Logic Design Principles

Throughout the text, the description of a design principle presents its name in a boldfaced display, and each place that the principle is used highlights it in underlined italics.

Design principles applicable to many areas of computer systems

• Adopt sweeping simplifications. So you can see what you are doing.
• Avoid excessive generality. If it is good for everything, it is good for nothing.
• Avoid rarely used components. Deterioration and corruption accumulate unnoticed—until the next use.
• Be explicit. Get all of the assumptions out on the table.
• Decouple modules with indirection. Indirection supports replaceability.
• Design for iteration. You won't get it right the first time, so make it easy to change.
• End-to-end argument. The application knows best.
• Escalating complexity principle. Adding a feature increases complexity out of proportion.
• Incommensurate scaling rule. Changing a parameter by a factor of ten requires a new design.
• Keep digging principle. Complex systems fail for complex reasons.
• Law of diminishing returns. The more one improves some measure of goodness, the more effort the next improvement will require.
• Open design principle. Let anyone comment on the design; you need all the help you can get.
• Principle of least astonishment. People are part of the system. Choose interfaces that match the user’s experience, expectations, and mental models.
• Robustness principle. Be tolerant of inputs, strict on outputs.
• Safety margin principle. Keep track of the distance to the edge of the cliff or you may fall over the edge.
• Unyielding foundations rule. It is easier to change a module than to change the modularity.

Design principles applicable to specific areas of computer systems

• Atomicity: Golden rule of atomicity. Never modify the only copy!
• Coordination: One-writer principle. If each variable has only one writer, coordination is simpler.
• Durability: The durability mantra. Multiple copies, widely separated and independently administered.
• Security: Minimize secrets. Because they probably won’t remain secret for long.
• Security: Complete mediation. Check every operation for authenticity, integrity, and authorization.
• Security: Fail-safe defaults. Most users won’t change them, so set defaults to do something safe.
• Security: Least privilege principle. Don’t store lunch in the safe with the jewels.
• Security: Economy of mechanism. The less there is, the more likely you will get it right.
• Security: Minimize common mechanism. Shared mechanisms provide unwanted communication paths.

Design Hints (useful but not as compelling as design principles)

• Exploit brute force
• Instead of reducing latency, hide it
• Optimize for the common case
• Separate mechanism from policy 

Source: Jerome H. Saltzer and M. Frans Kaashoek, https://ocw.mit.edu/resources/res-6-004-principles-of-computer-system-design-an-introduction-spring-2009/online-textbook/principles_open_5_0.pdf
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