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Server-Side System Architecture (NestJS Backend)

Executive Summary​

The Learnille Server (server/) is an enterprise-grade NestJS RESTful API backend engineered for multi-tenant online learning, 1-on-1 expert consultations, real-time presence, wallet payouts, and multi-currency transaction processing.

🏛 1. High-Level Server System Diagram​

                                      +------------------------------------+
                                      |   Client Apps (Student, Admin,     |
                                      |      Instructor, Consultant)       |
                                      +-----------------+------------------+
                                                        |
                                           HTTPS / REST / WebSockets
                                                        |
                                                        v
+---------------------------------------------------------------------------------------------------+
|                                 NESTJS SERVER BACKEND (`server/`)                                 |
|                                                                                                   |
|  +---------------------------------------------------------------------------------------------+  |
|  | ENTRY PIPELINE: Helmet -> Cors -> CookieParser -> ThrottlerGuard -> RequestLoggerMiddleware |  |
|  +----------------------------------------------+----------------------------------------------+  |
|                                                 |                                                 |
|  +----------------------------------------------v----------------------------------------------+  |
|  | GLOBAL INTERCEPTORS & FILTERS: ResponseInterceptor, AllExceptionsFilter, CurrencyConversion  |  |
|  +----------------------------------------------+----------------------------------------------+  |
|                                                 |                                                 |
|  +----------------------------------------------v----------------------------------------------+  |
|  | FEATURE MODULE DOMAINS:                                                                      |  |
|  | - AuthModule (JWT RS256, Passport, Google/Apple OAuth, CASL RBAC)                            |  |
|  | - CourseModule (Curriculum Editor, Sections, Video Progress)                                 |  |
|  | - ConsultationModule (Availability Calendar, Booking Engine, Slot Resolution)              |  |
|  | - WalletModule & WithdrawalModule (Escrow Hold, NGN Bank Resolution, OTP Verification)       |  |
|  | - ProductModule & ValidationModule (Quality Assurance Queue, State Machine)                  |  |
|  | - PaymentModule (Paystack & Flutterwave Gateways, Webhook Verification)                      |  |
|  | - OpenSearchModule (Vector Indexing, Full-Text Catalog Search)                               |  |
|  | - InstantdbModule (Real-Time Room State & Sync)                                              |  |
|  | - OutboxModule & BullmqModule (Transactional Outbox Pattern, Async Queues)                   |  |
|  | - NotificationModule (Novu Framework & Soketi WebSocket Channels)                            |  |
|  +----------------------------------------------+----------------------------------------------+  |
+-------------------------------------------------|-------------------------------------------------+
                                                  |
       +--------------------+---------------------+--------------------+--------------------+
       |                    |                     |                    |                    |
       v                    v                     v                    v                    v
+--------------+     +--------------+     +--------------+     +--------------+     +--------------+
| Self-Hosted  |     | Self-Hosted  |     | Cloudflare   |     | OpenSearch   |     | New Relic    |
|  PostgreSQL  |     |    Redis     |     |   R2 S3      |     | Index Engine |     | APM & Logs   |
| (TypeORM DB) |     |  (BullMQ)    |     |  (Storage)   |     | (Vector Srch)|     | & Sentry     |
+--------------+     +--------------+     +--------------+     +--------------+     +--------------+

🛠 2. Request Handling Pipeline & Lifecycle​

When an HTTP request enters the NestJS server, it passes through a deterministic processing chain:

[ Incoming HTTP Request ]
       |
       v
1. [ Security & Middleware Layer ]
   - Helmet (Security headers)
   - Cookie Parser
   - AppRequestLoggerMiddleware (Pino/Winston request logger)
       |
       v
2. [ Rate Limiting Guard ]
   - ThrottlerGuard (Redis-backed rate limiting with RedisThrottlerStorage)
       |
       v
3. [ Authentication & Authorization Guards ]
   - JwtAuthGuard (Passport JWT RS256/HS256 validation)
   - RolesGuard / CASL AbilityGuard (Role checks: STUDENT, INSTRUCTOR, CONSULTANT, ADMIN)
       |
       v
4. [ DTO Input Validation Pipe ]
   - ValidationPipe (class-validator DTO checking with transform: true)
       |
       v
5. [ Controller Action Handler ]
   - Extracts payload via @Body(), user via @CurrentUser()
   - Delegates business logic to domain service
       |
       v
6. [ Interceptors & Exception Filters ]
   - CurrencyConversionInterceptor (converts prices dynamically via dynamic header)
   - ResponseInterceptor (wraps output into standard { statusCode, message, data } JSON)
   - AllExceptionsFilter (catches and formats HTTP and system errors for client & Sentry)
       |
       v
[ Formatted HTTP Response ]

