# DBIS Core Banking System - Best Practices Guide

This document provides comprehensive best practices for developing, deploying, and maintaining the DBIS Core Banking System.

## Table of Contents

1. [Code Organization](#code-organization)
2. [Security Best Practices](#security-best-practices)
3. [Performance Best Practices](#performance-best-practices)
4. [Testing Best Practices](#testing-best-practices)
5. [Documentation Best Practices](#documentation-best-practices)
6. [Deployment Best Practices](#deployment-best-practices)

---

## Code Organization

### Directory Structure

```mermaid
graph TD
    subgraph "Recommended Structure"
        SRC[src/]
        
        CORE[core/<br/>Business Logic]
        INTEGRATION[integration/<br/>External Interfaces]
        INFRA[infrastructure/<br/>Infrastructure Services]
        SHARED[shared/<br/>Shared Utilities]
        
        CORE --> SERVICES[services/<br/>Service Layer]
        CORE --> MODELS[models/<br/>Domain Models]
        CORE --> ROUTES[routes/<br/>API Routes]
        
        INTEGRATION --> API_GW[api-gateway/]
        INTEGRATION --> ISO[iso20022/]
        INTEGRATION --> HSM[hsm/]
        
        INFRA --> MONITORING[monitoring/]
        INFRA --> ENCRYPTION[encryption/]
        INFRA --> QUANTUM[quantum/]
        
        SHARED --> UTILS[utils/]
        SHARED --> TYPES[types/]
        SHARED --> CONFIG[config/]
    end
```

### Best Practices

1. **Service Layer Pattern**
   - Keep business logic in service classes
   - Services should be stateless
   - Use dependency injection for testability

2. **Domain Models**
   - Use Prisma models for database entities
   - Create domain models for business logic
   - Separate data models from domain models

3. **Error Handling**
   - Use custom error classes
   - Provide meaningful error messages
   - Log errors with context

4. **Code Reusability**
   - Extract common functionality to shared utilities
   - Use TypeScript interfaces for contracts
   - Avoid code duplication

---

## Security Best Practices

### Authentication & Authorization

```mermaid
sequenceDiagram
    participant Client
    participant API as API Gateway
    participant Auth as Auth Service
    participant HSM
    participant Service as Business Service
    
    Client->>API: Request with JWT
    API->>Auth: Validate JWT
    Auth->>HSM: Verify Signature
    HSM-->>Auth: Signature Valid
    Auth->>Auth: Check Permissions
    Auth-->>API: Authorized
    API->>Service: Process Request
    Service-->>Client: Response
```

### Security Guidelines

**Priority: Critical**

1. **HSM Integration**
   - Use HSM for all cryptographic operations
   - Never store private keys in code or environment variables
   - Rotate keys regularly
   - Use hardware-backed key storage

2. **Zero-Trust Authentication**
   - Verify every request
   - Use request signatures
   - Implement token expiration
   - Validate timestamps to prevent replay attacks

3. **Input Validation**
   - Validate all input data
   - Use Zod for schema validation
   - Sanitize user inputs
   - Reject malformed requests

4. **Secrets Management**
   - Use environment variables for secrets
   - Never commit secrets to version control
   - Use secret management services (AWS Secrets Manager, HashiCorp Vault)
   - Rotate secrets regularly

5. **CORS Configuration**
   - Never use wildcards in production
   - Specify exact allowed origins
   - Validate origin headers
   - Use credentials only when necessary

### Data Protection

1. **Encryption**
   - Encrypt data in transit (TLS 1.3)
   - Encrypt sensitive data at rest
   - Use strong encryption algorithms
   - Implement key rotation

2. **Audit Logging**
   - Log all security events
   - Store logs in tamper-proof storage
   - Enable log analysis
   - Retain logs per regulatory requirements

---

## Performance Best Practices

### Database Optimization

```mermaid
graph LR
    subgraph "Database Optimization"
        INDEX[Proper Indexing]
        POOL[Connection Pooling]
        QUERY[Query Optimization]
        CACHE[Caching Strategy]
    end
    
    INDEX --> PERFORMANCE[Improved Performance]
    POOL --> PERFORMANCE
    QUERY --> PERFORMANCE
    CACHE --> PERFORMANCE
```

### Performance Guidelines

**Priority: High**

1. **Database Connection Management**
   - Use Prisma singleton pattern
   - Configure connection pooling
   - Monitor connection pool metrics
   - Implement connection retry logic

2. **Query Optimization**
   - Use database indexes
   - Avoid N+1 queries
   - Use select statements to limit fields
   - Implement pagination for large datasets

3. **Caching Strategy**
   - Cache frequently accessed data
   - Use Redis for distributed caching
   - Implement cache invalidation
   - Set appropriate TTL values

4. **Async Processing**
   - Use message queues for long-running tasks
   - Process notifications asynchronously
   - Implement background jobs
   - Use worker pools for CPU-intensive tasks

5. **API Response Optimization**
   - Minimize response payload size
   - Use compression (gzip)
   - Implement response caching
   - Use pagination for list endpoints

### Monitoring & Profiling

1. **Performance Metrics**
   - Track API response times
   - Monitor database query times
   - Measure cache hit rates
   - Track error rates

2. **Profiling**
   - Use APM tools (New Relic, Datadog)
   - Profile slow endpoints
   - Identify bottlenecks
   - Optimize hot paths

---

## Testing Best Practices

### Testing Pyramid

```mermaid
graph TD
    subgraph "Testing Pyramid"
        E2E[E2E Tests<br/>Few]
        INT[Integration Tests<br/>Some]
        UNIT[Unit Tests<br/>Many]
    end
    
    E2E --> INT
    INT --> UNIT
```

