STATELESS VS. COUPLED SYSTEMS
When systems work together, they either want to know the state of the other system (whether it??™s online
or offline), need to know, or couldn??™t care less. In large-scale system design, the terms are loosely coupled
vs. tightly coupled. Nobody talks about uncoupled systems, because that implies that they aren??™t a
part of a distributed application framework??”uncoupled systems are stand-alone systems.
Log shipping is almost an uncoupled system, but one piece of information is crucial to it functioning
properly (teaser: more on that coming up). Clustering, on the other hand, which I??™ll examine in the
next chapter, is a textbook case of a tightly coupled system. Other mitigation techniques in this book fall
somewhere in between.
Why are state and tightly coupled vs. loosely coupled systems important when it comes to disaster
recovery? When designing any type of distributed system, it is critical to understand when a tightly
coupled system is appropriate and when a loosely coupled system is advantageous. Tightly coupled
systems require that you consider different issues in disaster recovery planning than do loosely coupled
systems.
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