Parallel Feature Development

# Parallel Feature Development

Strategies for decomposing features into parallel work streams, establishing file ownership boundaries, avoiding conflicts, and integrating results from multiple implementer agents.

## When to Use This Skill

- Decomposing a feature for parallel implementation
- Establishing file ownership boundaries between agents
- Designing interface contracts between parallel work streams
- Choosing integration strategies (vertical slice vs horizontal layer)
- Managing branch and merge workflows for parallel development

## File Ownership Strategies

### By Directory

Assign each implementer ownership of specific directories:

```
implementer-1: src/components/auth/
implementer-2: src/api/auth/
implementer-3: tests/auth/
```

**Best for**: Well-organized codebases with clear directory boundaries.

### By Module

Assign ownership of logical modules (which may span directories):

```
implementer-1: Authentication module (login, register, logout)
implementer-2: Authorization module (roles, permissions, guards)
```

**Best for**: Feature-oriented architectures, domain-driven design.

### By Layer

Assign ownership of architectural layers:

```
implementer-1: UI layer (components, styles, layouts)
implementer-2: Business logic layer (services, validators)
implementer-3: Data layer (models, repositories, migrations)
```

**Best for**: Traditional MVC/layered architectures.

## Conflict Avoidance Rules

### The Cardinal Rule

**One owner per file.** No file should be assigned to multiple implementers.

### When Files Must Be Shared

If a file genuinely needs changes from multiple implementers:

1. **Designate a single owner** — One implementer owns the file
2. **Other implementers request changes** — Message the owner with specific change requests
3. **Owner applies changes sequentially** — Prevents merge conflicts
4. **Alternative: Extract interfaces** — Create a separate interface file that the non-owner can import without modifying

### Interface Contracts

When implementers need to coordinate at boundaries:

```typescript
// src/types/auth-contract.ts (owned by team-lead, read-only for implementers)
export interface AuthResponse {
  token: string;
  user: UserProfile;
  expiresAt: number;
}

export interface AuthService {
  login(email: string, password: string): Promise<AuthResponse>;
  register(data: RegisterData): Promise<AuthResponse>;
}
```

Both implementers import from the contract file but neither modifies it.

## Integration Patterns

### Vertical Slice

Each implementer builds a complete feature slice (UI + API + tests):

```
implementer-1: Login feature (login form + login API + login tests)
implementer-2: Register feature (register form + register API + register tests)
```

**Pros**: Each slice is independently testable, minimal integration needed.
**Cons**: May duplicate shared utilities, harder with tightly coupled features.

### Horizontal Layer

Each implementer builds one layer across all features:

```
implementer-1: All UI components (login form, register form, profile page)
implementer-2: All API endpoints (login, register, profile)
implementer-3: All tests (unit, integration, e2e)
```

**Pros**: Consistent patterns within each layer, natural specialization.
**Cons**: More integration points, layer 3 depends on layers 1 and 2.

### Hybrid

Mix vertical and horizontal based on coupling:

```
implementer-1: Login feature (vertical slice — UI + API + tests)
implementer-2: Shared auth infrastructure (horizontal — middleware, JWT utils, types)
```

**Best for**: Most real-world features with some shared infrastructure.

## Branch Management

### Single Branch Strategy

All implementers work on the same feature branch:

- Simple setup, no merge overhead
- Requires strict file ownership to avoid conflicts
- Best for: small teams (2-3), well-defined boundaries

### Multi-Branch Strategy

Each implementer works on a sub-branch:

```
feature/auth
  ├── feature/auth-login      (implementer-1)
  ├── feature/auth-register    (implementer-2)
  └── feature/auth-tests       (implementer-3)
```

- More isolation, explicit merge points
- Higher overhead, merge conflicts still possible in shared files
- Best for: larger teams (4+), complex features

## Troubleshooting

**Implementers are blocking each other waiting for shared code.**
Extract the shared piece into its own interface contract file owned by the team-lead and have implementers import from it. Neither implementer modifies the contract — they only implement against it.

**Merge conflicts appear even with clear ownership rules.**
A file was assigned to two agents, or a config/index file (e.g., `index.ts`, `__init__.py`) that auto-imports everything was modified by both. Designate one owner for all barrel/index files, or have the lead merge them at the end.

**An implementer finishes early but the integration step is blocked.**
Use a staging interface: the finished implementer writes a stub or mock of the downstream dependency so the other implementer can continue working. Replace with the real implementation at integration time.

**The feature decomposition turned out wrong mid-stream.**
Stop new work, have the lead redistribute files, and communicate the change via broadcast. Sunk cost on partially written code is acceptable — continuing with the wrong split is worse.

**Tests written by one implementer fail against code written by another.**
Interface contracts drifted: the implementer who owns the API changed a signature without notifying the test implementer. Enforce the rule that contract files require a broadcast before modification.

## Related Skills

- [team-composition-patterns](../team-composition-patterns/SKILL.md) — Choose the right team size and agent types before decomposing work
- [team-communication-protocols](../team-communication-protocols/SKILL.md) — Coordinate integration handoffs and plan approvals between implementers