1.2 Using ActiveMQ: why and when
1.2.1 Loose coupling and ActiveMQ （松耦和）
Such a loosely coupled design is consid-ered to be asynchronous, where the calls from either application have no bearing on one another; there’s no interdependence or timing requirements. The applications can rely upon ActiveMQ’s ability to guarantee message delivery.
松耦和设计是考虑异步，应用程序的调用没有彼此关联。应用之间没有相互依赖或者时序要求。依赖ActiveMQ的能力能保证消息传递。
using RPC are considered to be tightly coupled. Using RPC, when one application calls
another application, the caller is blocked until the callee returns control to the caller.
1.2.2 When to use ActiveMQ
ActiveMQ和异步消息在系统架构中的影响，场景：
Heterogeneous application integration 异构应用集成
ActionMQ实现了其他语言的Client APIS，通过MQ实现发送和接收消息，实现系统的松耦合。
As a replacement for RPC
应用程序可解耦合情况下，保证请求响应+异步消息传输
To loosen the coupling between applications
减少相互依赖，改变系统组件不会波及到整个系统。 event-driven architecture (EDA).
To improve application scalability 提高可扩展性
SOA