modular-design-principles

# Modular Design Principles

Use this skill when reasoning about **structure and boundaries** in any codebase. It intentionally avoids framework names, folder conventions, and tooling — map principles to your stack locally.

## What to load

| Task | Where |
|------|--------|
| Principles table + violations + workflows (this file) | `SKILL.md` |
| Per-principle definition, agent rules, abstract examples | `references/principles.md` |

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## Layered mental model

- **Composition roots** (applications, hosts, runners): wire modules together; keep orchestration thin.
- **Modules / bounded contexts**: cohesive units of behavior and data ownership; each should be understandable and testable on its own.
- **Shared kernels** (use sparingly): only stable, truly cross-cutting concepts; resist turning them into a grab-bag of “everything everyone needs.”

How you physically lay this out (mono repo, multi repo, packages, libraries) is a **delivery choice**, not the definition of modularity. The principles below still apply.

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## The ten principles

| # | Principle | Intent |
|---|-----------|--------|
| 1 | **Well-defined boundaries** | A small, stable **public surface**; everything else is internal. Consumers depend on contracts, not internals. |
| 2 | **Composability** | Modules can be used alone or combined without special knowledge of each other’s internals. |
| 3 | **Independence** | No hidden shared mutable state across boundaries; each module should be testable in isolation (with fakes or test doubles at the edges). |
| 4 | **Individual scale** | Resources (compute, storage, rate limits, batch size) can be tuned **per module** where it matters, without rewriting others. |
| 5 | **Explicit communication** | Cross-module interaction uses **documented contracts** (APIs, events, messages, shared types) — not incidental coupling. |
| 6 | **Replaceability** | Dependencies on other modules are expressed through **interfaces or protocols** so implementations can change. |
| 7 | **Deployment independence** | Modules do not assume they share a process, host, or release cadence unless that is an explicit architectural decision. |
| 8 | **State isolation** | Each module **owns** its persistent state and naming; no silent sharing of the same logical data store or ambiguous global names across boundaries. |
| 9 | **Observability** | Each module can be diagnosed on its own: logs, metrics, traces, health — attributable to the unit that emitted them. |
| 10 | **Fail independence** | Failures are **contained** (timeouts, bulkheads, circuit breaking, idempotency) so one module’s outage does not blindly cascade. |

**Principle 8** is often the hardest: ambiguous ownership of data or names is a frequent source of “works until it doesn’t” integration bugs.

For **depth** (rules for agents + abstract examples per principle), load `references/principles.md`.

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## Typical violations (stated abstractly)

1. **Colliding concepts** — the same name or schema for different things in different modules, or duplicate “global” definitions that diverge over time.
2. **Reach-through persistence** — one module reading or writing another module’s tables, buckets, or documents **without** going through an agreed contract.
3. **Centralized data ownership** — a single persistence layer that registers and exposes **all** stores for **all** modules, encouraging hidden coupling.
4. **Logic at the edge** — business rules in transport adapters (HTTP handlers, UI, CLI) instead of domain/application code.
5. **Edge talking to storage directly** — adapters depending on low-level persistence APIs instead of use cases or application services.
6. **Unscoped transactions** — writes that span boundaries without clear transaction ownership and failure semantics.
7. **Leaky exports** — repositories, internal services, or implementation types exposed as the module’s public API.
8. **Facades that aren’t thin** — “public” entry points that embed querying, mapping, or policy instead of delegating to the right layer inside the module.

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## Creating a bounded context (workflow)

Use when introducing a **new** cohesive area of the system (greenfield module or extracted domain).

1. **Scope and language** — Name the context; list core nouns/verbs (**ubiquitous language**). Reject vague names that collide with other contexts.
2. **Responsibilities** — What decisions happen **only** here? What is explicitly *out* of scope?
3. **State ownership** — Which facts are **authoritative** in this context? Where are they stored conceptually (even if storage tech is undecided)?
4. **Public contract** — Operations and/or events other contexts may use. Version or evolve this contract intentionally.
5. **Integrations** — For each neighbor: sync call, async message, shared read model, or batch sync? Document **consistency** (immediate, eventual) and **failure** behavior.
6. **Invariants and lifecycles** — What must always be true inside this boundary? What starts/completes a lifecycle?
7. **Isolation check** — Can you test core behavior **without** spinning up unrelated contexts (fakes at ports)?
8. **Observability** — How will you trace a request or job through **this** context with clear identifiers?

**Cross-module interaction** (while designing): prefer the **minimal** contract; define **timeouts**, **retries**, **idempotency** for async; avoid “temporary” direct store access as a shortcut.

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## When to split or merge

**Default:** **fewer boundaries** until real pain appears — “flat is often better” than premature fragmentation. Splitting adds coordination, versioning, and operational cost.

### Six-criteria test (favor split when several are true)

| # | Criterion | Question |
|---|-----------|----------|
| 1 | **Language** | Do the sub-areas use **different vocabulary** or conflicting definitions of the same word? |
| 2 | **Rate of change** | Do parts **change on different cadences** or for unrelated reasons (most edits touch one side)? |
| 3 |