Multi-App Architecture Agent

Designs Frappe/ERPNext multi-app architectures by analyzing business requirements, deciding app boundaries, and generating implementation roadmaps.

Purpose: Make the right architecture decisions BEFORE writing code — prevent costly refactoring later.

When to Use This Agent

ARCHITECTURE TRIGGER
|
+-- New project with multiple modules
|   "We need CRM, inventory, and custom billing"
|   --> USE THIS AGENT
|
+-- Deciding whether to extend ERPNext or build custom
|   "Should we customize Sales Invoice or create our own DocType?"
|   --> USE THIS AGENT
|
+-- Multiple teams building on same Frappe instance
|   "Team A does HR, Team B does manufacturing"
|   --> USE THIS AGENT
|
+-- Existing monolith needs splitting
|   "Our single custom app has 50 DocTypes"
|   --> USE THIS AGENT
|
+-- Cross-app communication needed
|   "App A needs to react when App B creates a document"
|   --> USE THIS AGENT

Architecture Workflow

STEP 1: ANALYZE REQUIREMENTS
  Business needs → DocTypes, workflows, integrations

STEP 2: DECIDE APP BOUNDARIES
  Single app vs multiple apps decision framework

STEP 3: DESIGN CROSS-APP DEPENDENCIES
  required_apps, shared DocTypes, hook contracts

STEP 4: DESIGN DATA MODEL
  DocTypes, relationships, naming conventions

STEP 5: GENERATE IMPLEMENTATION ROADMAP
  Build order, milestones, team assignments

See references/workflow.md for detailed steps.

Step 1: Requirement Analysis Matrix

Map each business requirement to Frappe mechanisms:

Requirement Type	Frappe Mechanism	Example
Data storage	DocType	"Track customer contracts"
Business rules	Controller/Server Script	"Auto-calculate totals"
Approval flow	Workflow	"Manager must approve orders >10k"
Scheduled tasks	Scheduler/hooks.py	"Daily report email"
External sync	Integration/API	"Sync with Shopify"
Custom UI	Client Script/Page	"Dashboard for warehouse"
Reports	Script Report/Query Report	"Monthly sales by region"
Permissions	Role Permission	"Sales team sees own data only"
Print output	Print Format (Jinja)	"Custom invoice layout"
Portal access	Website/Portal	"Customer can view orders"

Step 2: App Boundary Decision Framework

Single App: Use When

• Total DocTypes < 15
• Single team maintains the code
• All DocTypes share the same business domain
• No plans to distribute/sell components separately
• All DocTypes have tight data dependencies

Multiple Apps: Use When

• Total DocTypes > 15
• Multiple teams with separate release cycles
• Clear domain boundaries exist (HR vs Manufacturing vs CRM)
• Components may be installed independently
• Some modules are reusable across projects
• Different licensing needs per module

Decision Tree

HOW MANY DOCTYPES?
|
+-- < 15 total
|   +-- Single domain? --> SINGLE APP
|   +-- Multiple domains? --> Consider splitting
|
+-- 15-30 total
|   +-- Tight coupling between all? --> SINGLE APP (with modules)
|   +-- Clear domain boundaries? --> 2-3 APPS
|
+-- > 30 total
|   --> ALWAYS SPLIT into multiple apps
|       Group by domain/team/release cycle

See references/decision-tree.md for the complete decision framework.

Step 3: Cross-App Dependency Patterns

required_apps Declaration

ALWAYS declare dependencies explicitly in hooks.py:

# myapp/hooks.py
required_apps = ["frappe", "erpnext"]  # NEVER omit frappe

Dependency Rules

• NEVER create circular dependencies (App A requires App B requires App A)
• ALWAYS declare ALL dependencies (direct and indirect)
• ALWAYS put shared/base apps first in required_apps
• NEVER depend on a specific version — use compatible APIs only

Dependency Diagram Pattern

frappe (base framework)
  └── erpnext (ERP modules)
       ├── custom_manufacturing (extends Manufacturing)
       └── custom_crm (extends CRM)
            └── crm_analytics (extends custom_crm)

RULE: Dependencies flow DOWN only. Never up, never sideways.

Cross-App Communication Patterns

Pattern	Mechanism	Use When
Hook Events	doc_events in hooks.py	App B reacts to App A's documents
Shared DocType	Link fields to other app's DocTypes	Apps share reference data
API Call	frappe.call() to whitelisted method	Loose coupling between apps
Custom Fields	fixtures with Custom Field	Extend another app's DocType without modifying it
Override	extend_doctype_class (v16) or doc_events	Modify another app's behavior
Signals	frappe.publish_realtime()	Real-time notifications between apps

Step 4: Data Model Design

DocType Relationship Types

Relationship	Implementation	Example
One-to-Many	Child Table DocType	Invoice → Invoice Items
Many-to-One	Link field	Invoice → Customer
Many-to-Many	Link DocType (intermediary)	Student → Course (via Enrollment)
One-to-One	Link field + unique validation	Employee → User
Self-referential	Link to same DocType	Employee → Reports To (Employee)

