Ismel Brito - Academia.edu (original) (raw)

Papers by Ismel Brito

Autonomous Agents & Multiagent Systems/Agent Theories, Architectures, and Languages, 2010

DPOP is an algorithm for distributed constraint optimization which has, as main drawback, the exp... more DPOP is an algorithm for distributed constraint optimization which has, as main drawback, the exponential size of some of its messages. Recently, some algorithms for distributed cluster tree elimination have been proposed. They also suffer from exponential size messages. However, using the strategy of cost function filtering, in practice these algorithms obtain important reductions in maximum message size and total communication cost. In this paper, we explain the relation between DPOP and these algorithms, and show how cost function filtering can be combined with DPOP. We present experimental evidence of the benefits of this new approach.

Catalonian Conference on AI, 2008

Meetings are an important vehicle for human communication. The Meeting Scheduling problem (MS) is... more Meetings are an important vehicle for human communication. The Meeting Scheduling problem (MS) is a decision-making process affecting several people, in which it is necessary to decide when and where several meetings could be scheduled. MS is a naturally distributed problem which has a clear motivation to be tried using distributed techniques: people may desire to preserve the already planned meetings in their personal calendars during resolution. In this paper, we evaluate three distributed algorithms for MS according to efficiency and privacy loss. Two of these algorithms view MS as a Distributed Constraint Satisfaction problem.

The Stable Marriage Problem is a combinatorial problem which can be solved by a centralized algor... more The Stable Marriage Problem is a combinatorial problem which can be solved by a centralized algorithm in polynomial time. This requires to make public lists of preferences which agents would like to keep private. With this aim, we deflne the distributed version of this prob- lem, and we provide a constraint-based approach that solves it keeping privacy. We give empirical

Not every research paper in DisCSP evaluates algorithms in the same way. Motivated by this fact, ... more Not every research paper in DisCSP evaluates algorithms in the same way. Motivated by this fact, we revise some elements of the area of distributed algorithms as well as distributed constraints, which can help to develop a well- founded methodology for evaluation of DisCSP algorithms. Although prelimi- nary, we suggest a number of points which should be considered in such method- ology.

Lecture Notes in Computer Science, 2006

We consider the Stable Marriage Problem and the Stable Roommates Problem, two types of the genera... more We consider the Stable Marriage Problem and the Stable Roommates Problem, two types of the general class of Stable Matching Problems, in presence of ties and incomplete preference lists. They can be solved by centralized algorithms, but this requires to make public the preference lists of members, something that members would prefer to avoid for privacy reasons. This motivates a distributed formulation of these problems to keep privacy. We propose a distributed constraint approach that solves all the considered problems, keeping privacy.

Lecture Notes in Computer Science, 2005

We consider the Stable Marriage Problem and the Stable Roommates Problem, two well-known types of... more We consider the Stable Marriage Problem and the Stable Roommates Problem, two well-known types of the general class of Stable Matching Problems. They are combinatorial problems which can be solved by centralized algorithms in polynomial time. This requires to make public lists of preferences which agents would like to keep private. With this aim, we define the distributed version of these problems, and we provide a constraint-based approach that solves them keeping privacy. We give empirical results on the proposed approach.

Lecture Notes in Computer Science, 2008

ABT is the reference algorithm for asynchronous distributed constraint satisfaction. When searchi... more ABT is the reference algorithm for asynchronous distributed constraint satisfaction. When searching, ABT produces nogoods as justifications of deleted values. When one of such nogoods has an empty left-hand side, the considered value is eliminated unconditionally, once and for all. This value deletion can be propagated using standard arc consistency techniques, producing new deletions in the domains of other variables. This causes substantial reductions in the search effort required to solve a class of problems. We also extend this idea to the propagation of conditional deletions, something already proposed in the past. We provide experimental results that show the benefits of the proposed approach, especially considering communication cost.

