorithmically, we need to combine the multi-output Gaussian processes with the primal-dual algorithm. Indeed, one direction to explore is combining our method with the federated Bayesian optimization method.

Reviewer comment (Theory):
Is it possible to consider more complex interactions between agents? It might be a too strong assumption that the functions of individual agents $(f_1,g_1),\ldots, (f_N, g_N)$ come from independent Gaussian processes. Is it possible to study the setting where $f=(f_1,\ldots, f_N)$ and $g=(g_1,\ldots,g_N)$ follow a joint Gaussian process?

Author response:
Thanks for the great suggestion. Our current setting is more like a frequentist setting, where the unknown objective/constraint is only assumed to be smooth in some sense. Independence of the functions is not assumed. To incorporate the correlated functions setting algorithmically, we need to combine the multi-output Gaussian processes with the primal-dual algorithm. Indeed, one direction to explore is combining our method with the federated Bayesian optimization method.

Reviewer comment (Novelty):
There is a related line of work on federated Bayesian optimization ([1,2] below) and federated kernelized bandits [3,4] which should be discussed (perhaps in the Introduction). In fact, the problem setting of multi-agent BO shares many similarities with federated BO/kernelized bandits, including both the first point (the agents do not share the local observations) and second point (the function evaluations are done locally) discussed in the bottom paragraph of page 1. These previous works may not diminish the novelty of the current paper though, because as far as I know, no prior federated BO/kernelized bandit papers have considered constraints.       
[1] Federated Bayesian Optimization via Thompson Sampling, 2020,      
[2] Differentially Private Federated Bayesian Optimization with Distributed Exploration, 2021,      
[3] Communication Efficient Distributed Learning for Kernelized Contextual Bandits, 2022,      
[4] Kernel-Based Federated Learning with Personalization, 2022.

Author response:
Many thanks for pointing out this line of works to us. They are indeed very relevant to this paper and we have added some discussion on them highlighted in color blue in the introduction section.

Reviewer comment (Experiments):
The paper did give some real-world scenarios with black-box constraints and known affine constraints, however, are there use cases with both types of constraints? The experiments section also only has one experiment with only known affine constraints, so I think it will be helpful to apply the method to an experiment with both types of constraints.

Author response:
Thanks for the great suggestions. Yes, there are certainly use cases with both types of constraints. For example, in demand response of smart grid, the operator aims to maximize the sum of net profit from each agent, which can be modeled as black-box functions of price, subject to the black-box constraint of the total energy consumption of all the agents below some threshold. At the same time, the operator needs to keep the prices for different agents the same or at least no significant differences~(affine constraint) due to fairness reasons. The experiments in this paper aim to separately demonstrate the effectiveness of our algorithm in dealing with these two types of constraints. More realistic experiments with both types of constraints are left as future works.

Reviewer comment (Writing):
I think the example given at the top of page 5 can be further clarified.

Author response:
Thanks for the suggestion. We have added more clarifications to this example and updated the paper accordingly.

Reviewer comment (Theory):
Theorem 1: I think some discussions regarding the differences between the two cases would be helpful. Regarding the dependency on $\sum^N_{i=1}\sum^m_{j=1}\gamma^T_{i,j}$, if we substitute in the rate of $\gamma$ for either the SE kernel and Matern kernel, I guess this term would depend linearly on $N$ and $m$? If this is the case, then for case 1 of Theorem 1, the first term in the regret bound would have a dependency of $N^2$ and $m$. Also, it is mentioned in the paragraph below Theorem 1 that "we can trade violation for smaller regret", I think some elaborations would be helpful.

Author response:
Yes. And thanks for the suggestion. We have revised the manuscript accordingly by appending more discussions in the paragraph that follows the Thm. 1.

Reviewer comment (Theory):
Remark 4: if I understand correctly, the assumption $\lim_{T\rightarrow \infty}\sum^N_{i=1}\sum^m_{j=1}\gamma^T_{i,j} / \sqrt{T} = 0$ holds for the Matern kernel only when the function is smooth?

Author response:
Yes. It indeed requires the Matern kernel to have a certain level of smoothness. Actually, to satisfy this constraint, the smoothness parameter $\nu_{i,j}$ needs to be at least $\frac{d_{i,j}}{2}$, where $d_{i,j}$ is the input dimension of the corresponding black-box function. We have revised Remark 4 accordingly in the manuscript.

Reviewer comment (Evaluation):
Figure 1: I agree that the strong violation of the proposed DMABO grows slower and slower yet the baseline of DCEI suffers from linear growth, but in this figure, the strong violations of DMABO is always larger than DCEI. It seems the relative magnitude is only reversed when the number of steps is large enough.

Author response:
Yes. Our theoretical results hold asymptotically. It is possible that in the initial steps, our method has a larger violation.