New generation distributed software systems are expected to be able to meet changing user needs and to manage the variability of the open environment they operate in. This motivates current research on developing adaptive software applications, which recognizes a prominent role to middleware. In this paper, we discuss requirements for an agent-based middleware that enables adaptation. We illustrate these requirements as a result of modelling an application scenario that will be supported by our middleware, using an agent-oriented methodology. Middleware is conceived as web of agents, to overcome the challenge of runtime adaptation in an open environment.