Lecture Notes in Computer Science, 2004

There is some debate about the kind of algorithms that are most suitable to solve DisCSP. Synchro... more There is some debate about the kind of algorithms that are most suitable to solve DisCSP. Synchronous algorithms exchange updated information with a low degree of parallelism. Asynchronous algorithms use less updated information with a higher parallelism. Hybrid algorithms combine both features. Lately, there is some evidence that synchronous algorithms could be more efficient than asynchronous ones for one problem class. In this paper, we present some improvements on existing synchronous and asynchronous algorithms, as well as a new hybrid algorithm. We provide an empirical investigation of these algorithms on n-queens and binary random DisCSP.

Lecture Notes in Computer Science, 2008

Asynchronous Backtracking (ABT) is a reference algorithm for Distributed CSP (DisCSP). In ABT, ag... more Asynchronous Backtracking (ABT) is a reference algorithm for Distributed CSP (DisCSP). In ABT, agents assign values to their variables and exchange messages asynchronously and concurrently. When an ABT agent sends a backtracking message, it continues working without waiting for an answer. In this paper, we describe a case showing that this strategy may cause some inefficiency. To overcome this, we

Lecture Notes in Computer Science, 2007

DisFC is an ABT-like algorithm that, instead of sending the value taken by the high priority agen... more DisFC is an ABT-like algorithm that, instead of sending the value taken by the high priority agent, it sends the domain of the low priority agent that is compatible with that value. With this strategy, plus the use of sequence numbers, some privacy level is achieved. In particular, each agent knows its value in the solution, but ignores the values of the others. However, the idea of sending the whole compatible domain each time an agent changes its value may cause a privacy loss on shared constraints that was initially overlooked. To solve this issue, we propose DisFC lies , an algorithm that works like DisFC but it may lie about the compatible domains of other agents. It requires a single extra condition: if an agent sends a lie, it has to tell the truth in finite time afterwards. We prove that the algorithm is sound, complete and terminates. We provide experimental results on the increment in privacy achieved, at the extra cost of more search.

Autonomous Agents & Multiagent Systems/Agent Theories, Architectures, and Languages, 2009

In this paper we present an application build on top of electronic institutions in order to creat... more In this paper we present an application build on top of electronic institutions in order to create a remote experimental platform for human and virtual agents. Our software objectives are twofold: (1) provide experiment designers with a tool to design an experiment where human and virtual agents will interact, and, (2) provide experimental subjects with a friendly interface to interact with virtual agents through an electronic institution.

In the last years, the AI community has shown an increasing interest in distributed problem solvi... more In the last years, the AI community has shown an increasing interest in distributed problem solving. In the scope of distributed constraint reasoning, several asynchronous backtracking procedures have been proposed for nding solutions in a constraint network distributed among several computers. They dier in the way they store failing combinations of values (nogoods), and in the way they check the

Not every research paper in DisCSP evaluates algorithms in the same way. Motivated by this fact, ... more Not every research paper in DisCSP evaluates algorithms in the same way. Motivated by this fact, we revise some elements of the area of distributed algorithms as well as distributed constraints, which can help to develop a well- founded methodology for evaluation of DisCSP algorithms. Although prelimi- nary, we suggest a number of points which should be considered in such

Cluster and mini-cluster tree elimination are well-known solving meth- ods for constrained optimi... more Cluster and mini-cluster tree elimination are well-known solving meth- ods for constrained optimization problems, developed for the central- ized case. These methods, based on cost function combination, can be easily reformulated as synchronous algorithms to solve the distributed versions of the above mentioned problems. During solving they ex- change a linear number of messages, but these messages could be of exponential size, which is their main drawback that often limits their practical application. Filtering is a general technique to decrease the size of cost function combination when using upper and lower bounds. We combine this technique with the previous algorithms, producing a signicative decrement in message size and improving their practi- cal memory usage. As result, the improved algorithm is able to solve larger problems, keeping under control memory consumption.

