Computer System Design Principles
- Page ID
- 58700
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)Throughout the text, the description of a design principle presents its name in a bold-faced 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
- Have 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