Appears in: AI magazine Volume 19, No.2 Intelligent Agents Summer 1998. Articles Multiagent Systems Katia P. Sycara Agent-based systems technology has generated lots particularly complex, large, or unpredictable, then the of excitement in recent years because of its promise only way it can reasonably be addressed is to develop as a new paradigm for conceptualizing, designing, a number of functionally specific and (nearly) modular and implementing software systems. This promise is components (agents) that are specialized at solving a particularly attractive for creating software that particular problem aspect. This decomposition allows operates in environments that are distributed and each agent to use the most appropriate paradigm for open, such as the internet Currently, the great solving its particular problem. When interdependent majority of agent-based systems consist of a single problems arise, the agents in the system must agent However as the technology matures and coordinate with one another to ensure that addresses increasingly complex applications, the interdependencies are properly managed. need for systems that consist of multiple agents that Furthermore, real problems involve distributed, communicate in a peer-to-peer fashion is becoming open systems (Hewitt 1986). An open system is one in apparent. Central to the design and effective which the structure of the system itself is capable of operation of such multiagent systems (MASs) are a dynamically changing. The characteristics of such a core set of issues and research questions that have system are that its components are nor known in been studied over the years by the distributed AI advance; can change over time; and can consist of community. In this article, I present some of the highly heterogeneous agents implemented by differ- critical notions in MASs and the research work that ent people, at different times, with different software has addressed them- I organize these notions around tools and techniques. Perhaps the best-known example the concept of problem-solving coherence, which I of a highly open software environment is the internet. believe is one of the most critical overall The internet can be viewed as a large, distributed characteristics that an MAS should exhibit information resource, with nodes on the network designed and implemented by different organizations ost researchers in AI to date have dealt with and individuals. In an open environment, information developing theories, techniques, and sources, communication links, and agents could Msystems to study and understand the appear and disappear unexpectedly. Currently, agents behavior and reasoning on the internet mostly perform information retrieval properties of a single cognitive entity. AI has matured, and filtering. The next generation of agent technology and it endeavors to attack more complex, realistic, and will perform information gathering in context and large-scale problems. Such problems are beyond the sophisticated reasoning in support of user problem- capabilities of an individual agent. The capacity of an solving tasks. These capabilities require that agents be intelligent agent is limited by its knowledge, its able to interoperate and coordinate with each other in computing resources, and its perspective. This bounded peer-to-peer interactions. In addition, these rationality (Simon 1957) is one of the underlying capabilities will allow agents to increase the problem- reasons for creating problem-solving organizations. solving scope of single agents. Such functions will The most powerful tools for handling complexity are require techniques based on negotiation or modularity and abstraction. Multiagent systems cooperation, which lie firmly in the domain of MASs (MASs) offer modularity. If a problem domain is (Jennings, Sycara, and Wooldridge 1998; 79 AI Magazine Copyright 1998, American Association for Artificial Intelligence. AIl rights reserved 0738-4602-1998/ $2.00SUMMER 1998 The autonomous and interacting with other similar agents O’Hare and Jennings 1996; Bond and Gasser 1988). that manage calendars of different users (Garrido and characteristics It is becoming increasingly clear that to be successful, Sycara 1996; Dent et al. 1992). Such agents also can of MASs are increased research resources and attention should be be customized to reflect the preferences and constraint that given to systems consisting of not one but multiple agents. of their users. Other examples include air-traffic The distributed AI (DAD community that started forming in control (Kinny et al 1992; Cammarata, McArthur, and (1) each the early 1980s and was tiny compared to mainstream, single- Steeb 1983) and multiagent bargaining for buying and agent has agent AI is rapidly increasing. The growth of the MAS field is selling goods on the internet indisputable. Fourth is to provide solutions that efficiently use incomplete Research in MASs is concerned with the study, behavior, information sources that are spatially distributed. information and construction of a collection of possibly preexisting Examples of such domains include sensor networks autonomous agents that interact with each other and their (Corkill and Lesser 1983), seismic monitoring (Mason or capabilities environments. Study of such systems goes beyond the study of and Johnson 1989), and information gathering from for solving individual intelligence to consider, in addition, problem the internet (Sycara et al. 1996). the problem solving that has social components. An MAS can be defined as Fifth is to provide solutions in situations where a loosely coupled network of problem solvers that interact to expertise is distributed. Examples of such problems and, thus, solve problems that are beyond the individual capabilities or include concurrent engineering (Lewis and Sycara has a limited knowledge of each problem solver (Durfee and Lesser 1989). 1993), health care, and manufacturing. These problem solvers, often ailed agent, are autonomous and Sixth is to enhance performance along the viewpoint; (2) can be heterogeneous in nature. dimensions of (1) computational efficiency because there is no The characteristic of MASs are that (1) each agent has concurrency of computation is exploited (as long as incomplete information or capabilities for solving the problem communication is kept minimal, for example, by system global and. thus, has a limited viewpoint; (2) there is no system transmitting high-level information and results rather control; global control; (3) data are decentralized; and (4) computation than low-level data); (2) reliability, that is, graceful is asynchronous. The motivations for the increasing interest in recovery of component failures, because agents with (3) data are MAS research include the ability of MASs to do the redundant capabilities or appropriate interagent decentralized; following: coordination are found dynamically (for example, and (4) First is to solve problems that are too large for a centralized taking up responsibilities of agents that fail); (3) agent to solve because of resource limitations or the sheer risk extensibility because the number and the capabilities of computation of having one centralized system that could be a performance agents working on a problem can be altered; (4) is bottleneck or could fail at critical times. robustness, the system’s ability to tolerate uncertainty Second is to allow for the interconnection and because suitable information is exchanged among asynchronous. interoperation of multiple existing legacy systems. To keep agents; (5) maintainability because a system composed pace with changing business of multiple components-agents is easier to maintain needs, legacy systems must periodically be because of Its modularity; (6) responsiveness because updated. Completely rewriting such software tends to be modularity can handle anomalies locally, not prohibitively expensive and is often simply impossible. propagate them to the whole system; (7) flexibility Therefore, in the short to medium term, the only way that such because agents with different abilities can adaptively legacy systems can remain useful is to incorporate them into a organize to solve the current problem; and wider cooperating agent community in which they can be (8) reuse because functionally specific agents can be exploited by other pieces of software. Incorporating legacy reused in different agent teams to solve different systems into an agent society can be done, for example, by problems. building an agent wrapper around the software to enable it to MASs are now a research reality and are rapidly interoperate with other systems (Genesereth and Ketchpel having a critical presence in many human-computer 1994). environments. My purpose in this article is not to Third is to provide solutions to problems that can naturally be provide a detailed review of the field; I leave this task regarded as a society of autonomous interacting components- to others (see, for example, Huhns and Singh [1997], agents. O’Hare and Jennings [19961, Wooldridge and For example, in meeting scheduling a scheduling agent that Jennings [1995], Chaib-draa et al. [19921, and Bond manages the calendar of its user can be regarded as and AI MAGAZINE 80 AI Magazine Copyright 1998, American Association for Artificial Intelligence. AIl rights reserved 0738-4602-1998/ $2.00SUMMER 1998 Gasser [1988] for surveys). Rather than present an in- Ensuring that an MAS exhibits coherent collective depth analysis and critique of the field, I instead briefly