Principles and Practice of Constraint Programming, 2003

A reason to distribute constraint satisfaction is privacy: agents may not want to share their val... more A reason to distribute constraint satisfaction is privacy: agents may not want to share their values, and they may wish to keep constraints as private as possible. In this paper, we present the Distributed Forward Checking algorithm, a natural successor of Asynchronous Backtracking, where some privacy is achieved on agent values. Regarding constraints, we introduce the Partially Known Constraints model,

Constraints, 2009

Distributed constraint satisfaction problems (DisCSPs) are composed of agents connected by constr... more Distributed constraint satisfaction problems (DisCSPs) are composed of agents connected by constraints. The standard model for DisCSP search algorithms uses messages containing assignments of agents. It assumes that constraints are checked by one of the two agents involved in a binary constraint, hence the constraint is fully known to both agents. This paper presents a new DisCSP model in which constraints are kept private and are only partially known to agents. In addition, value assignments can also be kept private to agents and not be circulated in messages. Two versions of a new asynchronous backtracking algorithm that work with partially known constraints (PKC) are presented. One is a two-phase asynchronous backtracking algorithm and the other uses only a single phase.

Artificial Intelligence, 2005

Following the pioneer work of Yokoo and colleagues on the ABT (asynchronous backtracking) algorit... more Following the pioneer work of Yokoo and colleagues on the ABT (asynchronous backtracking) algorithm, several ABT-based procedures have been proposed for solving distributed constraint networks. They differ in the way they store nogoods, but they all use additional communication links between unconnected agents to detect obsolete information. In this paper, we propose a new asynchronous backtracking algorithm which does not need to add links between initially unconnected agents. To make the description simpler and to facilitate the comparisons between algorithms, we present a unifying framework from which the new algorithm we propose, as well as existing ones, are derived. We provide an experimental evaluation of these algorithms. * The work of Ismel Brito and Pedro Meseguer is supported by the Spanish project REPLI: TIC-2002-04470-C03.

Applied Artificial Intelligence, 2012

The execution of process models is usually presented through a graphical user interface, especial... more The execution of process models is usually presented through a graphical user interface, especially when users' input is required. Existing mechanisms, such as Electronic Institutions (EIs), provide means to easily specify and automatically execute process models. However, every time the specification is modified, the graphical user interface (GUI) needed during the execution stage should be manually modified accordingly. This paper proposes a system that helps maintain such GUIs in an efficient and automated manner. We present and test Charms, a system built on top of EIs that allows the automatic creation and update of GUIs based on the provided process model specification.

Autonomous Agents & Multiagent Systems/Agent Theories, Architectures, and Languages, 2010

DPOP is an algorithm for distributed constraint optimization which has, as main drawback, the exp... more DPOP is an algorithm for distributed constraint optimization which has, as main drawback, the exponential size of some of its messages. Recently, some algorithms for distributed cluster tree elimination have been proposed. They also suffer from exponential size messages. However, using the strategy of cost function filtering, in practice these algorithms obtain important reductions in maximum message size and total communication cost. In this paper, we explain the relation between DPOP and these algorithms, and show how cost function filtering can be combined with DPOP. We present experimental evidence of the benefits of this new approach.

Catalonian Conference on AI, 2008

Meetings are an important vehicle for human communication. The Meeting Scheduling problem (MS) is... more Meetings are an important vehicle for human communication. The Meeting Scheduling problem (MS) is a decision-making process affecting several people, in which it is necessary to decide when and where several meetings could be scheduled. MS is a naturally distributed problem which has a clear motivation to be tried using distributed techniques: people may desire to preserve the already planned meetings in their personal calendars during resolution. In this paper, we evaluate three distributed algorithms for MS according to efficiency and privacy loss. Two of these algorithms view MS as a Distributed Constraint Satisfaction problem.

The Stable Marriage Problem is a combinatorial problem which can be solved by a centralized algor... more The Stable Marriage Problem is a combinatorial problem which can be solved by a centralized algorithm in polynomial time. This requires to make public lists of preferences which agents would like to keep private. With this aim, we deflne the distributed version of this prob- lem, and we provide a constraint-based approach that solves it keeping privacy. We give empirical

Not every research paper in DisCSP evaluates algorithms in the same way. Motivated by this fact, ... more Not every research paper in DisCSP evaluates algorithms in the same way. Motivated by this fact, we revise some elements of the area of distributed algorithms as well as distributed constraints, which can help to develop a well- founded methodology for evaluation of DisCSP algorithms. Although prelimi- nary, we suggest a number of points which should be considered in such method- ology.

