# OAuth2 STS SOLID Refactoring Plan

## Overview
This document outlines refactoring opportunities to better align with SOLID principles.

## 1. Extract Grant Type Handlers (SRP + OCP)

### Current Issue
`OAuthServer::handle_token()` contains all grant type logic in one method.

### Proposed Solution
```rust
trait GrantHandler {
    fn can_handle(&self, grant_type: &str) -> bool;
    fn handle_grant(
        &self,
        params: &HashMap<String, String>,
        auth_header: Option<&str>,
        ip_address: Option<String>,
    ) -> Result<String, String>;
}

struct AuthorizationCodeGrantHandler {
    client_authenticator: Arc<dyn ClientAuthenticator>,
    token_generator: Arc<dyn TokenGenerator>,
    code_repository: Arc<dyn AuthCodeRepository>,
    token_repository: Arc<dyn TokenRepository>,
    audit_logger: Arc<dyn AuditLogger>,
}

struct RefreshTokenGrantHandler {
    // Similar dependencies
}

struct GrantHandlerRegistry {
    handlers: Vec<Arc<dyn GrantHandler>>,
}
```

### Benefits
- ✅ Easy to add new grant types (Client Credentials, Device Code, etc.)
- ✅ Each handler has single responsibility
- ✅ Testable in isolation

## 2. Create Repository Abstractions (DIP + SRP)

### Current Issue
Monolithic `Database` struct violates SRP and makes testing difficult.

### Proposed Solution
```rust
trait ClientRepository {
    fn get_client(&self, client_id: &str) -> Result<Option<OAuthClient>>;
    fn authenticate_client(&self, client_id: &str, secret: &str) -> Result<bool>;
    fn is_redirect_uri_valid(&self, client_id: &str, uri: &str) -> bool;
}

trait TokenRepository {
    fn create_access_token(&self, token: &DbAccessToken) -> Result<()>;
    fn get_access_token(&self, token_hash: &str) -> Result<Option<DbAccessToken>>;
    fn revoke_access_token(&self, token_hash: &str) -> Result<()>;
}

trait AuthCodeRepository {
    fn create_auth_code(&self, code: &DbAuthCode) -> Result<()>;
    fn get_auth_code(&self, code: &str) -> Result<Option<DbAuthCode>>;
    fn mark_auth_code_used(&self, code: &str) -> Result<()>;
}

// SQLite implementations
struct SqliteClientRepository {
    database: Arc<Mutex<Database>>,
}

struct SqliteTokenRepository {
    database: Arc<Mutex<Database>>,
}
```

### Benefits
- ✅ Easy to swap database backends (PostgreSQL, Redis, etc.)
- ✅ Better testability with mock repositories
- ✅ Clear separation of concerns

## 3. Extract Authentication Strategy (OCP + SRP)

### Current Issue
Client authentication logic scattered throughout token handlers.

### Proposed Solution
```rust
trait ClientAuthenticator {
    fn authenticate(
        &self,
        params: &HashMap<String, String>,
        auth_header: Option<&str>,
    ) -> Result<(String, String), String>; // Returns (client_id, client_secret)
}

struct BasicAuthenticator {
    client_repository: Arc<dyn ClientRepository>,
}

struct PostAuthenticator {
    client_repository: Arc<dyn ClientRepository>,
}

struct CompositeAuthenticator {
    authenticators: Vec<Arc<dyn ClientAuthenticator>>,
}
```

### Benefits
- ✅ Easy to add new authentication methods (JWT, mTLS, etc.)
- ✅ Clear separation of authentication concerns
- ✅ Configurable authentication strategies

