Designing an application for performance should be based on the use cases and patterns of anticipated system load and behavior. Measuring performance is extremely important to guide optimization in the process. Fortunately, there are several well-known optimization patterns to tap into, such as resource pooling, data sizing, pre-fetch and pre-compute, staging, batching, and so on. As it turns out, application performance is not only a function of the use cases and patterns—the system as a whole is a continuous stochastic turn of events that can be assessed statistically and is guided by probability. Clojure is a fun language to do high-performance programming. This book prescribes many pointers and practices for performance, but there is no mantra that can solve everything. The devil is in the details. Know the idioms and patterns, experiment to see what works for your applications, and know which rules you can bend for performance.