product-primitives

Product Primitives

Break complex systems into fundamental primitives — deep, information-hiding building blocks with clear boundaries and simple interfaces. Think from the software creator's perspective: what are the atomic capabilities this system needs, how do they connect, and where should the boundaries be?

When to Use

One-off analysis for big-picture thinking — new applications, large features, or understanding existing system architecture. While /dev-skills:product-thinker thinks from the user's perspective, product primitives thinks from the builder's perspective, identifying where to draw module boundaries and what each piece should encapsulate.

Use Explore sub-agents to research the codebase if working in an existing project. Use browser (Chrome DevTools MCP) to walk through the product if it's running.

How to Decompose

The quality of a decomposition depends on where you draw boundaries. Use these principles to guide every boundary decision:

Deep, not shallow

Each primitive should have a simple interface hiding significant functionality. If a primitive's interface is as complex as what it does internally, it's not a useful building block — it's just indirection. A good primitive absorbs complexity so that anything composing it stays simple.

Information hiding

Each primitive should encapsulate a design decision — a piece of knowledge that nothing outside it needs to know. The data format, the algorithm, the storage mechanism, the external API quirks — these should live inside one primitive, not leak across boundaries. When you can change a primitive's internals without affecting anything else, the boundary is right.

Split by knowledge, not by time

Don't decompose by execution order ("first we ingest, then we transform, then we store"). This is temporal decomposition — it leaks information because operations that happen at different times often share the same knowledge. Instead, ask: what distinct pieces of knowledge does this system need? Each piece of knowledge becomes a primitive.

Bring together or split apart?

• Merge when two pieces share information, are always used together, or can't be understood independently
• Split when pieces are truly independent and separating them reduces the total complexity
• Test: will developers need to read both to understand either? If yes, they belong together

Pull complexity downward

When complexity is unavoidable, push it into lower-level primitives so that higher-level composition stays simple. The primitives should suffer so that the system using them doesn't have to.

Layer placement: I/O / Function / State

Before finalizing primitives, run them through the I/O / Function / State lens. Each primitive should clearly belong to one layer: a boundary primitive (I/O — network, files, time, webhooks), a logic primitive (Function — pure computation, decisions, orchestration), or a persistence primitive (State — DB, durable queues, caches that outlive a request). A primitive that mixes layers is a decomposition smell — split it. A primitive that owns one layer cleanly almost always passes the depth and information-hiding tests too. See references/io-function-state.md for the lens, heuristics, worked example, and limits.

What Makes a Good Primitive

• Deep — simple interface, powerful functionality behind it
• Encapsulating — hides a design decision; internals can change without rippling outward
• Composable — combines with other primitives through simple, obvious interfaces
• Transferable — useful in multiple workflows, not tied to one specific scenario

Red Flags

• Shallow primitive — interface is as complex as implementation, not hiding anything useful
• Information leakage — same knowledge (format, protocol, business rule) appears in multiple primitives
• Temporal decomposition — boundaries mirror execution order instead of knowledge boundaries
• Pass-through primitive — does nothing but forward to another primitive with a similar interface
• Conjoined primitives — can't understand one without understanding the other

Output Format

## System: [Name]

[1-2 sentence description of what this system does]

## Primitives

### 1. [Primitive Name]
**Purpose**: What it does
**Encapsulates**: What knowledge/decisions it hides
**Interface**: What callers need to know (inputs → outputs)

### 2. [Next Primitive]
...

## How They Fit Together

[ASCII diagram showing relationships and data flow]

## Composition Examples

[Show how 2-3 user workflows are composed from these primitives]

Example

Input: "Break down an e-commerce marketing automation system"

Output:

## System: E-commerce Marketing Automation

Automate marketing campaigns for online stores — segment customers, create campaigns,
schedule sends, and measure results.

## Primitives

### 1. Audience Segmentation
**Purpose**: Group customers by behavior, demographics, or purchase history
**Encapsulates**: Segment rule evaluation logic, membership caching, refresh scheduling
**Interface**: (segment rules, customer data) → named segments with member lists

### 2. Campaign Composition
**Purpose**: Assemble a campaign from template, audience, and schedule
**Encapsulates**: Campaign state machine (draft → scheduled → sent → completed),
validation rules, duplication logic
**Interface**: (template + segment + schedule) → ready-to-send campaign

### 3. Content Rendering
**Purpose**: Produce personalized email/SMS content from templates + product data
**Encapsulates**: Template engine, personalization token resolution, product data lookup
**Interface**: (template + recipient + products) → rendered content

### 4. Send Orchestration
**Purpose**: Execute campaign delivery respecting rate limits and timing
**Encapsulates**: Channel-specific delivery (email vs SMS), rate limiting, retry logic,
bounce handling
**Interface**: (campaign + recipients + content) → delivery receipts

### 5. Performance Measurement
**Purpose**: Track opens, clicks, conversions, revenue attribution
**Encapsulates**: Event aggregation, attribution models, metric computation
**Interface**: (delivery events + interactions + orders) → campaign metrics

## How They Fit Together

┌──────────────────┐
│    Audience       │
│  Segmentation     │──────────────────────┐
└────────┬─────────┘                       │
         │ segments                        │ segment scores
         ▼                                 │
┌──────────────────┐    ┌───────────────┐  │
│    Campaign       │───▶│   Content     │  │
│   Composition     │    │  Rendering    │  │
└────────┬─────────┘    └───────┬───────┘  │
         │ campaign              │ content  │
         ▼                       ▼          │
      ┌──────────────────────────┐          │
      │    Send Orchestration    │          │
      └────────────┬─────────────┘          │
                   │ delivery events        │
                   ▼                        │
      ┌──────────────────────────┐          │
      │ Performance Measurement  │──────────┘
      └──────────────────────────┘
        (feeds back into segmentation)

## Composition Examples

**Scheduled newsletter**: Segmentation → Composition → Rendering → Orchestration → Measurement
**Triggered welcome series**: (event) → Composition → Rendering → Orchestration
**Re-engagement**: Measurement (inactive users) → Segmentation → Composition → ...

Note: each arrow represents a simple interface — the receiving primitive doesn't need to know
how the sending primitive works internally. Orchestration doesn't know how Rendering chose
which products to feature; Measurement doesn't know how Orchestration handled rate limits.

Handoffs

• Pair with /dev-skills:product-thinker for user-perspective analysis alongside system decomposition
• Feed individual primitives into /dev-skills:shaping-work to shape them as work items