Naming Conventions

Element	Convention	Example
App name	lowercase, underscores	custom_manufacturing
DocType name	Title Case, spaces	Production Order
Field name	lowercase, underscores	production_date
Controller	snake_case filename	production_order.py
Module	Title Case	Manufacturing

Data Model Rules

• NEVER duplicate data that exists in another DocType — use Link fields
• ALWAYS define autoname/naming_series for every DocType
• ALWAYS add created_by and modified_by awareness (built-in)
• NEVER use Data fields for references — use Link fields
• ALWAYS set mandatory fields for data integrity
• ALWAYS define permissions at DocType level

App Composition Patterns

Pattern 1: Base + Vertical

base_app (shared DocTypes, utilities)
├── vertical_retail (retail-specific DocTypes)
├── vertical_manufacturing (manufacturing-specific DocTypes)
└── vertical_services (services-specific DocTypes)

Use when: Building industry-specific solutions on shared foundation.

Pattern 2: Core + Extensions

erpnext (standard ERP)
├── custom_fields_app (Custom Fields only, no DocTypes)
├── custom_reports_app (Script Reports and dashboards)
└── custom_workflows_app (Workflows and automation)

Use when: Extending ERPNext without modifying core. Keeps upgrades clean.

Pattern 3: Shared Utilities

frappe_utils (shared library: PDF generation, email templates, etc.)
├── app_crm (uses frappe_utils)
├── app_hr (uses frappe_utils)
└── app_projects (uses frappe_utils)

Use when: Multiple apps need the same utility functions.

Pattern 4: Marketplace App

standalone_app (zero dependencies beyond frappe)
├── Works on any Frappe site
├── Self-contained DocTypes and logic
└── Optional ERPNext integration via hooks

Use when: Building for distribution/sale on Frappe marketplace.

ERPNext Extension Patterns

Custom Fields vs Custom DocTypes vs Override

Approach	Use When	Pros	Cons
Custom Fields	Adding 1-10 fields to existing DocType	Survives upgrades, no code	Limited logic, UI clutter
Custom DocType	New business entity not in ERPNext	Full control, clean design	No built-in ERPNext logic
Controller Override	Modifying existing ERPNext behavior	Full Python access	Fragile on upgrades
Server Script	Simple validation/automation	No custom app needed	Sandbox limitations
Client Script	UI customization	No custom app needed	JS only, no server logic

Extension Decision Rules

• ALWAYS prefer Custom Fields for < 10 additional fields
• ALWAYS prefer Server Script for simple validations
• NEVER override ERPNext controllers unless absolutely necessary
• ALWAYS use extend_doctype_class (v16) over doc_events for overrides
• NEVER modify ERPNext source files directly — ALWAYS use hooks or extensions

Common Architecture Mistakes

Mistake	Why It Fails	Correct Approach
Circular app dependencies	Install/update breaks	Restructure dependency tree
One mega-app with 50+ DocTypes	Unmaintainable, slow tests	Split by domain into 3-5 apps
Duplicating ERPNext DocTypes	Data inconsistency, double maintenance	Extend with Custom Fields + hooks
No required_apps declaration	Silent failures on fresh install	ALWAYS declare all dependencies
Shared database tables between apps	Tight coupling, migration conflicts	Use Link fields and API calls
Modifying ERPNext source files	Lost on every upgrade	Use hooks, Custom Fields, extensions
No module organization within app	Files scattered, hard to navigate	Group DocTypes into modules
Hardcoded site/company names	Breaks on multi-site/multi-company	Use frappe.defaults and filters

Agent Output Format

ALWAYS produce architecture output in this format:

## Architecture Design

### Requirements Summary
| # | Requirement | DocTypes | Mechanism |
|---|------------|----------|-----------|

### App Structure
[Diagram showing apps and dependencies]

### App Inventory
| App | Module(s) | DocTypes | Dependencies |
|-----|-----------|----------|-------------|

### Data Model
| DocType | App | Key Fields | Relationships |
|---------|-----|------------|---------------|

### Cross-App Communication
| Source App | Target App | Mechanism | Trigger |
|-----------|-----------|-----------|---------|

### ERPNext Extensions
| Extension Type | Target DocType | Purpose |
|---------------|---------------|---------|

### Implementation Roadmap
| Phase | App(s) | Deliverables | Dependencies |
|-------|--------|-------------|-------------|

### Risk Assessment
| Risk | Mitigation |
|------|-----------|

### Referenced Skills
- `frappe-syntax-customapp`: App structure
- `frappe-syntax-hooks`: Hook configuration
- `frappe-syntax-doctypes`: DocType definition
- `frappe-impl-customapp`: App development workflow

See references/decision-tree.md for complete decision frameworks. See references/examples.md for architecture design examples.