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copied!<p>Architecture and architectural decisions are primarily about the "non-functional" requirements of a system; <em>pace</em> RoadWarrier, but each of the things he mentions are consequences of the architectural decisions more than independent in and of themselves. (Proof: what leads to a particular choice in any of those domains? It's always the need to satisfy some nonfunctional requirement.)</p> <p>With that in mind, it's a two part issue. First, you need to decide what NFRs are important. preferably by stating them with some specificity using a rigorous method, eg, don't just say "highly available", say "the system must be available (MTTF/(MTTF+MTTR)) 99.99 percent, with the longest single duration outage being 4 minutes."</p> <p>Second, you need to consider what views will help you to design to satisfy these requirements <em>and justify your decisions</em>. Depending on the stringency of your requirements, this may be anything from a white board block diagram to a formal simulation study.</p> <p>In a <em>business</em> domain, say in an IT system available over a web interface, you might, for example, want:</p> <ul> <li>reliability (MMTF)</li> <li>availability (MTTF/(MTTF+MTTR))</li> <li>scaleability (system must be able to add 10 pct capacity within 72 hours at X cost)</li> <li>capacity (system must sustain 1 million active users)</li> <li>throughput (system must process 100 transactions per second mean with σ=2.5 tps)</li> <li>response time (under test load user must receive full page in ≤ 2 seconds)</li> <li>security (metrics here is a topic for a while article in itself)</li> </ul> <p>You should also, if you're specifying the performance etc characteristics, describe the <em>workload</em>, ie, the size of a user's data, the arrival rate of web requests, etc.</p>

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