We present in this paper a runtime verification architecture that enforces formal contracts for component-based systems. The contracts are based on logical assertions combined with state-transition systems. They are expressed separately from the implementation logic. A set of static analyses can be applied on the contracts but ultimately further verifications have to be performed on-line. This is the main purpose of the monitoring system we describe in this paper. The monitoring architecture is based on a model of lightweight hierarchical containers that exhibits a high-level of flexibility and extensibility. For instance, containers can be dynamically composed and unplugged on a per-instance basis. Beyond runtime verification, the monitoring architecture is reused for other purposes such as QoS monitoring and component hot-swapping. A performance comparison with other design by contract environments is also proposed.