### Testing Guidelines

**Priority: High**

1. **Unit Testing**
   - Test individual functions and methods
   - Use mocks for external dependencies
   - Aim for high code coverage (>80%)
   - Test edge cases and error scenarios

2. **Integration Testing**
   - Test service interactions
   - Use test database
   - Test API endpoints
   - Validate data flow

3. **E2E Testing**
   - Test complete user flows
   - Use realistic test data
   - Test critical paths
   - Validate business requirements

4. **Test Data Management**
   - Use factories for test data
   - Clean up test data after tests
   - Use database transactions for isolation
   - Create reusable test fixtures

5. **Test Automation**
   - Run tests in CI/CD pipeline
   - Fail builds on test failures
   - Generate coverage reports
   - Run tests on every commit

### Test Organization

```mermaid
graph TD
    TESTS[__tests__/]
    
    TESTS --> UNIT[unit/<br/>Unit Tests]
    TESTS --> INT[integration/<br/>Integration Tests]
    TESTS --> E2E[e2e/<br/>E2E Tests]
    TESTS --> UTILS[utils/<br/>Test Utilities]
    
    UNIT --> SERVICES[services/]
    UNIT --> MIDDLEWARE[middleware/]
    
    INT --> API[api/]
    INT --> DATABASE[database/]
    
    E2E --> FLOWS[flows/]
```

---

## Documentation Best Practices

### Documentation Structure

```mermaid
graph TD
    DOCS[docs/]
    
    DOCS --> ARCH[architecture/<br/>Architecture Docs]
    DOCS --> FLOWS[flows/<br/>Flow Documentation]
    DOCS --> API[api/<br/>API Documentation]
    DOCS --> DEPLOY[deployment/<br/>Deployment Guides]
    DOCS --> ADR[adr/<br/>Architecture Decisions]
    
    ARCH --> DIAGRAMS[diagrams/<br/>Visual Diagrams]
    ARCH --> VOLUMES[volume-*/<br/>Volume Documentation]
```

### Documentation Guidelines

**Priority: Medium**

1. **Code Documentation**
   - Use JSDoc comments for functions
   - Document complex algorithms
   - Include usage examples
   - Document parameters and return values

2. **API Documentation**
   - Use OpenAPI/Swagger
   - Document all endpoints
   - Include request/response examples
   - Document error responses

3. **Architecture Documentation**
   - Document system design decisions
   - Use diagrams (Mermaid, ASCII)
   - Keep documentation up to date
   - Include recommendations

4. **Flow Documentation**
   - Document all business flows
   - Use sequence diagrams
   - Include error scenarios
   - Document prerequisites

---

## Deployment Best Practices

### Deployment Architecture

```mermaid
graph TB
    subgraph "Production Environment"
        LB[Load Balancer]
        
        subgraph "Application Tier"
            APP1[App Instance 1]
            APP2[App Instance 2]
            APP3[App Instance N]
        end
        
        subgraph "Database Tier"
            DB_PRIMARY[Primary DB]
            DB_REPLICA1[Replica 1]
            DB_REPLICA2[Replica 2]
        end
        
        subgraph "Cache Tier"
            CACHE1[Redis 1]
            CACHE2[Redis 2]
        end
        
        LB --> APP1
        LB --> APP2
        LB --> APP3
        
        APP1 --> DB_PRIMARY
        APP2 --> DB_PRIMARY
        APP3 --> DB_PRIMARY
        
        DB_PRIMARY --> DB_REPLICA1
        DB_PRIMARY --> DB_REPLICA2
        
        APP1 --> CACHE1
        APP2 --> CACHE2
    end
```

### Deployment Guidelines

**Priority: Critical**

1. **Infrastructure as Code**
   - Use Terraform or CloudFormation
   - Version control infrastructure
   - Automate provisioning
   - Test infrastructure changes

2. **CI/CD Pipeline**
   - Automate builds and tests
   - Use blue-green deployments
   - Implement rollback procedures
   - Monitor deployment health

3. **Environment Management**
   - Separate dev, staging, production
   - Use environment-specific configs
   - Never use production data in dev
   - Secure environment variables

4. **Database Migrations**
   - Version control migrations
   - Test migrations in staging
   - Backup before migrations
   - Plan rollback procedures

5. **Monitoring & Alerting**
   - Monitor application health
   - Set up alerts for critical issues
   - Track key metrics
   - Use dashboards for visibility

6. **Disaster Recovery**
   - Implement automated backups
   - Test restore procedures
   - Document recovery plans
   - Maintain RTO/RPO targets

---

## Code Quality Standards

### Code Review Checklist

- [ ] Code follows style guide
- [ ] Tests are included and passing
- [ ] Documentation is updated
- [ ] Security considerations addressed
- [ ] Performance implications considered
- [ ] Error handling is comprehensive
- [ ] Logging is appropriate
- [ ] No hardcoded values
- [ ] Environment variables used correctly

### Linting & Formatting

1. **ESLint Configuration**
   - Use TypeScript ESLint rules
   - Enable strict mode
   - Fix linting errors before commit

2. **Prettier Configuration**
   - Consistent code formatting
   - Auto-format on save
   - Enforce in CI/CD

3. **Pre-commit Hooks**
   - Run linting
   - Run tests
   - Check formatting
   - Validate commits

---

## Summary

Following these best practices ensures:

- **Security**: Robust protection against threats
- **Performance**: Optimal system performance
- **Maintainability**: Easy to understand and modify
- **Reliability**: Consistent and predictable behavior
- **Scalability**: Ability to handle growth

For specific implementation details, refer to:
- [Development Guide](./development.md)
- [Deployment Guide](./deployment.md)
- [API Guide](./api-guide.md)
- [Recommendations](./RECOMMENDATIONS.md)