📦 3. Deep Domain Module Architecture​

3.1. Authentication & Security Subsystem (auth/, casl/, crypto/)​

  • Token Architecture: RS256 asymmetric key pairs (CryptoModule). Short-lived JWT access tokens + Redis-rotated refresh tokens.
  • Social Login: OAuth flow handling for Google (google/) and Apple (auth/). Account linking endpoints permit attaching social credentials to an existing user account.
  • CASL Role Permissions: Fine-grained access control supporting 4 primary roles (Student, Instructor, Consultant, Admin).

3.2. Course Curriculum Engine (course/, category/)​

  • Normalized Curriculum: Supports hierarchical structure: Course -> Section -> Subsection -> Item (Video, Document, Quiz).
  • Progress Tracking: Continuous video timestamp tracking via heartbeat endpoints (/course/video-progress) recalculating student completion percentage dynamically.
  • Category Hierarchy: Category tree mapping supporting parent-child categories, featured flags, and display order batch updating.

3.3. Consultation Booking & Availability (consultation/, consultant/)​

  • Dual Mode: Supports ONEOFF (specific single date/time) and RECURRING (weekly interval windows) consultations.
  • Availability Engine: Calculates available time slots dynamically by cross-referencing consultant weekly hours, booked slot reservations, minimum notice buffers (e.g. 2 hours notice required), and timezone offsets.
  • Reschedule Workflow: Multi-party rescheduling protocol allowing students or consultants to propose pending slot changes.

3.4. Escrow, Wallet & Withdrawal System (wallet/, withdrawal/, payments/)​

  • Escrow Payment Hold: Funds from course sales or bookings enter pending_hold in provider wallets, releasing to available_balance upon course refund window expiry or consultation completion.
  • Dual Payment Gateways: Paystack and Flutterwave adapters with strict webhook signature verification (x-paystack-signature, Flutterwave hash).
  • Multi-Step Withdrawal Flow:
    1. Add bank account method (/withdrawals/methods). Bank account holder name is resolved via Flutterwave NGN lookup.
    2. Verify account via SMS/email OTP (/withdrawals/methods/verify).
    3. Initiate withdrawal request (/withdrawals/request).
    4. Verify withdrawal with secondary OTP (/withdrawals/:id/verify).
    5. Admin approves and executes automated bank payout (/withdrawals/:id/approve).

3.5. Transactional Outbox & Background Processing (outbox/, bullmq/)​

  • Outbox Pattern: Changes to database entities write an outbox event in the same DB transaction (outbox_events).
  • BullMQ Workers: BullMQ worker polls and processes outbox events asynchronously, handling external webhook dispatches, email triggers via Novu, and real-time Soketi broadcasts with automatic retries and dead-letter queues.

3.6. Search & Vector Engine (opensearch/)​

  • Catalog Indexing: Auto-indexes published courses and consultations into OpenSearch.
  • Vector Search: Supports hybrid keyword search and semantic vector embeddings for recommendations and fast discovery.

🗄 4. Database & Persistence Standards​

  • Primary Database: PostgreSQL accessed via TypeORM.
  • Naming Standard: All database columns use snake_case transformation (created_at, user_id, is_published).
  • Migrations: TypeORM migration scripts (server/src/migrations/) maintain database schema changes. Direct schema alteration in production is strictly prohibited.

📊 5. Server Configuration & Observability​

  • Centralized Config: ConfigModule validates all environment variables on boot via validateConfig in server/src/config.ts.
  • Monitoring: Integrated with New Relic APM for transaction tracing, New Relic Log Enrichment for Winston logs, and Sentry for real-time exception reporting.
  • Health Diagnostics: HealthModule powered by NestJS Terminus monitoring DB connection status, Redis availability, and host memory usage (/health).