## 4. Extract Token Generation (SRP + OCP)

### Current Issue
Token generation logic embedded in `OAuthServer`.

### Proposed Solution
```rust
trait TokenGenerator {
    fn generate_access_token(
        &self,
        user_id: &str,
        client_id: &str,
        scope: &Option<String>,
        token_id: &str,
    ) -> Result<String>;
    
    fn generate_refresh_token(
        &self,
        client_id: &str,
        user_id: &str,
        scope: &Option<String>,
    ) -> Result<String>;
}

struct JwtTokenGenerator {
    key_manager: Arc<Mutex<KeyManager>>,
    config: Config,
}

struct OpaqueTokenGenerator {
    // For opaque token implementation
}
```

### Benefits
- ✅ Easy to switch token formats (JWT vs opaque)
- ✅ Configurable token generation strategies
- ✅ Better testability

## 5. Extract Cross-Cutting Concerns

### Rate Limiting
```rust
trait RateLimiter {
    fn check_rate_limit(&self, identifier: &str, endpoint: &str) -> Result<()>;
}

struct DatabaseRateLimiter {
    rate_repository: Arc<dyn RateRepository>,
    config: Config,
}

struct RedisRateLimiter {
    redis_client: redis::Client,
    config: Config,
}
```

### Audit Logging
```rust
trait AuditLogger {
    fn log_event(&self, event: AuditEvent) -> Result<()>;
}

struct DatabaseAuditLogger {
    audit_repository: Arc<dyn AuditRepository>,
}

struct JsonFileAuditLogger {
    file_path: PathBuf,
}
```

## 6. Refactor HTTP Layer (SRP)

### Current Issue
`Server` mixes HTTP protocol with OAuth business logic.

### Proposed Solution
```rust
struct HttpServer {
    router: Router,
    config: Config,
}

struct Router {
    oauth_handler: Arc<OAuthHandler>,
    static_handler: Arc<StaticHandler>,
}

struct OAuthHandler {
    oauth_service: Arc<OAuthService>, // Renamed from OAuthServer
}

// Clean separation between HTTP concerns and OAuth business logic
```

## 7. Dependency Injection Container

### Proposed Solution
```rust
struct ServiceContainer {
    // Repositories
    client_repository: Arc<dyn ClientRepository>,
    token_repository: Arc<dyn TokenRepository>,
    auth_code_repository: Arc<dyn AuthCodeRepository>,
    
    // Services
    client_authenticator: Arc<dyn ClientAuthenticator>,
    token_generator: Arc<dyn TokenGenerator>,
    rate_limiter: Arc<dyn RateLimiter>,
    audit_logger: Arc<dyn AuditLogger>,
    
    // Grant handlers
    grant_registry: Arc<GrantHandlerRegistry>,
}

impl ServiceContainer {
    fn new(config: &Config) -> Result<Self> {
        // Wire up all dependencies
    }
}
```

## Implementation Strategy

### Phase 1: Repository Extraction
1. Create repository traits
2. Move database methods to specific repositories
3. Update `OAuthServer` to use repositories

### Phase 2: Grant Handler Extraction
1. Create `GrantHandler` trait
2. Extract authorization code handler
3. Extract refresh token handler
4. Create registry

### Phase 3: Cross-Cutting Concerns
1. Extract rate limiting
2. Extract audit logging
3. Extract authentication

### Phase 4: HTTP Layer Cleanup
1. Separate HTTP protocol from business logic
2. Create clean request/response handlers

### Phase 5: Dependency Injection
1. Create service container
2. Wire up all dependencies
3. Update main.rs to use container

## Benefits of This Refactoring

### Maintainability
- ✅ Easier to understand and modify individual components
- ✅ Clear separation of concerns
- ✅ Reduced coupling between components

### Extensibility
- ✅ Easy to add new grant types
- ✅ Easy to swap implementations (database, token format, etc.)
- ✅ Configurable strategies for cross-cutting concerns

### Testability
- ✅ Each component can be tested in isolation
- ✅ Easy to create mock implementations
- ✅ Better unit test coverage

### Production Readiness
- ✅ Easy to scale different components independently
- ✅ Better observability and monitoring capabilities
- ✅ More flexible deployment options