Lecture Notes in Computer Science, 2006

We consider the Stable Marriage Problem and the Stable Roommates Problem, two types of the genera... more We consider the Stable Marriage Problem and the Stable Roommates Problem, two types of the general class of Stable Matching Problems, in presence of ties and incomplete preference lists. They can be solved by centralized algorithms, but this requires to make public the preference lists of members, something that members would prefer to avoid for privacy reasons. This motivates a distributed formulation of these problems to keep privacy. We propose a distributed constraint approach that solves all the considered problems, keeping privacy.

Lecture Notes in Computer Science, 2005

We consider the Stable Marriage Problem and the Stable Roommates Problem, two well-known types of... more We consider the Stable Marriage Problem and the Stable Roommates Problem, two well-known types of the general class of Stable Matching Problems. They are combinatorial problems which can be solved by centralized algorithms in polynomial time. This requires to make public lists of preferences which agents would like to keep private. With this aim, we define the distributed version of these problems, and we provide a constraint-based approach that solves them keeping privacy. We give empirical results on the proposed approach.

Lecture Notes in Computer Science, 2008

ABT is the reference algorithm for asynchronous distributed constraint satisfaction. When searchi... more ABT is the reference algorithm for asynchronous distributed constraint satisfaction. When searching, ABT produces nogoods as justifications of deleted values. When one of such nogoods has an empty left-hand side, the considered value is eliminated unconditionally, once and for all. This value deletion can be propagated using standard arc consistency techniques, producing new deletions in the domains of other variables. This causes substantial reductions in the search effort required to solve a class of problems. We also extend this idea to the propagation of conditional deletions, something already proposed in the past. We provide experimental results that show the benefits of the proposed approach, especially considering communication cost.

Lecture Notes in Computer Science, 2004

There is some debate about the kind of algorithms that are most suitable to solve DisCSP. Synchro... more There is some debate about the kind of algorithms that are most suitable to solve DisCSP. Synchronous algorithms exchange updated information with a low degree of parallelism. Asynchronous algorithms use less updated information with a higher parallelism. Hybrid algorithms combine both features. Lately, there is some evidence that synchronous algorithms could be more efficient than asynchronous ones for one problem class. In this paper, we present some improvements on existing synchronous and asynchronous algorithms, as well as a new hybrid algorithm. We provide an empirical investigation of these algorithms on n-queens and binary random DisCSP.

Lecture Notes in Computer Science, 2008

Asynchronous Backtracking (ABT) is a reference algorithm for Distributed CSP (DisCSP). In ABT, ag... more Asynchronous Backtracking (ABT) is a reference algorithm for Distributed CSP (DisCSP). In ABT, agents assign values to their variables and exchange messages asynchronously and concurrently. When an ABT agent sends a backtracking message, it continues working without waiting for an answer. In this paper, we describe a case showing that this strategy may cause some inefficiency. To overcome this, we

Lecture Notes in Computer Science, 2007

DisFC is an ABT-like algorithm that, instead of sending the value taken by the high priority agen... more DisFC is an ABT-like algorithm that, instead of sending the value taken by the high priority agent, it sends the domain of the low priority agent that is compatible with that value. With this strategy, plus the use of sequence numbers, some privacy level is achieved. In particular, each agent knows its value in the solution, but ignores the values of the others. However, the idea of sending the whole compatible domain each time an agent changes its value may cause a privacy loss on shared constraints that was initially overlooked. To solve this issue, we propose DisFC lies , an algorithm that works like DisFC but it may lie about the compatible domains of other agents. It requires a single extra condition: if an agent sends a lie, it has to tell the truth in finite time afterwards. We prove that the algorithm is sound, complete and terminates. We provide experimental results on the increment in privacy achieved, at the extra cost of more search.

Autonomous Agents & Multiagent Systems/Agent Theories, Architectures, and Languages, 2009

In this paper we present an application build on top of electronic institutions in order to creat... more In this paper we present an application build on top of electronic institutions in order to create a remote experimental platform for human and virtual agents. Our software objectives are twofold: (1) provide experiment designers with a tool to design an experiment where human and virtual agents will interact, and, (2) provide experimental subjects with a friendly interface to interact with virtual agents through an electronic institution.

