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Complexity is the single greatest constraint on software development over a system's lifetime. The job of design is to manage complexity, and the most powerful lever is modular design: split the system into modules whose interfaces are far simpler than their implementations, s...

Software delivery performance can be measured, it predicts organizational performance, and it is driven by specific capabilities a team can adopt. The headline result, established through survey research and statistical analysis rather than anecdote, is that speed and stabilit...

An affordance is a relationship between an object and an agent: the actions the object makes possible given that agent's capabilities. A chair affords sitting; a handle affords pulling. Affordances exist whether or not anyone perceives them. Designing the right affordances mak...

An initial value, even an arbitrary or irrelevant one, pulls subsequent estimates toward it. People adjust away from the anchor but insufficiently, so it contaminates the final judgment. Anchors shape estimates without awareness, which makes negotiation, pricing, and forecasti...

The structure of software systems: boundaries, components, and the tradeoffs between competing options. Architecture characteristics and the reasoning behind structural decisions.

The system qualities an architecture must support beyond its domain functionality, the " ilities": performance, scalability, availability, security, deployability, testability, and so on. They are the success criteria of an architecture, separate from what the system does. An...

A short, durable document for each significant architecture decision that records its context, the decision made, the alternatives considered, and the consequences. ADRs are usually kept in version control alongside the code and are immutable: a superseded decision gets a new...

Automated testing is the practice of encoding expected behavior as checks a machine runs, so that whether the system still works is answered by running the suite rather than by inspection or hope. Self testing code, where a fast, trustworthy suite accompanies the code, is the...

Judging the frequency or probability of something by how easily examples come to mind. Vivid, recent, or emotionally charged instances feel more common than they actually are. Ease of recall is driven by salience and exposure, not by real frequency, so availability systematica...

A flow control mechanism in which a consumer that cannot keep up signals the producer to slow down, rather than dropping data or exhausting memory. It bounds work in progress by propagating "not so fast" back up the pipeline. When producers outpace consumers, something has to...

A goal seeking, stabilizing loop that moves a stock toward a target by counteracting deviations. The larger the gap from the goal, the stronger the corrective flow. Balancing loops are the source of stability, regulation, and self correction without central control. They are a...

Ignoring the underlying frequency of a category in favor of specific, individuating detail (often a stereotype or vivid story) when judging probability. The representativeness of the detail overrides the statistics. Plausible detail feels more diagnostic than dull base rates,...

The amount of work moved through a process in one increment. Small batches mean changes are made, integrated, and released in frequent tiny pieces rather than large infrequent ones. Reducing batch size is a core lean principle: it shortens cycle time, smooths flow, and surface...

A blameless postmortem is a written, after the fact analysis of an incident that assumes everyone acted with good intent given what they knew at the time, and that looks for the systemic and latent causes that let the failure happen rather than for a person to fault. The outpu...

The resource whose capacity is less than the demand placed on it. A bottleneck, the constraint, sets the maximum throughput of the entire system: everything downstream waits on it and everything upstream piles up before it. A system's output is governed by its single binding c...

An explicit boundary within which a single domain model applies and its terms have one precise meaning. Outside the boundary the same word may mean something different, modeled differently. Bounded contexts make the limits of a model deliberate: each context owns its model and...

People make reasonable decisions based on the limited, often delayed information available from their position in a system, not on perfect global knowledge. Rational local choices can still add up to poor system wide outcomes. It explains why competent, well intentioned actors...

Software rots the way a building does: one visible, unrepaired flaw, a "broken window", signals that no one cares, and invites more neglect until decay accelerates. The remedy is to fix small problems promptly rather than letting them accumulate into entropy. Quality erodes gr...

Adding manpower to a late software project makes it later. Effort (measured in person months) and progress (measured in time) are not interchangeable, because tasks have sequential constraints and new people add training and communication overhead before they add output. It de...

Every change to a complex system, including changes that fix problems or add safety, creates new and often unforeseen paths to failure. Improvement and new risk arrive together. Changes alter the web of interactions and consume the margin that absorbed past variation, so the s...

The design of the environment in which a decision is presented: the order and number of options, the defaults, the wording, and the feedback. The person who arranges these is a "choice architect." There is no neutral arrangement; every design influences what people choose. Bec...

A code smell is a surface indication in code that usually corresponds to a deeper design problem. It is a heuristic, not a rule: a smell tells you where to look and that something may be worth improving, not what is wrong or how to fix it. Named smells (duplicated code, long f...

The feeling of mental fluency when information is easy to process, from familiarity, clarity, repetition, or good legibility. System 1 reads ease as a signal of truth, safety, and liking; strain signals effort and doubt. Things that are easy to process feel more true, more lik...

The total mental effort required to work in a given domain or system. A team has a finite cognitive capacity, and the amount of software and responsibility assigned to it should fit within that capacity. When a team's cognitive load exceeds its capacity, flow slows, quality dr...