Developing large multi-agent systems is a complex task involving the processes of the requirement, architecture, design and implementation of these systems. In particular, the architectural design is critical to cope with the increasing size and complexity of these systems. The multi-agent system that accesses a central repository is typically based on the Blackboard architectural pattern. However, the control strategies of the agents are inherently complex. In this paper, we present an eventbased control strategy to manage the access of central repository by multi-agents. In this approach, the Blackboard pattern is composed with the Implicit Invocation pattern to separate the control policies from the control component so that the architecture is ease of evolution, reuses and changes. Keywords Multi-agent systems, architectural pattern, implicit invocation, blackboard, software architecture.