In the last years, the AI community has shown an increasing interest in distributed problem solvi... more In the last years, the AI community has shown an increasing interest in distributed problem solving. In the scope of distributed constraint reasoning, several asynchronous backtracking procedures have been proposed for nding solutions in a constraint network distributed among several computers. They dier in the way they store failing combinations of values (nogoods), and in the way they check the

Not every research paper in DisCSP evaluates algorithms in the same way. Motivated by this fact, ... more Not every research paper in DisCSP evaluates algorithms in the same way. Motivated by this fact, we revise some elements of the area of distributed algorithms as well as distributed constraints, which can help to develop a well- founded methodology for evaluation of DisCSP algorithms. Although prelimi- nary, we suggest a number of points which should be considered in such

Cluster and mini-cluster tree elimination are well-known solving meth- ods for constrained optimi... more Cluster and mini-cluster tree elimination are well-known solving meth- ods for constrained optimization problems, developed for the central- ized case. These methods, based on cost function combination, can be easily reformulated as synchronous algorithms to solve the distributed versions of the above mentioned problems. During solving they ex- change a linear number of messages, but these messages could be of exponential size, which is their main drawback that often limits their practical application. Filtering is a general technique to decrease the size of cost function combination when using upper and lower bounds. We combine this technique with the previous algorithms, producing a signicative decrement in message size and improving their practi- cal memory usage. As result, the improved algorithm is able to solve larger problems, keeping under control memory consumption.

Principles and Practice of Constraint Programming, 2003

A reason to distribute constraint satisfaction is privacy: agents may not want to share their val... more A reason to distribute constraint satisfaction is privacy: agents may not want to share their values, and they may wish to keep constraints as private as possible. In this paper, we present the Distributed Forward Checking algorithm, a natural successor of Asynchronous Backtracking, where some privacy is achieved on agent values. Regarding constraints, we introduce the Partially Known Constraints model,

Constraints, 2009

Distributed constraint satisfaction problems (DisCSPs) are composed of agents connected by constr... more Distributed constraint satisfaction problems (DisCSPs) are composed of agents connected by constraints. The standard model for DisCSP search algorithms uses messages containing assignments of agents. It assumes that constraints are checked by one of the two agents involved in a binary constraint, hence the constraint is fully known to both agents. This paper presents a new DisCSP model in which constraints are kept private and are only partially known to agents. In addition, value assignments can also be kept private to agents and not be circulated in messages. Two versions of a new asynchronous backtracking algorithm that work with partially known constraints (PKC) are presented. One is a two-phase asynchronous backtracking algorithm and the other uses only a single phase.

Artificial Intelligence, 2005

Following the pioneer work of Yokoo and colleagues on the ABT (asynchronous backtracking) algorit... more Following the pioneer work of Yokoo and colleagues on the ABT (asynchronous backtracking) algorithm, several ABT-based procedures have been proposed for solving distributed constraint networks. They differ in the way they store nogoods, but they all use additional communication links between unconnected agents to detect obsolete information. In this paper, we propose a new asynchronous backtracking algorithm which does not need to add links between initially unconnected agents. To make the description simpler and to facilitate the comparisons between algorithms, we present a unifying framework from which the new algorithm we propose, as well as existing ones, are derived. We provide an experimental evaluation of these algorithms. * The work of Ismel Brito and Pedro Meseguer is supported by the Spanish project REPLI: TIC-2002-04470-C03.

Applied Artificial Intelligence, 2012

The execution of process models is usually presented through a graphical user interface, especial... more The execution of process models is usually presented through a graphical user interface, especially when users' input is required. Existing mechanisms, such as Electronic Institutions (EIs), provide means to easily specify and automatically execute process models. However, every time the specification is modified, the graphical user interface (GUI) needed during the execution stage should be manually modified accordingly. This paper proposes a system that helps maintain such GUIs in an efficient and automated manner. We present and test Charms, a system built on top of EIs that allows the automatic creation and update of GUIs based on the provided process model specification.