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:

// 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